Time Since Rewetting Defines Vegetation Composition and Carbon Dioxide Fluxes on Former Milled Peatlands - Comparison With Undisturbed Bogs

Time Since Rewetting Defines Vegetation Composition and Carbon Dioxide Fluxes on Former Milled Peatlands - Comparison With Undisturbed Bogs | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Time Since Rewetting Defines Vegetation Composition and Carbon Dioxide Fluxes on Former Milled Peatlands - Comparison With Undisturbed Bogs Anna-Helena Purre, Mati Ilomets This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-323253/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 5 You are reading this latest preprint version Abstract Rewetting is the most common restoration approach for milled peatlands in Europe, with the aim of creating suitable conditions for the development of peatland specific plant cover and carbon accumulation. Therefore, it is important to determine if time since rewetting is pivotal for milled peatlands to become functionally and structurally similar to their undisturbed counterparts. We investigate the temporal succession in rewetted peatlands in Estonia by a chronosequence of 4, 15, and 35 years before the measurements. Plant functional type (PFT) cover and biomass, bryophyte production and CO 2 fluxes were measured on two milled peatlands, as well as undisturbed bogs adjacent to milled peatlands. Differences in vegetation composition and CO 2 fluxes between the sites were greater for rewetted than undisturbed sites. The most recently rewetted site was mainly covered in bare peat and Eriophorum vaginatum and was a CO 2 source. On the rewetted site of 15 years, Sphagnum was present in addition to ombrotrophic sedges, and in the rewetted site of 35 years, lawn-hollow microtopography is starting to develop with various PFTs. Both of these sites were CO 2 sinks. Lawn Sphagnum was abundant on the two older rewetted sites, and was connected with CO 2 sink functioning in the rewetted sites. Still, hummock Sphagnum species, which were present in undisturbed bogs, were absent from all of the rewetted sites. With time, CO 2 fluxes, microtopography and vegetation develop after rewetting in the direction of undisturbed bogs, while vegetation composition still differs from the reference sites even 35 years after rewetting. Environmental Policy Milled peatlands above-ground biomass rewetting CO2 exchange reference ecosystem peatland restoration Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 1. Introduction Undisturbed peatlands are important carbon sinks in the long term (Yu 2012 ) as well as a suitable habitat for plant species that have adapted to survive in acidic and waterlogged conditions (Minayeva 2008). Northern peatlands have been widely affected by peat mining for horticulture or energy production (Leifeld et al. 2019 ), particularly since the 1950s when peat milling became the main technique for peat extraction. This method involves drainage and the removal of vegetation in large areas, so thin layers of peat can be extracted every summer season. Excavated peatlands have several negative environmental effects, such as peat loss through mineralisation, high CO 2 emissions, fire hazard, no plant diversity and low aesthetic value. The natural revegetation of those site takes a long time, depends on the environmental conditions of the site and usually does not lead to mire-specific plant communities (Lavoie et al. 2003 ; Graf et al. 2008 ; Orru et al. 2016 ). Unrestored milled peatlands are important CO 2 sources to the atmosphere due to the low water tables allowing peat mineralisation and sparse or absent vegetation (Strack et al. 2016 ; Rankin et al. 2018 ). The main mitigation possibility for those negative impacts is peatland rewetting, which through higher water tables creates suitable conditions for revegetation and thus reduces CO 2 emissions (Wilson et al. 2016 ) and the flammability of these sites (Granath et al. 2016 ). Although rewetting increases CH 4 emissions on restored peatlands, but as CH 4 is a short-lived gas in the atmosphere, rewetting of peatlands mitigates the climate change in long-term (Günther et al. 2020 ). Various criteria have been taken into account throughout the history of restoration ecology to assess restoration success. First, biodiversity measures and hydrology were engaged to indicate the success of restoring the ecosystem, which in recent decades have been integrated with greenhouse gas balances showing recovery of ecosystem functioning (Kløve et al. 2017 ; Renou-Wilson et al. 2018 ). The initial response of the plant community and its diversity to rewetting is complex and depends also on the pre-rewetting state of the peatland (Tuittila et al. 2000 ). Over the time-scale of several decades after peatland rewetting or self-recovery, plant cover increases with time (Orru et al. 2016 ; Priede et al. 2016 ). Furthermore, CO 2 fluxes change in time after rewetting (Kløve et al. 2017 ). Beyer and Höper ( 2015 ) estimate based on their experience of greenhouse gas measurements in temperate peatland that rewetted peat extraction sites may become peat accumulating ecosystems after about 30 years. Even after 30 years post rewetting, milled peatlands can remain CO 2 sources, but those emissions from rewetted sites tend to be smaller than from active peat extraction sites, especially if Sphagnum is dominating (Samaritani et al. 2011 ; Vanselow-Algan et al. 2015). The rate of vegetation recovery likely depends on successfully raising the water table (Konvalinková and Prach 2014 ; Strack et al. 2014 ; Priede et al. 2016 ), the presence of plant propagules (Campbell and Rochefort 2003 ; Konvalinková and Prach 2014 ) and the nutrient status of the site (Komulainen et al. 1999 ; Konvalinková and Prach, 2014 ; Kozlov et al. 2016 ; Priede et al. 2016 ). However, even after decades, rewetted boreal peatlands mostly differ from their reference ecosystem regarding carbon accumulation and vegetation structure (Moreno-Mateos et al. 2012 ). The quicker return of CO 2 sink function has been reported on milled peatlands restored using the moss-layer-transfer technique (Strack et al. 2016 ; Nugent et al. 2018 ). Milled peatlands have been ombrotrophic bogs and in some cases bog remnants border the peat extraction areas. Usually during peat extraction, layers of Sphagnum peat are removed, so more nutrient-rich peat deposits at the bottom of the former mire with varying thickness are left on the site. This leads to the development of wet minerotrophic vegetation (Tuittila et al. 2000 ; Renou-Wilson et al. 2018 ), which is different from typical raised bog vegetation (Renou-Wilson et al. 2018 ). With vegetation succession and distancing peatland surface from the water table minerotrophic vegetation will be replaced by bog species in time (Samaritani et al. 2011 ). Due to the challenges related to raising and keeping the stable water table close to the peat surface throughout the year (Price et al. 2003 ; Brown et al. 2017 ), functional dryness of the residual peat layer (Price and Whitehead, 2001 ; Price et al. 2003 ), the smaller water holding capacity of developed moss layer (Waddington et al. 2011 ; McCarter and Price 2015 ) and the moisture conditions being closely related with the microtopography of the site (Bugnon et al. 1997 ; Price et al. 1998 ; Price and Whitehead 2001 ; Purre and Ilomets, 2018 ) environmental conditions in restored sites may be drier than in undisturbed sites. Restoration sites have generally lower bryophyte and shrub but higher graminoid cover than in undisturbed peatlands (Soini et al. 2010 ; González et al. 2013 ; Strack et al. 2016 ). Some studies report that vegetation in rewetted sites is more heterogeneous than in pristine peatlands due to the patchiness and incompleteness of the plant cover on restoration sites as vegetation cover starts to develop near the drainage ditches and close to existing vegetation (Soini et al. 2010 ; Laine et al. 2016 ). Undisturbed peatlands are CO 2 sinks at least over longer time-scales, CO 2 exchange of the peatland can vary annually (Wilson et al. 2016 ; Nugent et al. 2018 ). Kløve et al. ( 2017 ) stress the lack of knowledge about the greenhouse gas fluxes in rewetted peatlands, especially on sites that have been rewetted for decades. Restored milled peatlands can have larger (Soini et al. 2010 ; Strack et al. 2016 ; Wilson et al. 2016 ; Nugent et al. 2018 , 2019 ) or smaller (Renou-Wilson et al. 2018 ; Nugent et al. 2019 ) CO 2 uptake than similar eco-hydrologically undisturbed sites. Similar carbon sequestration as in undisturbed peatlands is reported to return sooner to rewetted milled peatlands than the development of typical raised bog species composition (Soini et al. 2010 ; Renou-Wilson et al. 2018 ). Although photosynthesis on restored milled peatlands develops akin to undisturbed peatland, ecosystem respiration stays lower (Strack et al. 2016 ). Therefore, on rewetted sites, Net Ecosystem Exchange (NEE) is reported to be driven by respiration rather than by photosynthesis (Samaritani et al. 2011 ; Wilson et al. 2016 ). Due to their transitional stages, rewetted sites have large interannual and within-site variations in CO 2 fluxes (Wilson et al. 2016 ; Nugent et al. 2019 ), whereas undisturbed peatlands have reached a mature development stage regarding their vegetation, water retention capacity and CO 2 balance (Wilson et al. 2016 ). Plant communities vary by their photosynthesis and respiration rates. The highest photosynthesis on nutrient-poor peatlands is reported from Eriophorum communities (Beyer and Höper 2015 ; Wilson et al. 2016 ), but this is accompanied by higher respiration (Beyer and Höper 2015 ). Higher CO 2 net sink function (Kivimäki et al. 2008 ) and soil organic matter accumulation (Andersen et al. 2013 ) have been recorded on plots with mixed graminoid and Sphagnum patches compared to pure graminoid patches. Sphagnum species have lower photosynthetic capacities (Korrensalo et al. 2016 ) and lower respiration, therefore plots with Sphagnum are larger CO 2 sinks than plots with only graminoids (Beyer and Höper 2015 ). Vascular plants are also important in peatland CO 2 exchange, especially by mitigating the effect of drought on CO 2 sink functioning (Kuiper et al. 2014 ). In similar climatic conditions, correlating vegetation cover and CO 2 fluxes differs between land-use types (Laine et al. 2016 ; Strack et al. 2016 ) — in restored sites, CO 2 net uptake increases with vascular plant cover (Strack et al. 2014 ; 2016 ), while in undisturbed sites plots with higher moss cover are greater CO 2 sinks (Strack et al. 2016 ). In this paper we analyse the success of rewetting by comparing CO 2 fluxes and vegetation on relatively similar rewetted milled peatlands with somewhat different site conditions in different successional stages and initially eco-hydrologically similar nearby undisturbed bogs to assess if a longer time since rewetting or ecosystem recovery time leads to ecosystems that functionally converge to the state of reference bogs. For that, we established the following postulates: CO 2 fluxes and vegetation structure on rewetted milled peatlands develop in the direction of undisturbed reference bogs in time; Higher amount of plant above-ground, especially Sphagnum , biomass is related to higher CO 2 sink function on rewetted milled peatlands. 2. Methods 2.1. Study sites Two paired study sites (Kõrsa and Hara) were selected (Table 1 ), both of which include rewetted abandoned milled peatlands and remnant open raised bog areas (Fig. 1 ). The Kõrsa site is located in southwestern Estonia next to an active peat extraction site. The Kõrsa rewetted (Kõrsa R ) site has self-recovered after the end of peat extraction due to the water level being raised up to the peat surface in 1980 following a damming to create a firewater reservoir (Ramst et al. 2007 ). Revegetation began at Kõrsa R two years later with the self-establishment of E. vaginatum tussocks in 1982. The Hara site is located in Northern Estonia, in Lahemaa National Park. The Hara rewetted site divides into a self-recovered northern part (Hara RN ) and an actively rewetted southern part (Hara RS ). The water table in Hara RN rose in 2000–2002 due to closing of the bordering ditch, and vegetation started to develop earlier in that area. For conservative estimate of recovery time, year 2000 is considered as a start of recovery when analysing the effect of time since rewetting. Hara RS was rewetted in 2012 by the State Forestry Centre of Estonia. No additional restoration measures were applied in addition to rewetting. The rewetted and undisturbed sites of both paired sites initially had a similar ecohydrological status (raised bog), which is still present in the undisturbed parts of Kõrsa (Kõrsa P ) and Hara (Hara P ). Previously, the vegetation of the rewetted sites was described by Ramst et al. ( 2007 ), as the following: HaraRS: Eriophorum vaginatum and bare peat; Hara RN : E. vaginatum, Warnstorfia fluitans, Sphagnum riparium and Sphagnum cuspidatum ; Kõrsa R : E. vaginatum, Chiloscyphus pallescens, Pleurozium schreberi, Polytrichum strictum, W. fluitans, Brachythecium mildeanum, S. cuspidatum and Sphagnum balticum . The respective paired sites are located in the same mire-basin, and the distances between the rewetted and undisturbed sites range from 140 metres in Hara to 500 m in Kõrsa. In all of the study sites, four measurement plots per site were established during the previous year (2015). As two of the plots in Hara RN became flooded during the measurement period, they were omitted from the study, and data from two measurement plots in Hara RN were used. The locations of the permanent measurement plots were chosen based on the dominant vegetation and by taking into account its variability between micro-topographic levels. In undisturbed sites and Kõrsa R , where microtopography has already developed, two measurement plots were situated on the hummocks and two plots on the lawns of each site. In other sites, two replicates for each vegetation type were established. Table 1 Locations and descriptions of the study site Hara Kõrsa Coordinates N 59 o 33’, E 25 o 36’ N 58 o 24’, E 24 o 41’ End of extraction 1994 1980 Water table depth below surface (cm) a Hara RS : -10 Hara RN : 0 Hara P : -5 – -30 Kõrsa R : 0 – -15 Kõrsa P : -10 – -35 Long-term (1981–2010) average annual/growing season temperature ( o C) b,c 5.7/12.4 6.3/13.4 Long-term (1981–2010) average annual/growing season precipitation (mm annually/growing season) b,c 587/381 746/418 Average annual/growing season temperature ( o C) 2016 b,c 6.6/13.4 6.7/13.5 Average annual/growing season precipitation (mm annually) 2016 b,c 849/430 757/398 a - Average growing season water table according to manual measurements during the CO 2 measurement sessions; b - data from Estonian Weather Service; c - Growing season data for May-October. 2.2. Vegetation analysis Vegetation analyses were conducted on the plant functional type (PFT) level, and we used the PFT division described in Laine et al. ( 2012 ). In our study sites, the following PFTs were present from the larger number of PFTs described by Laine et al. ( 2012 ): Hummock Sphagnum : S. rubellum , S. fuscum and S. capillifolium ; Lawn Sphagnum : S. angustifolium , S. balticum , S. medium , S. fallax , S. papillosum , S. squarrosum, S. riparium and S. cuspidatum ; True mosses: Polytrichum strictum, Warnstorfia fluitans, Chiloscyphus pallescens, Pleurozium schreberi and Brachythecium mildeanum ; Evergreen shrubs: Calluna vulgaris and Vaccinium oxycoccus , Andromeda polifolia ; Ombrotrophic forbs: Drosera rotundifolia and Rubus chamaemorus ; Minerotrophic forbs: Melampyrum spp., Menyanthes trifoliate and Thelypteris palustris ; Ombrotrophic sedges: Eriophorum vaginatum ; Trees: Pinus sylvestris , Betula pubescens , and Salix spp. The nomenclature followed Ingerpuu and Vellak ( 1998 ) for bryophytes and Leht ( 2010 ) for vascular plants. We measured the plant species coverage (%), vascular plant leaf area index (LAI vasc ; m 2 m − 2 ), above-ground biomass of PFTs (AGB; g dm − 2 ) and moss production (AGP; g dm − 2 year − 1 ) as well as the length increment of mosses (LI; mm year − 1 ). The plant cover of measurement plots was determined visually at the peak of the 2016 growing season (end of July) from inside the CO 2 flux measurement collars (four plots per each site/management type combination, but two plots in Hara RN ). LAI vasc was determined according to Wilson et al. ( 2007a ) during the CO 2 flux measurement campaigns. Biomass samples were collected from near the measurement plots with vegetation as similar as possible to those in the collars. Vascular plant biomass samples were collected at the end of July 2016 and bryophyte samples at the beginning of October 2016 to capture the maximum biomass of each plant group. We used two plot sizes for the AGB measurements of vascular plants (15 cm radius circular plot) and bryophytes (2.5 cm circular round plot) and collected one vascular plant sample and three bryophyte samples per measurement point (a total of four vascular plant and 12 bryophyte samples from Hara RS , Hara P , Kõrsa R and Kõrsa P each, and two vascular plant and six bryophyte samples in Hara RN ). Only the capitula for Sphagnum species was used as there is no clear distinction between the live and dead material of Sphagnum (Clymo 1970 ), and the upper 2 cm layer for other bryophytes to obtain biomass samples up to the similar depth of biomass as Sphagnum were collected to determine bryophyte biomass similar to Moore et al. ( 2002 ), Laine et al. ( 2012 ) and Purre et al. ( 2019b ). The collected samples were divided into species level. The sampling and laboratory analysis of biomass is described in Purre et al. ( 2019b ). During the data analysis, the biomass of different species was compiled into PFTs. From the air-dried (65ºC) bryophyte samples, the border of AGP and LI was determined using the innate markers method (Clymo 1970 ; Pouliot et al. 2010 ), then separated from the rest of the biomass and weighed (AGP). The LI of ten individuals from the dominant species of each sample were measured with a digital caliper. In Hara RS , bryophytes were absent and thus biomass AGP and LI were considered to be zero. 2.3. CO 2 flux measurements and data processing CO 2 measurements were carried out at least once a month during the growing season (May-October 2016). NEE and ecosystem respiration (R ECO ) was measured on 60×60 cm square aluminium collars inserted to about a 20 cm depth, with the rim filled with water to ensure an air-tight fit during flux measurements. We measured CO 2 concentrations with the infrared gas analyser Li-6400 (Li-Cor (USA)) from transparent Plexiglas chamber (60×60×30 cm) with a cooling system. The measurements period was two minutes, and the CO 2 content in the chamber was recorded with an interval of 15 s. After measuring the CO 2 concentrations in full-light, NEE was measured on two lower irradiation levels by using one or two shades that reduced the photosynthetically active radiation (PAR (µmol m − 2 s − 1 )) reaching the vegetation in the chamber at an average of 65% and 88%, respectively. Lastly, R ECO was measured by covering the chamber with an opaque hood. Between each measurement period, the measurement chamber was ventilated. During the measurement campaigns, plant parameters for determining LAI vasc inside the measurement collars were measured according to Wilson et al. ( 2007a ) in addition to recording PAR, the temperature inside the chamber, peat temperatures at 5 cm and 15 cm depths and the water table (cm). Input data (PAR, T AIR ) for CO 2 flux reconstruction were measured with hourly intervals in stations belonging to the Estonian Weather Service. For Hara, the temperature data was obtained from the nearest station in Vanaküla (about 10 km from the site) and radiation data from Harku meteorological station (about 70 km from Hara). For Kõrsa, all meteorological data was obtained from Pärnu-Sauga meteorological station located about 15 km from the site. Those stations were the closest to the study sites where PAR and T AIR were continuously measured, and they were located within a 10 km distance from the sea similarly to the study sites. The flux rates were estimated based on linear change in CO 2 concentrations in time. The linear method was chosen, as this method was considered suitable by Kandel et al. ( 2016 ) for CO 2 flux calculations in the case of short (few minutes) chamber closure periods (2 min in current study). The measured NEE and R ECO fluxes were considered suitable according to the following quality criteria: variation of PAR during the flux measurement not exceeding ± 15%, variation of inside temperature of the chamber not varying more than ± 5°C and the determination coefficient (R 2 ) of the measured flux of at least 0.9. Very small fluxes (± 0.2 ppm s − 1 ) were accepted regardless of their R 2 value. Similar quality criteria in respect of R 2 values were used by Järveoja et al. ( 2016 ). A total of 215 CO 2 flux measurements fulfilled the set criteria and were used for CO 2 flux reconstructions. Photosynthesis (P g ) was calculated by adding R ECO to NEE. CO 2 fluxes were reconstructed for the period from the beginning of May until the end of September 2016 at each site. With these reconstructions, based on measured and calculated CO 2 fluxes and other parameters (PAR, LAI vasc and air temperature (T AIR )), models were created for relating differences in measured CO 2 fluxes with differences in input parameters for reconstructing the whole growing season CO 2 fluxes. CO 2 flux and LAI vasc reconstruction was carried out in program R version 3.2.2 package nlme (Linear and Nonlinear Mixed Effects Models, ver. 3.1–121; Pinheiro et al. ( 2015 )). Gaussian curves were fitted to LAI vasc values, which were calculated according to the vegetation parameters measured during the CO 2 measurement campaigns for reconstructing the change in LAI vasc during the vegetation season as described by Wilson et al. ( 2007a ) in each measurement collar. The gross photosynthesis (P g (mg CO 2 m − 2 h − 1 )) model uses the saturating response to PAR (Eq. 1 ) and records the change in LAI vasc during the vegetation season: where P max is the maximum photosynthesis at light saturation, k and s are respectively the PAR and LAI vasc values when P g reaches half of its maximum level. The respiration model (Eq. 2 ) expresses an exponential response of ecosystem respiration (R ECO (mg CO 2 m − 2 h − 1 )) to the temperature inside the chamber (T AIR ). Where parameters r0 and b are respectively the respiration at the 0°C temperature and the sensitivity of respiration to air temperature, and T AIR is the air temperature (°C). CO 2 measurements and reconstructions are described in more detail in Purre et al. ( 2019a , 2019b ). 2.4. Data analysis Data analyses were conducted with IBM SPSS ver. 23. As the data did not fulfil the requirements for parametric data analysis according to the Shapiro-Wilk test, non-parametric data analysis methods were chosen. The Kruskal-Wallis and Mann-Whitney tests with Bonferroni correction for the pairwise comparison of vegetation parameters and CO 2 fluxes between the sites were applied. Spearman correlations were used to relate separate plant group abundances with different parameters of CO 2 fluxes (NEE, P g , R ECO ) in rewetted and undisturbed peatlands. Generalized linear mixed models (GLMMs) were applied on data from rewetted milled peatlands to determine the effect of site, microtopography and time since rewetting (fixed factors) on CO 2 fluxes (growing season NEE, P g or R ECO as target variables), biomass of studied plant functional types (PFTs) were incorporated in the models as random factors. For information criterion of the GLMMs log-likelihood was used, lower log-likelihood values showing better model fit. The results were considered statistically significant if p < 0.05. Average values are reported with standard errors. The multivariate analysis methods Redundancy Analysis (RDA) and Detrended Correspondence Analysis (DCA) were applied in PC-ORD ver. 7 to relate the abundances of PFTs and CO 2 fluxes on rewetted and undisturbed sites, and to analyse the changes in those variables with time since rewetting, respectively. In RDA, the response variables were standardised and a randomisation test was applied to test for any significant relationship between the PFT and CO 2 flux matrices. DCA was used to find the main gradients in PFT and CO 2 flux data using time since rewetting and the site as supplementary variables. 3. Results 3.1. Vegetation Vegetation varied significantly between rewetted and undisturbed sites and between all rewetted sites, while small differences also occurred between both undisturbed sites (Fig. 2 ). More PFTs were present on undisturbed and older rewetted sites, while many PFTs such as Sphagnum and evergreen shrubs were absent from the recently rewetted Hara RS . Evergreen shrubs such as C. vulgaris and A. polifolia had higher cover in undisturbed sites, while V. oxycoccus was present with low cover only in Kõrsa R . Evergreen shrub biomass was absent or significantly lower in rewetted sites compared to undisturbed sites (Appendix S1). Ombrotrophic forbs R. chamaemorus and D. rotundifolia were only present in undisturbed plots, but with relatively low cover (0.5-3%). Only in Kõrsa R minerotrophic forbs like Melampyrum species and T. palustris were present. Tree seedling of Salix spp., Betula spp. and P. sylvestris had about 1% cover on all sites, or were absent. In undisturbed sites, hummock ( S. fuscum , S. rubellum, S. angustifolium ) and lawn ( S. medium , S. balticum , S. papillosum ) Sphagnum species were present in relatively similar cover (ranging from 5% ( S. balticum in Kõrsa) to 45% ( S. rubellum in Kõrsa)). Only lawn species ( S. medium , S. fallax , and S. squarrosum ) were present in Kõrsa R and Hara RN site. True mosses ( P. strictum and P. schreberi ) had low cover (1–3%) on Kõrsa R and Hara RN but were absent from all of the other study sites. Small differences in plant cover and AGB occurred between the measurement plots in hummocks and lawns. Hummocks had higher AGB (15.3 ± 1.3 g dm − 2 ), AGP of Sphagnum (3.4 ± 0.5 g dm − 2 year − 1 ) and mosses (3.5 ± 0.5 g dm − 2 year − 1 ) than lawns (AGB 9.9 ± 0.8 g dm − 2 ; AGP of Sphagnum 1.5 ± 0.5 g dm − 2 year − 1 and mosses 1.5 ± 0.4 g dm − 2 year − 1 ) (p < 0.05). In reverse, the cover of lawn Sphagna was higher in lawns (72 ± 14%) than in hummocks (21 ± 15%) (p < 0.05). 3.2. Carbon dioxide fluxes Measured NEE and R ECO varied spatially to a larger extent in rewetted rather than in undisturbed sites (Appendix S2). A higher CO 2 net uptake with higher PAR was measured on both rewetted and undisturbed sites. Reconstructed P g and NEE did not differ statistically significantly (p > 0.05) between the rewetted and the undisturbed sites, whereas R ECO was significantly higher in the rewetted than in the undisturbed sites (p < 0.05; Fig. 3 ). The respiration model’s parameter r0 was significantly (p < 0.05) higher in the rewetted (41.8 ± 11.4 mg CO 2 m − 2 h − 1 ) than in the undisturbed sites (9.4 ± 3.1 mg CO 2 m − 2 h − 1 ; Appendix S3). In Kõrsa R the reconstructed P g was significantly higher than in the undisturbed sites and at Hara RS (p < 0.05). Also, Kõrsa R had significantly higher R ECO than Kõrsa P , whereas all of the other sites had a similar R ECO . Although there were no differences in the model parameters between Hara RN, Hara RS and Hara P (p > 0.05), P max and r0 were higher in Kõrsa R than in Kõrsa P (p < 0.05) indicating a higher maximum CO 2 uptake in case of light saturation and also a higher minimum respiration rate in rewetted sites. Undisturbed sites did not differ significantly according to their CO 2 fluxes (p > 0.05). There were no significant differences in CO 2 fluxes between the hummocks and the lawns in the undisturbed sites and Kõrsa R (p < 0.05). GLMMs were used specify the effect of site, microtopography and time since rewetting on growing season CO 2 flux components (R ECO , P g , NEE) on rewetted peatlands. Although none of the fixed effects and GLMMs were statistically significant, time since rewetting had strongest effect on all of the CO 2 flux components (Table 2 ). In addition to time since rewetting, microtopography and combination of microtopography and site had also relatively strong, but still statistically insignificant effect on R ECO . Table 2 Statistical results of general linear mixed models (GLMM) determining effect of site conditions (site, microtopography, time since rewetting) on CO 2 flux components (NEE, P g , R ECO ) in rewetted milled peatlands. CO 2 flux component Effect F P NEE Site F 1,10 =0,03 0,87 Microtopography F 1,3 =0,00 0,95 Time since rewetting F 1,1 =2,93 0,40 Site*Microtopography F 1,8 =0,59 0,47 P g Site F 1,0 =0,03 1,00 Microtopography F 1,2 =0,12 0,76 Time since rewetting F 1,6 =1,87 0,22 Site*Microtopography F 1,10 =0,40 0,54 R ECO Site F 1,0 =0,00 1,00 Microtopography F 1,10 =3,50 0,09 Time since rewetting F 1,8 =3,74 0,09 Site*Microtopography F 1,9 =1,09 0,32 3.3. CO 2 fluxes and vegetation CO 2 fluxes correlate with every PFT differently between the undisturbed and rewetted plots (Fig. 4 , Appendix. S4). In the undisturbed sites, P g was higher in measurement plots with higher ombrotrophic sedge ( E. vaginatum ) cover and biomass but lower with higher tree cover, which was related with the higher values of the parameter k indicating the PAR value when P g reaches half of its maximum value. In the rewetted sites, NEE was higher in the case of higher Sphagnum abundance, and higher photosynthesis rates were connected with the cover of minerotrophic forbs. P g increases with higher bryophyte and vascular biomass in rewetted sites, whereas this correlation was insignificant in the undisturbed sites (Fig. 5 ). In the undisturbed sites, higher R ECO was measured on plots with higher vascular plant biomass, whereas this correlation was insignificant in the rewetted sites. There were no other statistically significant correlations between vascular plant, bryophyte and plant biomass, and P g , R ECO and NEE in the rewetted nor in the undisturbed plots. With time since rewetting, communities evolve in the direction of undisturbed mires, where several PFTs are present, including Sphagnum and evergreen trees (Fig. 6 ). Hara RS is characterised by high R ECO and biomass of ombrotrophic sedges, Hara RN and Kõrsa R contain lawn Sphagnum and P g , while undisturbed sites (Hara P and Kõrsa P ) have higher NEE along with the presence of hummock Sphagnum , ombrotrophic forbs and evergreen shrubs. With this transition, high R ECO is replaced with higher P g , and eventually with higher NEE, indicating CO 2 sink function during the growing season. 4. Discussion 4.1. Vegetation Vegetation differed significantly between the undisturbed and rewetted sites. When undisturbed sites had oligotrophic raised-bog vegetation, vegetation in rewetted sites was typical to more nutrient rich environmental conditions and higher water table as reported previously (Tuittila et al. 2000 ; Samaritani et al. 2011 ; Renou-Wilson et al. 2018 ). Commonly, the less humified Sphagnum peat has been removed from abandoned milled peatlands, as the mineral-rich substrate supports the establishment and development of more nutrient demanding plant species. Contrary, oligotrophic vegetation is prevailing in bogs where the peat layer is more nutrient-poor and the water level deeper. After rewetting, vegetation establishment is more rapid and species rich in sites with more nutrients (Komulainen et al. 1999 ; Kozlov et al. 2016 ). This could have caused the relatively rapid vegetation succession on Kõrsa R where peat ash content is reported to be about twice higher (2–3%) than in Hara rewetted sites (about 1–2%; Orru 1995 ). In Kõrsa R , a rather diverse peatland community with a thick Sphagnum mat had developed in about 35 years. Actually, in Kõrsa R a thin layer of new peat – an acrotelm – has formed, which means that the site is functionally (but not structurally) quite similar to a pristine bog. According to results reported by Lucchese et al. ( 2010 ), about a 19 cm thick bryophyte layer would be needed in the Bois-des-Bel restored milled peatland in Canada to mitigate summer water level drawdown; this could be reached about 17 years after restoration. Throughout the study period in Kõrsa R and Hara RS , the water level stayed inside the moss layer, mainly near the moss surface, therefore not decreasing the moss growth during the summer period. In the rewetted sites with thick moss layer in the current study, the moss layer was looser than in the undisturbed reference sites. This was probably due to the higher water table along with the high abundance of hollow Sphagna in the rewetted sites. Hollow Sphagnum could be affected from extreme droughts to a larger degree due to their larger pore size and less connectivity with the residual peat layer (McCarter and Price, 2015 ) than the denser Sphagnum cover of undisturbed bogs, therefore making CO 2 exchange on rewetted sites more susceptible to drought impacts. Some PFTs were lacking or had very low abundances in the rewetted sites but were present in the reference sites. We found significantly lower biomass and cover of evergreen shrubs on the rewetted than in the undisturbed sites, similar to results by Soini et al. ( 2010 ) and González et al. ( 2013 ), and they were absent from the most recently rewetted sites. Hummock Sphagna , which was present in both undisturbed bog sites was completely absent from the rewetted sites. The low occurrence and dying-off of hummock Sphagnum due to high water tables has been reported previously by Soini et al. ( 2010 ) and González et al. ( 2013 ). In reverse, Karofeld et al. ( 2015 ) recorded relatively high cover of hummock Sphagna and the presence of shrubs on restored milled peatland site where those species were dispersed using the moss-layer-transfer technique (Rochefort et al. 2003 ). Therefore, the application of this technique could lead to a more diverse vegetation composition of restoration sites. While vegetation differs significantly between the rewetted sites, being more diverse in the older sites, the vegetation in both undisturbed sites with a similar hummock and hollow vegetation pattern did not differ from each other. Hummocks on the two undisturbed sites are typical Calluna-vulgaris-Sphagnum fuscum communities, the most common plant associations in Estonian bogs (Masing 1982 ), and are comparable to the high hummock communities described by Korrensalo et al. ( 2018 ). Lawns in the undisturbed sites belong to the tussocky Eriophorum community or the Sphagnum balticum-Sphagnum rubellum community (Masing 1982 ), described also by Korrensalo et al. ( 2018 ) in an undisturbed bog in central Finland as lawn and high lawn communities. A large variation in vegetation occurred in rewetted, especially in the most recently rewetted site of Hara RS between the measurement plots. However, this could also be caused by the relatively low number of measurement plots in each study site and their positioning on the site. In recovering milled peatlands, vegetation is developing in patterns due to large variations in suitable substrate conditions for plant growth (Tuittila et al. 2000 ; Purre and Ilomets 2018 ) and the presence of nurse-plant species (Tuittila et al. 2000 ; Groeneveld et al. 2007 ), whereas in undisturbed bogs microtopography explains the largest portion of variation in vegetation composition (Korrensalo et al. 2018 ; Mežaka et al. 2018 ). Sphagnum has been considered a keystone genus of peatland restoration (Rochefort 2000 ). In the newly rewetted Hara site, Sphagnum was not yet present in the measurement plots, although some patches of lawn Sphagnum (mainly Sphagnum cuspidatum ) were present in depressions with high water level. After rewetting, the height of the water table should remain a few centimetres below the peat surface, which leads to optimal conditions for Sphagnum growth and peat accumulation (Beyer and Höper 2015 ). Sphagnum has relatively high immigration potential (Campbell et al. 2003 ) and is abundant on the undisturbed plots bordering the rewetted ones, so further colonisation of Sphagna in recently rewetted sites is expected. In both older rewetting sites, Sphagnum had almost total cover. In addition, in the oldest Kõrsa R site, lawn Sphagnum species have created some relatively high hummocks and overgrow E. vaginatum tussocks. The AGP and IL of Sphagnum in the rewetted sites was similar to those reported by Ilomets ( 1982 ) in Estonian undisturbed peatlands, while we measured about double the production and somewhat higher IL of Sphagna on the undisturbed sites. This probably results from different methods used for growth measurements (Pouliot et al. 2010 ), variations in weather conditions (Vitt 1990 ; Bengtsson et al. 2020 ) and species composition (Lindholm and Vasander 1990 ; Bengtsson et al. 2020 ). 4.2. Carbon dioxide fluxes Both the undisturbed sites and the older rewetted sites were CO 2 net sinks during the growing season, while the more recently rewetted site was still a CO 2 source. Variations in CO 2 fluxes between the rewetted sites are large due to differences in vegetation, weather and water levels — while some sites are important CO 2 sinks (Tuittila et al. 1999 ; Beyer and Höper 2015 ; Wilson et al. 2016 ; Lee et al. 2017 ; Purre et al. 2019a ), others could be small CO 2 sources (Tuittila et al. 1999 ; Waddington and Warner 2001 ; Beyer and Höper 2015 ; Purre et al. 2019a ). Although rewetted sites could be CO 2 sources in the first decades after rewetting, they should become a CO 2 net sink with time (Samaritani et al. 2011 ). Similar (Komulainen et al. 1999 ) or higher (Soini et al. 2010 ; Strack et al. 2016 ) CO 2 net uptake on rewetted sites as in reference sites has been reported about ten years after rewetting, which is consistent with our results. NEE in the rewetted sites is rather connected with differences in R ECO than photosynthesis (Samaritani et al. 2011 ; Wilson et al. 2016 ). Similarly to our results from the Hara rewetted site, lower CO 2 net uptake due to higher respiration has been reported from newly rewetted sites than from undisturbed bogs (Urbanová et al. 2012 ). In reverse, in the studies by Soini et al. ( 2010 ), Christen et al. ( 2016 ) and Strack et al. ( 2016 ), higher P g compensated for high R ECO , therefore leading to a higher CO 2 net uptake on a rewetted site, which is consistent with our results from the Kõrsa R . CO 2 fluxes and model parameters varied stronger between the measurement plots of the rewetted sites compared to undisturbed sites, as also reported by Soini et al. ( 2010 ), Laine et al. ( 2016 ) and Strack et al. ( 2016 ). This could likely be driven by larger variations in PFT cover in the rewetted sites. Unvegetated plots on rewetted sites remain CO 2 sources (Wilson et al. 2016 ; Purre et al. 2019a ) but measurement plots turn from a CO 2 source to a sink with increasing plant cover (Strack et al. 2016 ; Purre et al. 2019a ). Respiration on younger sites with still fragmented vegetation cover and lower diversity of plant species is largely influenced by peat temperature and water table depth, whereas those factors have a smaller effect on sites where vegetation has recovered well (Waddington and Warner 2001 ; Samaritani et al. 2011 ; Vanselow-Algan et al. 2015). Therefore, it could be expected that the CO 2 sink function will increase and be more stable with secondary succession after rewetting, especially as the actual acrotelm is formed with time. We detected some effect of site status on plant above-ground biomass, which on rewetted sites had a strong positive correlation with photosynthesis, whereas in undisturbed plots the correlation between plant biomass and P g was insignificant. Similarly to our rewetted sites, Marinier et al. ( 2004 ) reported higher photosynthesis in plots with higher AGB, but plots with high AGB have also been reported to have higher R ECO (Marinier et al. 2004 ; Strack et al. 2016 ; Brown et al. 2017 ). This was not the case in our rewetted sites, although in the undisturbed sites, R ECO and vascular plant biomass had a strong positive correlation. The lack of correlations between the R ECO and vascular plant biomass on rewetted milled peatlands is probably due to the domination of heterotrophic respiration on such sites (Wilson et al. 2007b ; Järveoja et al. 2016 ; Purre et al. 2019a ). Laine et al. ( 2016 ), Strack et al. ( 2016 ) and Purre et al. ( 2019b ) also reported interaction between peatland management (undisturbed, rewetted), PFTs and carbon sequestration. According to Järveoja et al. ( 2016 ), those correlations depend on water level depth — if the water level is high in restored milled peatlands, bryophyte cover correlates with NEE, P g and autotrophic respiration, whereas with deeper water table CO 2 fluxes correlated with vascular plant cover. Therefore, the different correlations on rewetted and undisturbed sites are consistent with previous studies (Strack et al. 2016 ) and could be related to differences in water table height and fluctuations on sites with different management. There are large differences in photosynthetic capacities between PFTs. In the undisturbed sites, we measured higher photosynthesis and maximum photosynthesis rates (P max ) in the case of higher E. vaginatum cover. Vascular plant, especially graminoid biomass, has a relatively large impact on NEE in comparison with their abundance (Laine et al. 2012 ; Hassanpour Fard et al. 2020 ), due to their high photosynthetic capacity (Komulainen et al. 1999 ; Kivimäki et al. 2008 ; Urbanová et al. 2012 ; Strack et al. 2014 ; Laine et al. 2016 ). As E. vaginatum was present or abundant on most of the rewetted plots, the lack of correlation between the sedge cover and photosynthesis on the rewetted sites was unexpected. In addition to having high maximum photosynthesis (P max ), this sedge species also has high light use efficiency (parameter k in the photosynthesis model) (Kivimäki et al. 2008 ) and high respiration rate (Jordan et al. 2016 ). Still, in the case of a high water table, rewetted sites with high E. vaginatum cover have a CO 2 net sink function, even in unfavourable habitat conditions such as the occasionally lower water table during drought periods (Tuittila et al. 1999 ). In the rewetted sites, higher photosynthesis and P max were measured with higher evergreen shrub cover. Evergreen shrubs stand out from other vascular plants with low photosynthesis and respiration rates (Laine et al. 2016 ), while in reverse Korrensalo et al. ( 2016 ) reported high maximum photosynthesis rates on evergreen shrubs like A. polifolia , C. vulgaris and V. oxycoccus , which are also present in the undisturbed sites and Kõrsa rewetted site in our study. According to Korrensalo et al. ( 2016 ), the P max of evergreen shrubs varies between species belonging to the same PFT. Still, the cause of controversies between different studies remains unclear and can be result of a rather low number of measurements that do not cover the whole ecosystem variation. High photosynthesis in the case of higher evergreen shrub cover in this study could also be connected with higher plant cover and the number of PFTs on the measurement plots in Kõrsa R where evergreen shrubs were present. According to Kivimäki et al. ( 2008 ), the presence of different PFTs lowers the R ECO /P g ratio, so creating conditions for higher CO 2 net uptake as in Kõrsa, while in monostands of E. vaginatum this ratio is higher, which also explains a lower CO 2 net uptake, as well as CO 2 net emissions from the younger site in this study. According to Hassanpour Fard et al. ( 2020 ), the presence of some key species or PFTs either in monostand or in mixed community support the larger carbon accumulation during the growing season than the mixed communities with a different number of PFTs lacking such certain species. Whereas most vascular plants, especially sedges, have high photosynthesis rates during summer when their LAI is highest, the importance of Sphagnum in CO 2 sequestration expresses itself during spring and autumn, when LAI vasc is low (Korrensalo et al. 2017 ). In the rewetted sites, CO 2 net sink function was larger in plots with higher Sphagnum cover. Sphagnum has lower photosynthetic capacities than vascular plants (Laine et al. 2012 ; Christen et al. 2016 ; Korrensalo et al. 2016 ) and also low respiration rates (Waddington and Warner 2001 ; Laine et al. 2016 ), and by increasing soil moisture content, a Sphagnum carpet could reduce soil respiration (Waddington and Warner 2001 ). However, restoring the Sphagnum carpet may not be enough for CO 2 sequestering (Samaritani et al. 2011 ), especially as a newly formed Sphagnum carpet is sensitive to drier conditions (Tuittila et al. 2004 ). Therefore, constant high water tables are necessary, which support CO 2 accumulation of those sites early on after restoration activities (Günther et al. 2017 ). Limitations of the study This paper contributes to the growing but rather sparse knowledge base surrounding peatland restoration, engaging peatlands with different stages after rewetting and also several vegetation variables in addition to CO 2 flux measurements. However, some limitations of the study must be taken into account when considering the results. First, the study was conducted at a relatively low number of measurement points in the rewetted sites, especially in Hara RN . This could have affected the statistical analysis results regarding CO 2 fluxes as well as the vegetation variables to some extent, especially in case of GLMMs. For each vegetation type in each site, there were two true replicates, and one (Hara RN ) or two (all other sites) dominant vegetation types were covered in each study site. Also, the different rewetted milled peatlands or their fields had different time since rewetting, therefore the site conditions could have been affected somewhat the conclusions about the effect of time since rewetted. Still according to GLMMs time since rewetting was the main factor explaining the CO2 flux components on the rewetted study sites. Second, the study covered only one growing season, so the annual balances of CO 2 cannot be derived from this. The CO 2 sequestration of the sites presented here are also strongly affected by weather conditions during that year, so they can differ from other years with varying conditions as shown at the Hara rewetted site by Purre et al. ( 2019a ). Also, although all of the sites were open peatland sites, the CO 2 exchange and biomass related with scarce tree cover were not accounted for in any of the studied sites. In addition, uncertainties related to flux measurements and reconstructions could affect the source or sink function of the sites during the growing seasons, especially if fluxes are very low and uncertainties higher (Bubier et al. 1999 ). Third, the methane emissions, along with dissolved organic carbon and dissolved inorganic carbon, were not measured from the study sites in this paper, as the general aim of the study was to analyse the differences in plant production parameters and PFT composition closely related with the CO 2 fluxes. Therefore, the results presented here do not provide information about the full carbon balance of the sites, as methane emissions for such sites have been reported to be high (Strack et al. 2014 , 2016 ; Vanselow-Algan et al. 2015; Beyer and Höper 2015 ; Günther et al. 2017 ). Within these limitations, we still hope the paper will be of interest for a wide audience of peatland ecologists. Conclusion Although vegetation structure on rewetted milled peatlands approaches this on reference sites with time, some plant functional types present in the undisturbed reference sites, e.g., shrubs, colonise these sites in the later development stages and hummock Sphagnum could be absent even decades after rewetting. Vegetation composition developing with time affects the carbon accumulation of rewetted sites. During the studied growing season, over a decade ago rewetted milled peatlands were carbon sinks similarly to the reference sites, whereas the most recently rewetted site was still a carbon source to the atmosphere. Although graminoids play an important role in the photosynthesis of rewetted sites, as they do in undisturbed reference bogs, the carbon accumulation of rewetted peatlands is related with development of the Sphagnum mat, which is present in the reference sites. A well-developed Sphagnum mat also reflects the development of other environmental variables, of a functioning acrotelm and the development of a C sink function. Thus, a well-developed Sphagnum lawn could be used as an indicator of successful restoration. However, general plant functional type composition can still differ from reference sites in some accounts even several decades after rewetting. Conclusion Although vegetation structure on rewetted milled peatlands approaches this on reference sites with time, some plant functional types present in the undisturbed reference sites, e.g., shrubs, colonise these sites in the later development stages and hummock Sphagnum could be absent even decades after rewetting. Vegetation composition developing with time affects the carbon accumulation of rewetted sites. During the studied growing season, over a decade ago rewetted milled peatlands were carbon sinks similarly to the reference sites, whereas the most recently rewetted site was still a carbon source to the atmosphere. Although graminoids play an important role in the photosynthesis of rewetted sites, as they do in undisturbed reference bogs, the carbon accumulation of rewetted peatlands is related with development of the Sphagnum mat, which is present in the reference sites. A well-developed Sphagnum mat also reflects the development of other environmental variables, of a functioning acrotelm and the development of a C sink function. Thus, a well-developed Sphagnum lawn could be used as an indicator of successful restoration. However, general plant functional type composition can still differ from reference sites in some accounts even several decades after rewetting. Declarations Funding: No funding was received for conducting this study. Conflicts of interests: The authors have no conflicts of interest to declare that are relevant to the content of this article. Availability of data and material: The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request. Code availability: The codes used for data modelling and analysis are available from the corresponding author upon reasonable request. Author contributions: Both authors contributed to the study conception and design. Anna-Helena Purre conducted the material preparation, fieldwork, laboratory and data analysis, and she wrote the first draft of the manuscript. Mati Ilomets commented on previous versions of the manuscript. Both authors read and approved the final manuscript. 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Wetl Ecol Manag 11:65-83 Price J, Rochefort L, Quinty F (1998) Energy and moisture considerations on cutover peatlands: surface microtopography, mulch cover and Sphagnum Ecol Eng 10:293-312 Price J, Whitehead GS (2001) Developing hydrologic thresholds for Sphagnum recolonization on an abandoned cutover bog. Wetlands 21:32-40 Priede A, Mežaka A, Dobkeviča L, Grīnberga L (2016) Spontaneous revegetation of cutaway fens: can it result in valuable habitats? Mires Peat 18:1-14 Purre A-H, Ilomets M (2018) Relationships between bryophyte production and substrate properties in restored milled peatlands. Restor Ecol 26:858-864 Purre A-H, Pajula R, Ilomets M (2019a) Carbon dioxide sink function in restored milled peatlands - the significance of weather and vegetation. Geoderma 346:30-42 Purre A-H, Penttilä T, Ojanen P, Minkkinen K, Aurela M, Lohila A, Ilomets M (2019b) Carbon dioxide fluxes and vegetation structure in rewetted and pristine peatlands in Finland and Estonia. Boreal Environ Res 24:243-261 Ramst R, Orru M, Salo V, Halliste L (2007) Eesti mahajäetud turbatootmisalade revisjon. Etapp. Viljandi, Pärnu, Saare ja Hiiu maakond. OÜ Eesti Geoloogiakeskus, Tallinn Rankin T, Strachan IB, Strack M (2018) Carbon dioxide and methane exchange at a post-extraction, unrestored peatland. Ecol Eng 122:241-251 Renou-Wilson F, Moser G, Fallon D, Farrell CA, Müller C, Wilson D (2018) Rewetting degraded peatlands for climate and biodiversity benefits: Results from two raised bogs. Ecol Eng DOI 10.1016/j.ecoleng.2018.02.014. Rochefort L (2000) Sphagnum — A keystone genus in habitat restoration. Bryologist 103:503-508 Rochefort L, Quinty F, Campeau S, Johnson K, Malterer T (2003) North American approach to the restoration of Sphagnum dominated peatlands. Wetl Ecol Manag 11:3-20 Samaritani E, Siegenthaler A, Yli-Petäys M, Buttler A, Christin P-A, Mitchell EAD (2011) Seasonal net ecosystem carbon exchange of a regenerating cutaway bog: How long does it take to restore the C-sequestration function? Restor Ecol 19:440-449 Soini P, Riutta T, Yli-Petäys M, Vasander H (2010) Comparison of vegetation and CO 2 dynamics between a restored cut-away peatland and a pristine fen: Evaluation of the restoration success. Restor Ecol 18:894-903 Strack M, Cagampan J, Hassanpour Fard G, Keith AM, Nugent K, Rankin T, Robinson C, Strachan IB, Waddington JM, Xu B (2016) Controls on plot-scale growing season CO 2 and CH 4 fluxes in restored peatlands: Do they differ from unrestored and natural sites? Mires Peat 17: 1-18 Strack M, Keith AM, Xu B (2014) Growing season carbon dioxide and methane exchange at a restored peatland on the Western Boreal Plain. Ecol Eng 64:231-239 Tuittila E-S, Komulainen V-M, Vasander H, Laine J (1999) Restored cut-away peatland as a sink for atmospheric CO 2 . Oecol 120:563-574 Tuittila E-S, Vasander H, Laine J (2004) Sensitivity of C Sequestration in Reintroduced Sphagnum to Water-Level Variation in a Cutaway Peatland. Restor Ecol 12:483-493 Tuittila E-S, Vasander H, Laine J (2000) Impact of rewetting on the vegetation of a cut-away peatland. App Veg Sci 3:205-212 Urbanová Z, Picek T, Hájek T, Bufková I, Tuittila, E-S (2012) Vegetation and carbon gas dynamics under a changed hydrological regime in central European peatlands. Plant Ecol Divers 5:89-103 Vaneslow-Algan M, Schmidt SR, Greven M, Fiencke C, Kutzbach L, Pfeiffer E.-M (2015) High methane emissions dominated annual greenhouse gas balances 30 years after bog rewetting. Biogeosciences 12:4361-4371 Vitt DH (1990) Growth and production dynamics of boreal mosses over climatic, chemical and topographic gradients. Bot J Linn Soc 104:35-59 Waddington JM, Lucchese MC, Duval TP (2011) Sphagnum moss moisture retention following the re-vegetation of degraded peatlands. Ecohydrology 4:359-366 Waddington JM, Warner KD (2001) Atmospheric CO 2 sequestration in restored mined peatlands. Ecoscience 8:359-368 Wilson D, Alm J, Riutta T, Laine J, Byrne KA, Farrell EP, Tuittila E-S (2007a) A high resolution green area index for modelling the seasonal dynamics of CO 2 exchange in peatland vascular plant communities. Plant Ecol 190:37-51 Wilson D, Farrell CA, Fallon D, Moser G, Müller C, Renou-Wilson F (2016) Multiyear greenhouse gas balances at a rewetted temperate peatland. Glob Chang Biol 22:4080-4095 Wilson D, Tuittila E-S, Alm J, Laine J, Farrell EP, Byrne KA (2007b) Carbon dynamics of a restored maritime peatland. Ecoscience 14:71-80 Yu ZC (2012) Northern peatland carbon stocks and dynamics: a review. Biogeosciences 9:4071-4085 Supplementary Files Supplements.docx Cite Share Download PDF Status: Under Review Version 1 posted Reviewers invited by journal 21 Jun, 2021 Reviews received at journal 22 Mar, 2021 Editor invited by journal 17 Mar, 2021 Editor assigned by journal 15 Mar, 2021 First submitted to journal 11 Mar, 2021 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-323253","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":18230408,"identity":"844512f9-e5a1-401d-9db2-ffa7908d55e4","order_by":0,"name":"Anna-Helena Purre","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAtklEQVRIiWNgGAWjYJACZgYGCwZ+BuYGkrRIMEi2MZKqxeAYsVr4Z/ce/FxQISFnfL+xTYKhwoawFok755KlZ5yRMDY7xgjUciaNCGtu5BhI87ZJJG4DaWFsO0xYh/yNHOPfvP8k6je3gbT8+09Yi8GNHDNp3gaJBAM2kJaGA4S1GAK1WPMckzCccSyx2SLhWDJhLXJAh93mqbGR528+fPDGhxo7wlpQQQKpGkbBKBgFo2AUYAcAp0kzfZwuycEAAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0001-9827-0794","institution":"Tallinn University: Tallinna Ulikool","correspondingAuthor":true,"submittingAuthor":false,"prefix":"","firstName":"Anna-Helena","middleName":"","lastName":"Purre","suffix":""},{"id":18230409,"identity":"9d4e183c-1e17-44a6-a320-7cce53306d31","order_by":1,"name":"Mati Ilomets","email":"","orcid":"","institution":"Tallinn University: Tallinna Ulikool","correspondingAuthor":false,"submittingAuthor":false,"prefix":"","firstName":"Mati","middleName":"","lastName":"Ilomets","suffix":""}],"badges":[],"createdAt":"2021-03-13 06:16:03","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-323253/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-323253/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":7350087,"identity":"1bcd66a1-f9d3-49dd-a8a1-4f8c4e8742b5","added_by":"auto","created_at":"2021-03-25 13:55:42","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":833150,"visible":true,"origin":"","legend":"Kõrsa (a) and Hara (b) undisturbed reference sites, Kõrsa 35 years before rewetted (c) sites, and Hara rewetted fields 15 (d) and 4 (e) years after rewetting, respectively ","description":"","filename":"Fig01.png","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/053c37761a47b22723950d16.png"},{"id":7350086,"identity":"cdcfbd42-b010-43e7-b7bb-5efd8d4a71ca","added_by":"auto","created_at":"2021-03-25 13:55:42","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":56545,"visible":true,"origin":"","legend":"Average cover (in %) of vascular plant (a) and bryophyte (b) plant functional types of the study sites (±SE). Different small case letters indicate statistically significant differences between the sites (p \u003c 0.05). Statistical significance was tested using the Kruskal-Wallis test, and pairwise comparison was concluded using the Mann-Whitney test with Bonferroni correction.","description":"","filename":"Fig02.png","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/06473100ad6232d30b6be503.png"},{"id":7349880,"identity":"c83c4d16-c94d-4505-8da3-3bbf59dfaebc","added_by":"auto","created_at":"2021-03-25 13:52:42","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":63507,"visible":true,"origin":"","legend":"Cumulative reconstructed growing season CO2 fluxes in study sites ±SE. Different lower-case letters indicate a statistically significant (p \u003c 0.05) difference in NEE between the sites. The statistical significance was tested using the Kruskal-Wallis test, and pairwise comparison was concluded with the Mann-Whitney test and Bonferroni correction. NEE = Pg – RECO, note that Pg and RECO are always positive for clarity. ","description":"","filename":"Fig03.png","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/bbbc5b69e063ecaaa46f5ed9.png"},{"id":7350165,"identity":"67e64046-8dd4-4530-aeb2-44dae81feb7b","added_by":"auto","created_at":"2021-03-25 13:58:43","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":42338,"visible":true,"origin":"","legend":"Redundancy analysis of plant functional type cover, CO2 fluxes (NEE, Pg, RECO) and CO2 model parameters (Pmax, k, s, r0, b) in the undisturbed (a) and rewetted (b) sites. Only plant functional types, which are significant predictors explaining CO2 fluxes and model parameters, are shown. Minerotrophic forbs and true mosses in the case of undisturbed sites, and ombrotrophic forbs and hummock Sphagnum in the case of rewetted sites, were omitted from the analysis due to their absence from the respective sites. NEE – net ecosystem exchange, Pg – gross photosynthesis, RECO – ecosystem respiration, ombro_sedge – ombrotrophic sedges, Minero_forb – minerotrophic forbs, True_moss – true mosses, Lawn_Sph – lawn Sphagnum","description":"","filename":"Fig04.png","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/1430e24236d8a1e86ca67b78.png"},{"id":7350293,"identity":"c1db551c-bbf3-4f84-b316-21de6c8b111e","added_by":"auto","created_at":"2021-03-25 14:01:43","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":46931,"visible":true,"origin":"","legend":"Bryophyte (a), plant (b) and vascular plant (c) biomass related with average growing season photosynthesis (a, b), and ecosystem respiration (c) in rewetted and undisturbed plots","description":"","filename":"Fig05.png","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/0fbcdf0cd4312fffd603d572.png"},{"id":7350092,"identity":"e00b2695-d7e2-4aea-9713-301bd98a1d4b","added_by":"auto","created_at":"2021-03-25 13:55:43","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":38466,"visible":true,"origin":"","legend":"Detrended correspondence analysis (DCA) of plant functional types and CO2 fluxes (NEE, Pg, RECO) and its model parameters (Pmax, k, s, r0, b) in the study sites. The eigenvalues for the first and second axes are 0.181 and 0.067, respectively. NEE – net ecosystem exchange, Pg – gross photosynthesis, RECO – ecosystem respiration, Ombro_sedge – ombrotrophic sedges, Omb_forb – ombrotrophic forbs, Minero_forb – minerotrophic forbs, Ever_shrub – evergreen shrubs, True_moss – true mosses, Lawn_Sph – lawn Sphagnum, Hummock_Sph – hummock Sphagnum","description":"","filename":"Fig06.png","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/60e862f70bfb92dc89786f8a.png"},{"id":13682445,"identity":"95645955-98b8-4ac6-89fd-f6b29e77fc90","added_by":"auto","created_at":"2021-09-17 11:57:21","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1458790,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/5350a25f-199a-4788-9b9e-4a23e7d59df6.pdf"},{"id":7349884,"identity":"54ccbb34-c03b-464c-9269-807801fbdc7b","added_by":"auto","created_at":"2021-03-25 13:52:43","extension":"docx","order_by":6,"title":"","display":"","copyAsset":false,"role":"supplement","size":108571,"visible":true,"origin":"","legend":"","description":"","filename":"Supplements.docx","url":"https://assets-eu.researchsquare.com/files/rs-323253/v1/6b57316960ae9bdf5252ee84.docx"}],"financialInterests":"","formattedTitle":"\u003cp\u003eTime Since Rewetting Defines Vegetation Composition and Carbon Dioxide Fluxes on Former Milled Peatlands - Comparison With Undisturbed Bogs\u003c/p\u003e","fulltext":[{"header":"1. Introduction","content":" \u003cp\u003eUndisturbed peatlands are important carbon sinks in the long term (Yu \u003cspan citationid=\"CR81\" class=\"CitationRef\"\u003e2012\u003c/span\u003e) as well as a suitable habitat for plant species that have adapted to survive in acidic and waterlogged conditions (Minayeva 2008). Northern peatlands have been widely affected by peat mining for horticulture or energy production (Leifeld et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), particularly since the 1950s when peat milling became the main technique for peat extraction. This method involves drainage and the removal of vegetation in large areas, so thin layers of peat can be extracted every summer season.\u003c/p\u003e \u003cp\u003eExcavated peatlands have several negative environmental effects, such as peat loss through mineralisation, high CO\u003csub\u003e2\u003c/sub\u003e emissions, fire hazard, no plant diversity and low aesthetic value. The natural revegetation of those site takes a long time, depends on the environmental conditions of the site and usually does not lead to mire-specific plant communities (Lavoie et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Graf et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Orru et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Unrestored milled peatlands are important CO\u003csub\u003e2\u003c/sub\u003e sources to the atmosphere due to the low water tables allowing peat mineralisation and sparse or absent vegetation (Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Rankin et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The main mitigation possibility for those negative impacts is peatland rewetting, which through higher water tables creates suitable conditions for revegetation and thus reduces CO\u003csub\u003e2\u003c/sub\u003e emissions (Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and the flammability of these sites (Granath et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Although rewetting increases CH\u003csub\u003e4\u003c/sub\u003e emissions on restored peatlands, but as CH\u003csub\u003e4\u003c/sub\u003e is a short-lived gas in the atmosphere, rewetting of peatlands mitigates the climate change in long-term (G\u0026uuml;nther et al. \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Various criteria have been taken into account throughout the history of restoration ecology to assess restoration success. First, biodiversity measures and hydrology were engaged to indicate the success of restoring the ecosystem, which in recent decades have been integrated with greenhouse gas balances showing recovery of ecosystem functioning (Kl\u0026oslash;ve et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Renou-Wilson et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe initial response of the plant community and its diversity to rewetting is complex and depends also on the pre-rewetting state of the peatland (Tuittila et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). Over the time-scale of several decades after peatland rewetting or self-recovery, plant cover increases with time (Orru et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Priede et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Furthermore, CO\u003csub\u003e2\u003c/sub\u003e fluxes change in time after rewetting (Kl\u0026oslash;ve et al. \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Beyer and H\u0026ouml;per (\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) estimate based on their experience of greenhouse gas measurements in temperate peatland that rewetted peat extraction sites may become peat accumulating ecosystems after about 30 years. Even after 30 years post rewetting, milled peatlands can remain CO\u003csub\u003e2\u003c/sub\u003e sources, but those emissions from rewetted sites tend to be smaller than from active peat extraction sites, especially if \u003cem\u003eSphagnum\u003c/em\u003e is dominating (Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Vanselow-Algan et al. 2015).\u003c/p\u003e \u003cp\u003eThe rate of vegetation recovery likely depends on successfully raising the water table (Konvalinkov\u0026aacute; and Prach \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Strack et al. \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Priede et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), the presence of plant propagules (Campbell and Rochefort \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Konvalinkov\u0026aacute; and Prach \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2014\u003c/span\u003e) and the nutrient status of the site (Komulainen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Konvalinkov\u0026aacute; and Prach, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Kozlov et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Priede et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). However, even after decades, rewetted boreal peatlands mostly differ from their reference ecosystem regarding carbon accumulation and vegetation structure (Moreno-Mateos et al. \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). The quicker return of CO\u003csub\u003e2\u003c/sub\u003e sink function has been reported on milled peatlands restored using the moss-layer-transfer technique (Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Nugent et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eMilled peatlands have been ombrotrophic bogs and in some cases bog remnants border the peat extraction areas. Usually during peat extraction, layers of \u003cem\u003eSphagnum\u003c/em\u003e peat are removed, so more nutrient-rich peat deposits at the bottom of the former mire with varying thickness are left on the site. This leads to the development of wet minerotrophic vegetation (Tuittila et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Renou-Wilson et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2018\u003c/span\u003e), which is different from typical raised bog vegetation (Renou-Wilson et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). With vegetation succession and distancing peatland surface from the water table minerotrophic vegetation will be replaced by bog species in time (Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDue to the challenges related to raising and keeping the stable water table close to the peat surface throughout the year (Price et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2003\u003c/span\u003e; Brown et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2017\u003c/span\u003e), functional dryness of the residual peat layer (Price and Whitehead, \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Price et al. \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2003\u003c/span\u003e), the smaller water holding capacity of developed moss layer (Waddington et al. \u003cspan citationid=\"CR76\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; McCarter and Price \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) and the moisture conditions being closely related with the microtopography of the site (Bugnon et al. \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1997\u003c/span\u003e; Price et al. \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1998\u003c/span\u003e; Price and Whitehead \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Purre and Ilomets, \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) environmental conditions in restored sites may be drier than in undisturbed sites. Restoration sites have generally lower bryophyte and shrub but higher graminoid cover than in undisturbed peatlands (Soini et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Gonz\u0026aacute;lez et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Some studies report that vegetation in rewetted sites is more heterogeneous than in pristine peatlands due to the patchiness and incompleteness of the plant cover on restoration sites as vegetation cover starts to develop near the drainage ditches and close to existing vegetation (Soini et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Laine et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eUndisturbed peatlands are CO\u003csub\u003e2\u003c/sub\u003e sinks at least over longer time-scales, CO\u003csub\u003e2\u003c/sub\u003e exchange of the peatland can vary annually (Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Nugent et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Kl\u0026oslash;ve et al. (\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e2017\u003c/span\u003e) stress the lack of knowledge about the greenhouse gas fluxes in rewetted peatlands, especially on sites that have been rewetted for decades. Restored milled peatlands can have larger (Soini et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Nugent et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2018\u003c/span\u003e, \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) or smaller (Renou-Wilson et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Nugent et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2019\u003c/span\u003e) CO\u003csub\u003e2\u003c/sub\u003e uptake than similar eco-hydrologically undisturbed sites. Similar carbon sequestration as in undisturbed peatlands is reported to return sooner to rewetted milled peatlands than the development of typical raised bog species composition (Soini et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Renou-Wilson et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Although photosynthesis on restored milled peatlands develops akin to undisturbed peatland, ecosystem respiration stays lower (Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Therefore, on rewetted sites, Net Ecosystem Exchange (NEE) is reported to be driven by respiration rather than by photosynthesis (Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Due to their transitional stages, rewetted sites have large interannual and within-site variations in CO\u003csub\u003e2\u003c/sub\u003e fluxes (Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Nugent et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), whereas undisturbed peatlands have reached a mature development stage regarding their vegetation, water retention capacity and CO\u003csub\u003e2\u003c/sub\u003e balance (Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003ePlant communities vary by their photosynthesis and respiration rates. The highest photosynthesis on nutrient-poor peatlands is reported from \u003cem\u003eEriophorum\u003c/em\u003e communities (Beyer and H\u0026ouml;per \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), but this is accompanied by higher respiration (Beyer and H\u0026ouml;per \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Higher CO\u003csub\u003e2\u003c/sub\u003e net sink function (Kivim\u0026auml;ki et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and soil organic matter accumulation (Andersen et al. \u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e2013\u003c/span\u003e) have been recorded on plots with mixed graminoid and \u003cem\u003eSphagnum\u003c/em\u003e patches compared to pure graminoid patches. \u003cem\u003eSphagnum\u003c/em\u003e species have lower photosynthetic capacities (Korrensalo et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and lower respiration, therefore plots with \u003cem\u003eSphagnum\u003c/em\u003e are larger CO\u003csub\u003e2\u003c/sub\u003e sinks than plots with only graminoids (Beyer and H\u0026ouml;per \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Vascular plants are also important in peatland CO\u003csub\u003e2\u003c/sub\u003e exchange, especially by mitigating the effect of drought on CO\u003csub\u003e2\u003c/sub\u003e sink functioning (Kuiper et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). In similar climatic conditions, correlating vegetation cover and CO\u003csub\u003e2\u003c/sub\u003e fluxes differs between land-use types (Laine et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) \u0026mdash; in restored sites, CO\u003csub\u003e2\u003c/sub\u003e net uptake increases with vascular plant cover (Strack et al. \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), while in undisturbed sites plots with higher moss cover are greater CO\u003csub\u003e2\u003c/sub\u003e sinks (Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn this paper we analyse the success of rewetting by comparing CO\u003csub\u003e2\u003c/sub\u003e fluxes and vegetation on relatively similar rewetted milled peatlands with somewhat different site conditions in different successional stages and initially eco-hydrologically similar nearby undisturbed bogs to assess if a longer time since rewetting or ecosystem recovery time leads to ecosystems that functionally converge to the state of reference bogs. For that, we established the following postulates:\u003c/p\u003e \u003cp\u003e \u003col\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e fluxes and vegetation structure on rewetted milled peatlands develop in the direction of undisturbed reference bogs in time;\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003cspan\u003e \u003cli\u003e \u003cp\u003eHigher amount of plant above-ground, especially \u003cem\u003eSphagnum\u003c/em\u003e, biomass is related to higher CO\u003csub\u003e2\u003c/sub\u003e sink function on rewetted milled peatlands.\u003c/p\u003e \u003c/li\u003e \u003c/span\u003e \u003c/ol\u003e \u003c/p\u003e "},{"header":"2. Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\n\u003ch2\u003e2.1. Study sites\u003c/h2\u003e\n\u003cp\u003eTwo paired study sites (K\u0026otilde;rsa and Hara) were selected (Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e), both of which include rewetted abandoned milled peatlands and remnant open raised bog areas (Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). The K\u0026otilde;rsa site is located in southwestern Estonia next to an active peat extraction site. The K\u0026otilde;rsa rewetted (K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e) site has self-recovered after the end of peat extraction due to the water level being raised up to the peat surface in 1980 following a damming to create a firewater reservoir (Ramst et al. \u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e). Revegetation began at K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e two years later with the self-establishment of \u003cem\u003eE. vaginatum\u003c/em\u003e tussocks in 1982. The Hara site is located in Northern Estonia, in Lahemaa National Park. The Hara rewetted site divides into a self-recovered northern part (Hara\u003csub\u003eRN\u003c/sub\u003e) and an actively rewetted southern part (Hara\u003csub\u003eRS\u003c/sub\u003e). The water table in Hara\u003csub\u003eRN\u003c/sub\u003e rose in 2000\u0026ndash;2002 due to closing of the bordering ditch, and vegetation started to develop earlier in that area. For conservative estimate of recovery time, year 2000 is considered as a start of recovery when analysing the effect of time since rewetting. Hara\u003csub\u003eRS\u003c/sub\u003e was rewetted in 2012 by the State Forestry Centre of Estonia. No additional restoration measures were applied in addition to rewetting.\u003c/p\u003e\n\u003cp\u003eThe rewetted and undisturbed sites of both paired sites initially had a similar ecohydrological status (raised bog), which is still present in the undisturbed parts of K\u0026otilde;rsa (K\u0026otilde;rsa\u003csub\u003eP\u003c/sub\u003e) and Hara (Hara\u003csub\u003eP\u003c/sub\u003e). Previously, the vegetation of the rewetted sites was described by Ramst et al. (\u003cspan class=\"CitationRef\"\u003e2007\u003c/span\u003e), as the following:\u003c/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cp\u003eHaraRS: \u003cem\u003eEriophorum vaginatum\u003c/em\u003e and bare peat;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eHara\u003csub\u003eRN\u003c/sub\u003e: \u003cem\u003eE. vaginatum, Warnstorfia fluitans, Sphagnum riparium\u003c/em\u003e and \u003cem\u003eSphagnum cuspidatum\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eK\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e: \u003cem\u003eE. vaginatum, Chiloscyphus pallescens, Pleurozium schreberi, Polytrichum strictum, W. fluitans, Brachythecium mildeanum, S. cuspidatum\u003c/em\u003e and \u003cem\u003eSphagnum balticum\u003c/em\u003e.\u003c/p\u003e\n\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eThe respective paired sites are located in the same mire-basin, and the distances between the rewetted and undisturbed sites range from 140 metres in Hara to 500 m in K\u0026otilde;rsa.\u003c/p\u003e\n\u003cp\u003eIn all of the study sites, four measurement plots per site were established during the previous year (2015). As two of the plots in Hara\u003csub\u003eRN\u003c/sub\u003e became flooded during the measurement period, they were omitted from the study, and data from two measurement plots in Hara\u003csub\u003eRN\u003c/sub\u003e were used. The locations of the permanent measurement plots were chosen based on the dominant vegetation and by taking into account its variability between micro-topographic levels. In undisturbed sites and K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e, where microtopography has already developed, two measurement plots were situated on the hummocks and two plots on the lawns of each site. In other sites, two replicates for each vegetation type were established.\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003ctable id=\"Tab1\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eLocations and descriptions of the study site\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr\u003e\n\u003cth align=\"left\"\u003e\u0026nbsp;\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eHara\u003c/p\u003e\n\u003c/th\u003e\n\u003cth align=\"left\"\u003e\n\u003cp\u003eK\u0026otilde;rsa\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eCoordinates\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN 59\u003csup\u003eo\u003c/sup\u003e33\u0026rsquo;, E 25\u003csup\u003eo\u003c/sup\u003e36\u0026rsquo;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eN 58\u003csup\u003eo\u003c/sup\u003e24\u0026rsquo;, E 24\u003csup\u003eo\u003c/sup\u003e41\u0026rsquo;\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eEnd of extraction\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1994\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e1980\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eWater table depth below surface (cm)\u003csup\u003ea\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eHara\u003csub\u003eRS\u003c/sub\u003e: -10\u003c/p\u003e\n\u003cp\u003eHara\u003csub\u003eRN\u003c/sub\u003e : 0\u003c/p\u003e\n\u003cp\u003eHara\u003csub\u003eP\u003c/sub\u003e: -5 \u0026ndash; -30\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eK\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e: 0 \u0026ndash; -15\u003c/p\u003e\n\u003cp\u003eK\u0026otilde;rsa\u003csub\u003eP\u003c/sub\u003e: -10 \u0026ndash; -35\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eLong-term (1981\u0026ndash;2010) average annual/growing season temperature (\u003csup\u003eo\u003c/sup\u003eC)\u003csup\u003eb,c\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e5.7/12.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6.3/13.4\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eLong-term (1981\u0026ndash;2010) average annual/growing season precipitation (mm annually/growing season)\u003csup\u003eb,c\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e587/381\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e746/418\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAverage annual/growing season temperature (\u003csup\u003eo\u003c/sup\u003eC) 2016\u003csup\u003eb,c\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6.6/13.4\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e6.7/13.5\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003eAverage annual/growing season precipitation (mm annually) 2016\u003csup\u003eb,c\u003c/sup\u003e\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e849/430\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd align=\"left\"\u003e\n\u003cp\u003e757/398\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003ctfoot\u003e\n\u003ctr\u003e\n\u003ctd colspan=\"3\"\u003e\u003csup\u003ea\u003c/sup\u003e- Average growing season water table according to manual measurements during the CO\u003csub\u003e2\u003c/sub\u003e measurement sessions; \u003csup\u003eb\u003c/sup\u003e- data from Estonian Weather Service; \u003csup\u003ec\u003c/sup\u003e- Growing season data for May-October.\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tfoot\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec4\" class=\"Section2\"\u003e\n\u003ch2\u003e2.2. Vegetation analysis\u003c/h2\u003e\n\u003cp\u003eVegetation analyses were conducted on the plant functional type (PFT) level, and we used the PFT division described in Laine et al. (\u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e). In our study sites, the following PFTs were present from the larger number of PFTs described by Laine et al. (\u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e):\u003c/p\u003e\n\u003cul\u003e\n\u003cli\u003e\n\u003cp\u003eHummock \u003cem\u003eSphagnum\u003c/em\u003e: \u003cem\u003eS. rubellum\u003c/em\u003e, \u003cem\u003eS. fuscum\u003c/em\u003e and \u003cem\u003eS. capillifolium\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eLawn \u003cem\u003eSphagnum\u003c/em\u003e: \u003cem\u003eS. angustifolium\u003c/em\u003e, \u003cem\u003eS. balticum\u003c/em\u003e, \u003cem\u003eS. medium\u003c/em\u003e, \u003cem\u003eS. fallax\u003c/em\u003e, \u003cem\u003eS. papillosum\u003c/em\u003e, \u003cem\u003eS. squarrosum, S. riparium\u003c/em\u003e and \u003cem\u003eS. cuspidatum\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eTrue mosses: \u003cem\u003ePolytrichum strictum, Warnstorfia fluitans, Chiloscyphus pallescens, Pleurozium schreberi\u003c/em\u003e and \u003cem\u003eBrachythecium mildeanum\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eEvergreen shrubs: \u003cem\u003eCalluna vulgaris\u003c/em\u003e and \u003cem\u003eVaccinium oxycoccus\u003c/em\u003e, \u003cem\u003eAndromeda polifolia\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eOmbrotrophic forbs: \u003cem\u003eDrosera rotundifolia\u003c/em\u003e and \u003cem\u003eRubus chamaemorus\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eMinerotrophic forbs: \u003cem\u003eMelampyrum\u003c/em\u003e spp., \u003cem\u003eMenyanthes trifoliate\u003c/em\u003e and \u003cem\u003eThelypteris palustris\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eOmbrotrophic sedges: \u003cem\u003eEriophorum vaginatum\u003c/em\u003e;\u003c/p\u003e\n\u003c/li\u003e\n\u003cli\u003e\n\u003cp\u003eTrees: \u003cem\u003ePinus sylvestris\u003c/em\u003e, \u003cem\u003eBetula pubescens\u003c/em\u003e, and \u003cem\u003eSalix\u003c/em\u003e spp.\u003c/p\u003e\n\u003c/li\u003e\n\u003c/ul\u003e\n\u003cp\u003eThe nomenclature followed Ingerpuu and Vellak (\u003cspan class=\"CitationRef\"\u003e1998\u003c/span\u003e) for bryophytes and Leht (\u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e) for vascular plants.\u003c/p\u003e\n\u003cp\u003eWe measured the plant species coverage (%), vascular plant leaf area index (LAI\u003csub\u003evasc\u003c/sub\u003e; m\u003csup\u003e2\u003c/sup\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), above-ground biomass of PFTs (AGB; g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e) and moss production (AGP; g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) as well as the length increment of mosses (LI; mm year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e). The plant cover of measurement plots was determined visually at the peak of the 2016 growing season (end of July) from inside the CO\u003csub\u003e2\u003c/sub\u003e flux measurement collars (four plots per each site/management type combination, but two plots in Hara\u003csub\u003eRN\u003c/sub\u003e). LAI\u003csub\u003evasc\u003c/sub\u003e was determined according to Wilson et al. (\u003cspan class=\"CitationRef\"\u003e2007a\u003c/span\u003e) during the CO\u003csub\u003e2\u003c/sub\u003e flux measurement campaigns.\u003c/p\u003e\n\u003cp\u003eBiomass samples were collected from near the measurement plots with vegetation as similar as possible to those in the collars. Vascular plant biomass samples were collected at the end of July 2016 and bryophyte samples at the beginning of October 2016 to capture the maximum biomass of each plant group. We used two plot sizes for the AGB measurements of vascular plants (15 cm radius circular plot) and bryophytes (2.5 cm circular round plot) and collected one vascular plant sample and three bryophyte samples per measurement point (a total of four vascular plant and 12 bryophyte samples from Hara\u003csub\u003eRS\u003c/sub\u003e, Hara\u003csub\u003eP\u003c/sub\u003e, K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e and K\u0026otilde;rsa\u003csub\u003eP\u003c/sub\u003e each, and two vascular plant and six bryophyte samples in Hara\u003csub\u003eRN\u003c/sub\u003e). Only the capitula for \u003cem\u003eSphagnum\u003c/em\u003e species was used as there is no clear distinction between the live and dead material of \u003cem\u003eSphagnum\u003c/em\u003e (Clymo \u003cspan class=\"CitationRef\"\u003e1970\u003c/span\u003e), and the upper 2 cm layer for other bryophytes to obtain biomass samples up to the similar depth of biomass as \u003cem\u003eSphagnum\u003c/em\u003e were collected to determine bryophyte biomass similar to Moore et al. (\u003cspan class=\"CitationRef\"\u003e2002\u003c/span\u003e), Laine et al. (\u003cspan class=\"CitationRef\"\u003e2012\u003c/span\u003e) and Purre et al. (\u003cspan class=\"CitationRef\"\u003e2019b\u003c/span\u003e). The collected samples were divided into species level. The sampling and laboratory analysis of biomass is described in Purre et al. (\u003cspan class=\"CitationRef\"\u003e2019b\u003c/span\u003e). During the data analysis, the biomass of different species was compiled into PFTs. From the air-dried (65\u0026ordm;C) bryophyte samples, the border of AGP and LI was determined using the innate markers method (Clymo \u003cspan class=\"CitationRef\"\u003e1970\u003c/span\u003e; Pouliot et al. \u003cspan class=\"CitationRef\"\u003e2010\u003c/span\u003e), then separated from the rest of the biomass and weighed (AGP). The LI of ten individuals from the dominant species of each sample were measured with a digital caliper. In Hara\u003csub\u003eRS\u003c/sub\u003e, bryophytes were absent and thus biomass AGP and LI were considered to be zero.\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\n\u003ch2\u003e2.3. CO\u003csub\u003e2\u003c/sub\u003e flux measurements and data processing\u003c/h2\u003e\n\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e measurements were carried out at least once a month during the growing season (May-October 2016). NEE and ecosystem respiration (R\u003csub\u003eECO\u003c/sub\u003e) was measured on 60\u0026times;60 cm square aluminium collars inserted to about a 20 cm depth, with the rim filled with water to ensure an air-tight fit during flux measurements. We measured CO\u003csub\u003e2\u003c/sub\u003e concentrations with the infrared gas analyser Li-6400 (Li-Cor (USA)) from transparent Plexiglas chamber (60\u0026times;60\u0026times;30 cm) with a cooling system. The measurements period was two minutes, and the CO\u003csub\u003e2\u003c/sub\u003e content in the chamber was recorded with an interval of 15 s. After measuring the CO\u003csub\u003e2\u003c/sub\u003e concentrations in full-light, NEE was measured on two lower irradiation levels by using one or two shades that reduced the photosynthetically active radiation (PAR (\u0026micro;mol m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)) reaching the vegetation in the chamber at an average of 65% and 88%, respectively. Lastly, R\u003csub\u003eECO\u003c/sub\u003e was measured by covering the chamber with an opaque hood. Between each measurement period, the measurement chamber was ventilated. During the measurement campaigns, plant parameters for determining LAI\u003csub\u003evasc\u003c/sub\u003e inside the measurement collars were measured according to Wilson et al. (\u003cspan class=\"CitationRef\"\u003e2007a\u003c/span\u003e) in addition to recording PAR, the temperature inside the chamber, peat temperatures at 5 cm and 15 cm depths and the water table (cm).\u003c/p\u003e\n\u003cp\u003eInput data (PAR, T\u003csub\u003eAIR\u003c/sub\u003e) for CO\u003csub\u003e2\u003c/sub\u003e flux reconstruction were measured with hourly intervals in stations belonging to the Estonian Weather Service. For Hara, the temperature data was obtained from the nearest station in Vanak\u0026uuml;la (about 10 km from the site) and radiation data from Harku meteorological station (about 70 km from Hara). For K\u0026otilde;rsa, all meteorological data was obtained from P\u0026auml;rnu-Sauga meteorological station located about 15 km from the site. Those stations were the closest to the study sites where PAR and T\u003csub\u003eAIR\u003c/sub\u003e were continuously measured, and they were located within a 10 km distance from the sea similarly to the study sites.\u003c/p\u003e\n\u003cp\u003eThe flux rates were estimated based on linear change in CO\u003csub\u003e2\u003c/sub\u003e concentrations in time. The linear method was chosen, as this method was considered suitable by Kandel et al. (\u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e) for CO\u003csub\u003e2\u003c/sub\u003e flux calculations in the case of short (few minutes) chamber closure periods (2 min in current study). The measured NEE and R\u003csub\u003eECO\u003c/sub\u003e fluxes were considered suitable according to the following quality criteria: variation of PAR during the flux measurement not exceeding\u0026thinsp;\u0026plusmn;\u0026thinsp;15%, variation of inside temperature of the chamber not varying more than \u0026plusmn;\u0026thinsp;5\u0026deg;C and the determination coefficient (R\u003csup\u003e2\u003c/sup\u003e) of the measured flux of at least 0.9. Very small fluxes (\u0026plusmn;\u0026thinsp;0.2 ppm s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) were accepted regardless of their R\u003csup\u003e2\u003c/sup\u003e value. Similar quality criteria in respect of R\u003csup\u003e2\u003c/sup\u003e values were used by J\u0026auml;rveoja et al. (\u003cspan class=\"CitationRef\"\u003e2016\u003c/span\u003e). A total of 215 CO\u003csub\u003e2\u003c/sub\u003e flux measurements fulfilled the set criteria and were used for CO\u003csub\u003e2\u003c/sub\u003e flux reconstructions. Photosynthesis (P\u003csub\u003eg\u003c/sub\u003e) was calculated by adding R\u003csub\u003eECO\u003c/sub\u003e to NEE.\u003c/p\u003e\n\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e fluxes were reconstructed for the period from the beginning of May until the end of September 2016 at each site. With these reconstructions, based on measured and calculated CO\u003csub\u003e2\u003c/sub\u003e fluxes and other parameters (PAR, LAI\u003csub\u003evasc\u003c/sub\u003e and air temperature (T\u003csub\u003eAIR\u003c/sub\u003e)), models were created for relating differences in measured CO\u003csub\u003e2\u003c/sub\u003e fluxes with differences in input parameters for reconstructing the whole growing season CO\u003csub\u003e2\u003c/sub\u003e fluxes. CO\u003csub\u003e2\u003c/sub\u003e flux and LAI\u003csub\u003evasc\u003c/sub\u003e reconstruction was carried out in program R version 3.2.2 package nlme (Linear and Nonlinear Mixed Effects Models, ver. 3.1\u0026ndash;121; Pinheiro et al. (\u003cspan class=\"CitationRef\"\u003e2015\u003c/span\u003e)). Gaussian curves were fitted to LAI\u003csub\u003evasc\u003c/sub\u003e values, which were calculated according to the vegetation parameters measured during the CO\u003csub\u003e2\u003c/sub\u003e measurement campaigns for reconstructing the change in LAI\u003csub\u003evasc\u003c/sub\u003e during the vegetation season as described by Wilson et al. (\u003cspan class=\"CitationRef\"\u003e2007a\u003c/span\u003e) in each measurement collar.\u003c/p\u003e\n\u003cp\u003eThe gross photosynthesis (P\u003csub\u003eg\u003c/sub\u003e (mg CO\u003csub\u003e2\u003c/sub\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)) model uses the saturating response to PAR (Eq.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e) and records the change in LAI\u003csub\u003evasc\u003c/sub\u003e during the vegetation season:\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" alt=\"\" /\u003e\u003c/p\u003e\n\u003cp\u003ewhere P\u003csub\u003emax\u003c/sub\u003e is the maximum photosynthesis at light saturation, k and s are respectively the PAR and LAI\u003csub\u003evasc\u003c/sub\u003e values when P\u003csub\u003eg\u003c/sub\u003e reaches half of its maximum level.\u003c/p\u003e\n\u003cp\u003eThe respiration model (Eq.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003e) expresses an exponential response of ecosystem respiration (R\u003csub\u003eECO\u003c/sub\u003e (mg CO\u003csub\u003e2\u003c/sub\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e)) to the temperature inside the chamber (T\u003csub\u003eAIR\u003c/sub\u003e).\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\" alt=\"\" /\u003e\u003c/p\u003e\n\u003cp\u003eWhere parameters r0 and b are respectively the respiration at the 0\u0026deg;C temperature and the sensitivity of respiration to air temperature, and T\u003csub\u003eAIR\u003c/sub\u003e is the air temperature (\u0026deg;C). CO\u003csub\u003e2\u003c/sub\u003e measurements and reconstructions are described in more detail in Purre et al. (\u003cspan class=\"CitationRef\"\u003e2019a\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e2019b\u003c/span\u003e).\u003c/p\u003e\n\u003c/div\u003e\n\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\n\u003ch2\u003e2.4. Data analysis\u003c/h2\u003e\n\u003cp\u003eData analyses were conducted with IBM SPSS ver. 23. As the data did not fulfil the requirements for parametric data analysis according to the Shapiro-Wilk test, non-parametric data analysis methods were chosen. The Kruskal-Wallis and Mann-Whitney tests with Bonferroni correction for the pairwise comparison of vegetation parameters and CO\u003csub\u003e2\u003c/sub\u003e fluxes between the sites were applied. Spearman correlations were used to relate separate plant group abundances with different parameters of CO\u003csub\u003e2\u003c/sub\u003e fluxes (NEE, P\u003csub\u003eg\u003c/sub\u003e, R\u003csub\u003eECO\u003c/sub\u003e) in rewetted and undisturbed peatlands. Generalized linear mixed models (GLMMs) were applied on data from rewetted milled peatlands to determine the effect of site, microtopography and time since rewetting (fixed factors) on CO\u003csub\u003e2\u003c/sub\u003e fluxes (growing season NEE, P\u003csub\u003eg\u003c/sub\u003e or R\u003csub\u003eECO\u003c/sub\u003e as target variables), biomass of studied plant functional types (PFTs) were incorporated in the models as random factors. For information criterion of the GLMMs log-likelihood was used, lower log-likelihood values showing better model fit. The results were considered statistically significant if \u003cem\u003ep\u003c/em\u003e\u0026thinsp;\u0026lt;\u0026thinsp;0.05. Average values are reported with standard errors.\u003c/p\u003e\n\u003cp\u003eThe multivariate analysis methods Redundancy Analysis (RDA) and Detrended Correspondence Analysis (DCA) were applied in PC-ORD ver. 7 to relate the abundances of PFTs and CO\u003csub\u003e2\u003c/sub\u003e fluxes on rewetted and undisturbed sites, and to analyse the changes in those variables with time since rewetting, respectively. In RDA, the response variables were standardised and a randomisation test was applied to test for any significant relationship between the PFT and CO\u003csub\u003e2\u003c/sub\u003e flux matrices. DCA was used to find the main gradients in PFT and CO\u003csub\u003e2\u003c/sub\u003e flux data using time since rewetting and the site as supplementary variables.\u003c/p\u003e\n\u003c/div\u003e"},{"header":"3. Results","content":" \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.1. Vegetation\u003c/h2\u003e \u003cp\u003eVegetation varied significantly between rewetted and undisturbed sites and between all rewetted sites, while small differences also occurred between both undisturbed sites (Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). More PFTs were present on undisturbed and older rewetted sites, while many PFTs such as \u003cem\u003eSphagnum\u003c/em\u003e and evergreen shrubs were absent from the recently rewetted Hara\u003csub\u003eRS\u003c/sub\u003e. Evergreen shrubs such as \u003cem\u003eC. vulgaris\u003c/em\u003e and \u003cem\u003eA. polifolia\u003c/em\u003e had higher cover in undisturbed sites, while \u003cem\u003eV. oxycoccus\u003c/em\u003e was present with low cover only in K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e. Evergreen shrub biomass was absent or significantly lower in rewetted sites compared to undisturbed sites (Appendix S1). Ombrotrophic forbs \u003cem\u003eR. chamaemorus\u003c/em\u003e and \u003cem\u003eD. rotundifolia\u003c/em\u003e were only present in undisturbed plots, but with relatively low cover (0.5-3%). Only in K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e minerotrophic forbs like \u003cem\u003eMelampyrum\u003c/em\u003e species and \u003cem\u003eT. palustris\u003c/em\u003e were present. Tree seedling of \u003cem\u003eSalix\u003c/em\u003e spp., \u003cem\u003eBetula\u003c/em\u003e spp. and \u003cem\u003eP. sylvestris\u003c/em\u003e had about 1% cover on all sites, or were absent.\u003c/p\u003e \u003cp\u003eIn undisturbed sites, hummock (\u003cem\u003eS. fuscum\u003c/em\u003e, \u003cem\u003eS. rubellum, S. angustifolium\u003c/em\u003e) and lawn (\u003cem\u003eS. medium\u003c/em\u003e, \u003cem\u003eS. balticum\u003c/em\u003e, \u003cem\u003eS. papillosum\u003c/em\u003e) \u003cem\u003eSphagnum\u003c/em\u003e species were present in relatively similar cover (ranging from 5% (\u003cem\u003eS. balticum\u003c/em\u003e in K\u0026otilde;rsa) to 45% (\u003cem\u003eS. rubellum\u003c/em\u003e in K\u0026otilde;rsa)). Only lawn species (\u003cem\u003eS. medium\u003c/em\u003e, \u003cem\u003eS. fallax\u003c/em\u003e, and \u003cem\u003eS. squarrosum\u003c/em\u003e) were present in K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e and Hara\u003csub\u003eRN\u003c/sub\u003e site. True mosses (\u003cem\u003eP. strictum\u003c/em\u003e and \u003cem\u003eP. schreberi\u003c/em\u003e) had low cover (1\u0026ndash;3%) on K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e and Hara\u003csub\u003eRN\u003c/sub\u003e but were absent from all of the other study sites.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eSmall differences in plant cover and AGB occurred between the measurement plots in hummocks and lawns. Hummocks had higher AGB (15.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.3 g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e), AGP of \u003cem\u003eSphagnum\u003c/em\u003e (3.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) and mosses (3.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) than lawns (AGB 9.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e; AGP of \u003cem\u003eSphagnum\u003c/em\u003e 1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.5 g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e and mosses 1.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4 g dm\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e year\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). In reverse, the cover of lawn \u003cem\u003eSphagna\u003c/em\u003e was higher in lawns (72\u0026thinsp;\u0026plusmn;\u0026thinsp;14%) than in hummocks (21\u0026thinsp;\u0026plusmn;\u0026thinsp;15%) (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.2. Carbon dioxide fluxes\u003c/h2\u003e \u003cp\u003eMeasured NEE and R\u003csub\u003eECO\u003c/sub\u003e varied spatially to a larger extent in rewetted rather than in undisturbed sites (Appendix S2). A higher CO\u003csub\u003e2\u003c/sub\u003e net uptake with higher PAR was measured on both rewetted and undisturbed sites.\u003c/p\u003e \u003cp\u003eReconstructed P\u003csub\u003eg\u003c/sub\u003e and NEE did not differ statistically significantly (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05) between the rewetted and the undisturbed sites, whereas R\u003csub\u003eECO\u003c/sub\u003e was significantly higher in the rewetted than in the undisturbed sites (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05; Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). The respiration model\u0026rsquo;s parameter r0 was significantly (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) higher in the rewetted (41.8\u0026thinsp;\u0026plusmn;\u0026thinsp;11.4 mg CO\u003csub\u003e2\u003c/sub\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e) than in the undisturbed sites (9.4\u0026thinsp;\u0026plusmn;\u0026thinsp;3.1 mg CO\u003csub\u003e2\u003c/sub\u003e m\u003csup\u003e\u0026minus;\u0026thinsp;2\u003c/sup\u003e h\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e; Appendix S3). In K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e the reconstructed P\u003csub\u003eg\u003c/sub\u003e was significantly higher than in the undisturbed sites and at Hara\u003csub\u003eRS\u003c/sub\u003e (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05). Also, K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e had significantly higher R\u003csub\u003eECO\u003c/sub\u003e than K\u0026otilde;rsa\u003csub\u003eP\u003c/sub\u003e, whereas all of the other sites had a similar R\u003csub\u003eECO\u003c/sub\u003e. Although there were no differences in the model parameters between Hara\u003csub\u003eRN,\u003c/sub\u003e Hara\u003csub\u003eRS\u003c/sub\u003e and Hara\u003csub\u003eP\u003c/sub\u003e (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05), P\u003csub\u003emax\u003c/sub\u003e and r0 were higher in K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e than in K\u0026otilde;rsa\u003csub\u003eP\u003c/sub\u003e (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05) indicating a higher maximum CO\u003csub\u003e2\u003c/sub\u003e uptake in case of light saturation and also a higher minimum respiration rate in rewetted sites. Undisturbed sites did not differ significantly according to their CO\u003csub\u003e2\u003c/sub\u003e fluxes (p\u0026thinsp;\u0026gt;\u0026thinsp;0.05). There were no significant differences in CO\u003csub\u003e2\u003c/sub\u003e fluxes between the hummocks and the lawns in the undisturbed sites and K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e (p\u0026thinsp;\u0026lt;\u0026thinsp;0.05).\u003c/p\u003e \u003cp\u003eGLMMs were used specify the effect of site, microtopography and time since rewetting on growing season CO\u003csub\u003e2\u003c/sub\u003e flux components (R\u003csub\u003eECO\u003c/sub\u003e, P\u003csub\u003eg\u003c/sub\u003e, NEE) on rewetted peatlands. Although none of the fixed effects and GLMMs were statistically significant, time since rewetting had strongest effect on all of the CO\u003csub\u003e2\u003c/sub\u003e flux components (Table\u0026nbsp;\u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e). In addition to time since rewetting, microtopography and combination of microtopography and site had also relatively strong, but still statistically insignificant effect on R\u003csub\u003eECO\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e \u003ccaption language=\"En\"\u003e \u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e \u003cdiv class=\"CaptionContent\"\u003e \u003cp\u003eStatistical results of general linear mixed models (GLMM) determining effect of site conditions (site, microtopography, time since rewetting) on CO\u003csub\u003e2\u003c/sub\u003e flux components (NEE, P\u003csub\u003eg\u003c/sub\u003e, R\u003csub\u003eECO\u003c/sub\u003e) in rewetted milled peatlands.\u003c/p\u003e \u003c/div\u003e \u003c/caption\u003e \u003ccolgroup cols=\"4\"\u003e \u003cthead\u003e \u003ctr\u003e \u003cth align=\"left\" colname=\"c1\"\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e flux component\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eEffect\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eP\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eNEE\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,10\u003c/sub\u003e=0,03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,87\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMicrotopography\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,3\u003c/sub\u003e=0,00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,95\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTime since rewetting\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,1\u003c/sub\u003e=2,93\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,40\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSite*Microtopography\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,8\u003c/sub\u003e=0,59\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,47\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eP\u003csub\u003eg\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,0\u003c/sub\u003e=0,03\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1,00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMicrotopography\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,2\u003c/sub\u003e=0,12\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,76\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTime since rewetting\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,6\u003c/sub\u003e=1,87\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,22\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSite*Microtopography\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,10\u003c/sub\u003e=0,40\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,54\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eR\u003csub\u003eECO\u003c/sub\u003e\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSite\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,0\u003c/sub\u003e=0,00\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e1,00\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eMicrotopography\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,10\u003c/sub\u003e=3,50\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,09\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eTime since rewetting\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,8\u003c/sub\u003e=3,74\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,09\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/td\u003e \u003ctd align=\"left\" colname=\"c2\"\u003e \u003cp\u003eSite*Microtopography\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"left\" colname=\"c3\"\u003e \u003cp\u003eF\u003csub\u003e1,9\u003c/sub\u003e=1,09\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0,32\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.3. CO\u003csub\u003e2\u003c/sub\u003e fluxes and vegetation\u003c/h2\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e fluxes correlate with every PFT differently between the undisturbed and rewetted plots (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e, Appendix. S4). In the undisturbed sites, P\u003csub\u003eg\u003c/sub\u003e was higher in measurement plots with higher ombrotrophic sedge (\u003cem\u003eE. vaginatum\u003c/em\u003e) cover and biomass but lower with higher tree cover, which was related with the higher values of the parameter k indicating the PAR value when P\u003csub\u003eg\u003c/sub\u003e reaches half of its maximum value. In the rewetted sites, NEE was higher in the case of higher \u003cem\u003eSphagnum\u003c/em\u003e abundance, and higher photosynthesis rates were connected with the cover of minerotrophic forbs.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eP\u003csub\u003eg\u003c/sub\u003e increases with higher bryophyte and vascular biomass in rewetted sites, whereas this correlation was insignificant in the undisturbed sites (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). In the undisturbed sites, higher R\u003csub\u003eECO\u003c/sub\u003e was measured on plots with higher vascular plant biomass, whereas this correlation was insignificant in the rewetted sites. There were no other statistically significant correlations between vascular plant, bryophyte and plant biomass, and P\u003csub\u003eg\u003c/sub\u003e, R\u003csub\u003eECO\u003c/sub\u003e and NEE in the rewetted nor in the undisturbed plots.\u003c/p\u003e \u003cp\u003eWith time since rewetting, communities evolve in the direction of undisturbed mires, where several PFTs are present, including \u003cem\u003eSphagnum\u003c/em\u003e and evergreen trees (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Hara\u003csub\u003eRS\u003c/sub\u003e is characterised by high R\u003csub\u003eECO\u003c/sub\u003e and biomass of ombrotrophic sedges, Hara\u003csub\u003eRN\u003c/sub\u003e and K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e contain lawn \u003cem\u003eSphagnum\u003c/em\u003e and P\u003csub\u003eg\u003c/sub\u003e, while undisturbed sites (Hara\u003csub\u003eP\u003c/sub\u003e and K\u0026otilde;rsa\u003csub\u003eP\u003c/sub\u003e) have higher NEE along with the presence of hummock \u003cem\u003eSphagnum\u003c/em\u003e, ombrotrophic forbs and evergreen shrubs. With this transition, high R\u003csub\u003eECO\u003c/sub\u003e is replaced with higher P\u003csub\u003eg\u003c/sub\u003e, and eventually with higher NEE, indicating CO\u003csub\u003e2\u003c/sub\u003e sink function during the growing season.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e "},{"header":"4. Discussion","content":" \u003cdiv id=\"Sec12\" class=\"Section2\"\u003e \u003ch2\u003e4.1. Vegetation\u003c/h2\u003e \u003cp\u003eVegetation differed significantly between the undisturbed and rewetted sites. When undisturbed sites had oligotrophic raised-bog vegetation, vegetation in rewetted sites was typical to more nutrient rich environmental conditions and higher water table as reported previously (Tuittila et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Renou-Wilson et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Commonly, the less humified \u003cem\u003eSphagnum\u003c/em\u003e peat has been removed from abandoned milled peatlands, as the mineral-rich substrate supports the establishment and development of more nutrient demanding plant species. Contrary, oligotrophic vegetation is prevailing in bogs where the peat layer is more nutrient-poor and the water level deeper. After rewetting, vegetation establishment is more rapid and species rich in sites with more nutrients (Komulainen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Kozlov et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). This could have caused the relatively rapid vegetation succession on K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e where peat ash content is reported to be about twice higher (2\u0026ndash;3%) than in Hara rewetted sites (about 1\u0026ndash;2%; Orru \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e1995\u003c/span\u003e). In K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e, a rather diverse peatland community with a thick \u003cem\u003eSphagnum\u003c/em\u003e mat had developed in about 35 years.\u003c/p\u003e \u003cp\u003eActually, in K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e a thin layer of new peat \u0026ndash; an acrotelm \u0026ndash; has formed, which means that the site is functionally (but not structurally) quite similar to a pristine bog. According to results reported by Lucchese et al. (\u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), about a 19 cm thick bryophyte layer would be needed in the Bois-des-Bel restored milled peatland in Canada to mitigate summer water level drawdown; this could be reached about 17 years after restoration. Throughout the study period in K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e and Hara\u003csub\u003eRS\u003c/sub\u003e, the water level stayed inside the moss layer, mainly near the moss surface, therefore not decreasing the moss growth during the summer period. In the rewetted sites with thick moss layer in the current study, the moss layer was looser than in the undisturbed reference sites. This was probably due to the higher water table along with the high abundance of hollow \u003cem\u003eSphagna\u003c/em\u003e in the rewetted sites. Hollow \u003cem\u003eSphagnum\u003c/em\u003e could be affected from extreme droughts to a larger degree due to their larger pore size and less connectivity with the residual peat layer (McCarter and Price, \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) than the denser \u003cem\u003eSphagnum\u003c/em\u003e cover of undisturbed bogs, therefore making CO\u003csub\u003e2\u003c/sub\u003e exchange on rewetted sites more susceptible to drought impacts.\u003c/p\u003e \u003cp\u003eSome PFTs were lacking or had very low abundances in the rewetted sites but were present in the reference sites. We found significantly lower biomass and cover of evergreen shrubs on the rewetted than in the undisturbed sites, similar to results by Soini et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) and Gonz\u0026aacute;lez et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e), and they were absent from the most recently rewetted sites. Hummock \u003cem\u003eSphagna\u003c/em\u003e, which was present in both undisturbed bog sites was completely absent from the rewetted sites. The low occurrence and dying-off of hummock \u003cem\u003eSphagnum\u003c/em\u003e due to high water tables has been reported previously by Soini et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e) and Gonz\u0026aacute;lez et al. (\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). In reverse, Karofeld et al. (\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2015\u003c/span\u003e) recorded relatively high cover of hummock \u003cem\u003eSphagna\u003c/em\u003e and the presence of shrubs on restored milled peatland site where those species were dispersed using the moss-layer-transfer technique (Rochefort et al. \u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e2003\u003c/span\u003e). Therefore, the application of this technique could lead to a more diverse vegetation composition of restoration sites.\u003c/p\u003e \u003cp\u003eWhile vegetation differs significantly between the rewetted sites, being more diverse in the older sites, the vegetation in both undisturbed sites with a similar hummock and hollow vegetation pattern did not differ from each other. Hummocks on the two undisturbed sites are typical \u003cem\u003eCalluna-vulgaris-Sphagnum fuscum\u003c/em\u003e communities, the most common plant associations in Estonian bogs (Masing \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1982\u003c/span\u003e), and are comparable to the high hummock communities described by Korrensalo et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). Lawns in the undisturbed sites belong to the tussocky \u003cem\u003eEriophorum\u003c/em\u003e community or the \u003cem\u003eSphagnum balticum-Sphagnum rubellum\u003c/em\u003e community (Masing \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e1982\u003c/span\u003e), described also by Korrensalo et al. (\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) in an undisturbed bog in central Finland as lawn and high lawn communities. A large variation in vegetation occurred in rewetted, especially in the most recently rewetted site of Hara\u003csub\u003eRS\u003c/sub\u003e between the measurement plots. However, this could also be caused by the relatively low number of measurement plots in each study site and their positioning on the site. In recovering milled peatlands, vegetation is developing in patterns due to large variations in suitable substrate conditions for plant growth (Tuittila et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Purre and Ilomets \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2018\u003c/span\u003e) and the presence of nurse-plant species (Tuittila et al. \u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; Groeneveld et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2007\u003c/span\u003e), whereas in undisturbed bogs microtopography explains the largest portion of variation in vegetation composition (Korrensalo et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2018\u003c/span\u003e; Mežaka et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2018\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cem\u003eSphagnum\u003c/em\u003e has been considered a keystone genus of peatland restoration (Rochefort \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). In the newly rewetted Hara site, \u003cem\u003eSphagnum\u003c/em\u003e was not yet present in the measurement plots, although some patches of lawn \u003cem\u003eSphagnum\u003c/em\u003e (mainly \u003cem\u003eSphagnum cuspidatum\u003c/em\u003e) were present in depressions with high water level. After rewetting, the height of the water table should remain a few centimetres below the peat surface, which leads to optimal conditions for \u003cem\u003eSphagnum\u003c/em\u003e growth and peat accumulation (Beyer and H\u0026ouml;per \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). \u003cem\u003eSphagnum\u003c/em\u003e has relatively high immigration potential (Campbell et al. \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e2003\u003c/span\u003e) and is abundant on the undisturbed plots bordering the rewetted ones, so further colonisation of \u003cem\u003eSphagna\u003c/em\u003e in recently rewetted sites is expected. In both older rewetting sites, \u003cem\u003eSphagnum\u003c/em\u003e had almost total cover. In addition, in the oldest K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e site, lawn \u003cem\u003eSphagnum\u003c/em\u003e species have created some relatively high hummocks and overgrow \u003cem\u003eE. vaginatum\u003c/em\u003e tussocks. The AGP and IL of \u003cem\u003eSphagnum\u003c/em\u003e in the rewetted sites was similar to those reported by Ilomets (\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e1982\u003c/span\u003e) in Estonian undisturbed peatlands, while we measured about double the production and somewhat higher IL of \u003cem\u003eSphagna\u003c/em\u003e on the undisturbed sites. This probably results from different methods used for growth measurements (Pouliot et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), variations in weather conditions (Vitt \u003cspan citationid=\"CR75\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Bengtsson et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e) and species composition (Lindholm and Vasander \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e1990\u003c/span\u003e; Bengtsson et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2020\u003c/span\u003e).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section2\"\u003e \u003ch2\u003e4.2. Carbon dioxide fluxes\u003c/h2\u003e \u003cp\u003eBoth the undisturbed sites and the older rewetted sites were CO\u003csub\u003e2\u003c/sub\u003e net sinks during the growing season, while the more recently rewetted site was still a CO\u003csub\u003e2\u003c/sub\u003e source. Variations in CO\u003csub\u003e2\u003c/sub\u003e fluxes between the rewetted sites are large due to differences in vegetation, weather and water levels \u0026mdash; while some sites are important CO\u003csub\u003e2\u003c/sub\u003e sinks (Tuittila et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Beyer and H\u0026ouml;per \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Lee et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Purre et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e), others could be small CO\u003csub\u003e2\u003c/sub\u003e sources (Tuittila et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Waddington and Warner \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Beyer and H\u0026ouml;per \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Purre et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e). Although rewetted sites could be CO\u003csub\u003e2\u003c/sub\u003e sources in the first decades after rewetting, they should become a CO\u003csub\u003e2\u003c/sub\u003e net sink with time (Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Similar (Komulainen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1999\u003c/span\u003e) or higher (Soini et al. \u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) CO\u003csub\u003e2\u003c/sub\u003e net uptake on rewetted sites as in reference sites has been reported about ten years after rewetting, which is consistent with our results.\u003c/p\u003e \u003cp\u003eNEE in the rewetted sites is rather connected with differences in R\u003csub\u003eECO\u003c/sub\u003e than photosynthesis (Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Similarly to our results from the Hara rewetted site, lower CO\u003csub\u003e2\u003c/sub\u003e net uptake due to higher respiration has been reported from newly rewetted sites than from undisturbed bogs (Urbanov\u0026aacute; et al. \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). In reverse, in the studies by Soini et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), Christen et al. (\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and Strack et al. (\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), higher P\u003csub\u003eg\u003c/sub\u003e compensated for high R\u003csub\u003eECO\u003c/sub\u003e, therefore leading to a higher CO\u003csub\u003e2\u003c/sub\u003e net uptake on a rewetted site, which is consistent with our results from the K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e.\u003c/p\u003e \u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e fluxes and model parameters varied stronger between the measurement plots of the rewetted sites compared to undisturbed sites, as also reported by Soini et al. (\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e2010\u003c/span\u003e), Laine et al. (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and Strack et al. (\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). This could likely be driven by larger variations in PFT cover in the rewetted sites. Unvegetated plots on rewetted sites remain CO\u003csub\u003e2\u003c/sub\u003e sources (Wilson et al. \u003cspan citationid=\"CR79\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Purre et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e) but measurement plots turn from a CO\u003csub\u003e2\u003c/sub\u003e source to a sink with increasing plant cover (Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Purre et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e). Respiration on younger sites with still fragmented vegetation cover and lower diversity of plant species is largely influenced by peat temperature and water table depth, whereas those factors have a smaller effect on sites where vegetation has recovered well (Waddington and Warner \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Vanselow-Algan et al. 2015). Therefore, it could be expected that the CO\u003csub\u003e2\u003c/sub\u003e sink function will increase and be more stable with secondary succession after rewetting, especially as the actual acrotelm is formed with time.\u003c/p\u003e \u003cp\u003eWe detected some effect of site status on plant above-ground biomass, which on rewetted sites had a strong positive correlation with photosynthesis, whereas in undisturbed plots the correlation between plant biomass and P\u003csub\u003eg\u003c/sub\u003e was insignificant. Similarly to our rewetted sites, Marinier et al. (\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2004\u003c/span\u003e) reported higher photosynthesis in plots with higher AGB, but plots with high AGB have also been reported to have higher R\u003csub\u003eECO\u003c/sub\u003e (Marinier et al. \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Brown et al. \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). This was not the case in our rewetted sites, although in the undisturbed sites, R\u003csub\u003eECO\u003c/sub\u003e and vascular plant biomass had a strong positive correlation. The lack of correlations between the R\u003csub\u003eECO\u003c/sub\u003e and vascular plant biomass on rewetted milled peatlands is probably due to the domination of heterotrophic respiration on such sites (Wilson et al. \u003cspan citationid=\"CR80\" class=\"CitationRef\"\u003e2007b\u003c/span\u003e; J\u0026auml;rveoja et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Purre et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e). Laine et al. (\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), Strack et al. (\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and Purre et al. (\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2019b\u003c/span\u003e) also reported interaction between peatland management (undisturbed, rewetted), PFTs and carbon sequestration. According to J\u0026auml;rveoja et al. (\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), those correlations depend on water level depth \u0026mdash; if the water level is high in restored milled peatlands, bryophyte cover correlates with NEE, P\u003csub\u003eg\u003c/sub\u003e and autotrophic respiration, whereas with deeper water table CO\u003csub\u003e2\u003c/sub\u003e fluxes correlated with vascular plant cover. Therefore, the different correlations on rewetted and undisturbed sites are consistent with previous studies (Strack et al. \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and could be related to differences in water table height and fluctuations on sites with different management.\u003c/p\u003e \u003cp\u003eThere are large differences in photosynthetic capacities between PFTs. In the undisturbed sites, we measured higher photosynthesis and maximum photosynthesis rates (P\u003csub\u003emax\u003c/sub\u003e) in the case of higher \u003cem\u003eE. vaginatum\u003c/em\u003e cover. Vascular plant, especially graminoid biomass, has a relatively large impact on NEE in comparison with their abundance (Laine et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Hassanpour Fard et al. \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), due to their high photosynthetic capacity (Komulainen et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e1999\u003c/span\u003e; Kivim\u0026auml;ki et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Urbanov\u0026aacute; et al. \u003cspan citationid=\"CR73\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Strack et al. \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2014\u003c/span\u003e; Laine et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). As \u003cem\u003eE. vaginatum\u003c/em\u003e was present or abundant on most of the rewetted plots, the lack of correlation between the sedge cover and photosynthesis on the rewetted sites was unexpected. In addition to having high maximum photosynthesis (P\u003csub\u003emax\u003c/sub\u003e), this sedge species also has high light use efficiency (parameter k in the photosynthesis model) (Kivim\u0026auml;ki et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e) and high respiration rate (Jordan et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Still, in the case of a high water table, rewetted sites with high \u003cem\u003eE. vaginatum\u003c/em\u003e cover have a CO\u003csub\u003e2\u003c/sub\u003e net sink function, even in unfavourable habitat conditions such as the occasionally lower water table during drought periods (Tuittila et al. \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e1999\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the rewetted sites, higher photosynthesis and P\u003csub\u003emax\u003c/sub\u003e were measured with higher evergreen shrub cover. Evergreen shrubs stand out from other vascular plants with low photosynthesis and respiration rates (Laine et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), while in reverse Korrensalo et al. (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) reported high maximum photosynthesis rates on evergreen shrubs like \u003cem\u003eA. polifolia\u003c/em\u003e, \u003cem\u003eC. vulgaris\u003c/em\u003e and \u003cem\u003eV. oxycoccus\u003c/em\u003e, which are also present in the undisturbed sites and K\u0026otilde;rsa rewetted site in our study. According to Korrensalo et al. (\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), the P\u003csub\u003emax\u003c/sub\u003e of evergreen shrubs varies between species belonging to the same PFT. Still, the cause of controversies between different studies remains unclear and can be result of a rather low number of measurements that do not cover the whole ecosystem variation.\u003c/p\u003e \u003cp\u003eHigh photosynthesis in the case of higher evergreen shrub cover in this study could also be connected with higher plant cover and the number of PFTs on the measurement plots in K\u0026otilde;rsa\u003csub\u003eR\u003c/sub\u003e where evergreen shrubs were present. According to Kivim\u0026auml;ki et al. (\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2008\u003c/span\u003e), the presence of different PFTs lowers the R\u003csub\u003eECO\u003c/sub\u003e/P\u003csub\u003eg\u003c/sub\u003e ratio, so creating conditions for higher CO\u003csub\u003e2\u003c/sub\u003e net uptake as in K\u0026otilde;rsa, while in monostands of \u003cem\u003eE. vaginatum\u003c/em\u003e this ratio is higher, which also explains a lower CO\u003csub\u003e2\u003c/sub\u003e net uptake, as well as CO\u003csub\u003e2\u003c/sub\u003e net emissions from the younger site in this study. According to Hassanpour Fard et al. (\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2020\u003c/span\u003e), the presence of some key species or PFTs either in monostand or in mixed community support the larger carbon accumulation during the growing season than the mixed communities with a different number of PFTs lacking such certain species. Whereas most vascular plants, especially sedges, have high photosynthesis rates during summer when their LAI is highest, the importance of \u003cem\u003eSphagnum\u003c/em\u003e in CO\u003csub\u003e2\u003c/sub\u003e sequestration expresses itself during spring and autumn, when LAI\u003csub\u003evasc\u003c/sub\u003e is low (Korrensalo et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eIn the rewetted sites, CO\u003csub\u003e2\u003c/sub\u003e net sink function was larger in plots with higher \u003cem\u003eSphagnum\u003c/em\u003e cover. \u003cem\u003eSphagnum\u003c/em\u003e has lower photosynthetic capacities than vascular plants (Laine et al. \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Christen et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Korrensalo et al. \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2016\u003c/span\u003e) and also low respiration rates (Waddington and Warner \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2001\u003c/span\u003e; Laine et al. \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2016\u003c/span\u003e), and by increasing soil moisture content, a \u003cem\u003eSphagnum\u003c/em\u003e carpet could reduce soil respiration (Waddington and Warner \u003cspan citationid=\"CR77\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). However, restoring the \u003cem\u003eSphagnum\u003c/em\u003e carpet may not be enough for CO\u003csub\u003e2\u003c/sub\u003e sequestering (Samaritani et al. \u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), especially as a newly formed \u003cem\u003eSphagnum\u003c/em\u003e carpet is sensitive to drier conditions (Tuittila et al. \u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e2004\u003c/span\u003e). Therefore, constant high water tables are necessary, which support CO\u003csub\u003e2\u003c/sub\u003e accumulation of those sites early on after restoration activities (G\u0026uuml;nther et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003e \u003cb\u003eLimitations of the study\u003c/b\u003e \u003c/p\u003e \u003cp\u003eThis paper contributes to the growing but rather sparse knowledge base surrounding peatland restoration, engaging peatlands with different stages after rewetting and also several vegetation variables in addition to CO\u003csub\u003e2\u003c/sub\u003e flux measurements. However, some limitations of the study must be taken into account when considering the results. First, the study was conducted at a relatively low number of measurement points in the rewetted sites, especially in Hara\u003csub\u003eRN\u003c/sub\u003e. This could have affected the statistical analysis results regarding CO\u003csub\u003e2\u003c/sub\u003e fluxes as well as the vegetation variables to some extent, especially in case of GLMMs. For each vegetation type in each site, there were two true replicates, and one (Hara\u003csub\u003eRN\u003c/sub\u003e) or two (all other sites) dominant vegetation types were covered in each study site. Also, the different rewetted milled peatlands or their fields had different time since rewetting, therefore the site conditions could have been affected somewhat the conclusions about the effect of time since rewetted. Still according to GLMMs time since rewetting was the main factor explaining the CO2 flux components on the rewetted study sites.\u003c/p\u003e \u003cp\u003eSecond, the study covered only one growing season, so the annual balances of CO\u003csub\u003e2\u003c/sub\u003e cannot be derived from this. The CO\u003csub\u003e2\u003c/sub\u003e sequestration of the sites presented here are also strongly affected by weather conditions during that year, so they can differ from other years with varying conditions as shown at the Hara rewetted site by Purre et al. (\u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2019a\u003c/span\u003e). Also, although all of the sites were open peatland sites, the CO\u003csub\u003e2\u003c/sub\u003e exchange and biomass related with scarce tree cover were not accounted for in any of the studied sites. In addition, uncertainties related to flux measurements and reconstructions could affect the source or sink function of the sites during the growing seasons, especially if fluxes are very low and uncertainties higher (Bubier et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e1999\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThird, the methane emissions, along with dissolved organic carbon and dissolved inorganic carbon, were not measured from the study sites in this paper, as the general aim of the study was to analyse the differences in plant production parameters and PFT composition closely related with the CO\u003csub\u003e2\u003c/sub\u003e fluxes. Therefore, the results presented here do not provide information about the full carbon balance of the sites, as methane emissions for such sites have been reported to be high (Strack et al. \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e2014\u003c/span\u003e, \u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Vanselow-Algan et al. 2015; Beyer and H\u0026ouml;per \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; G\u0026uuml;nther et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Within these limitations, we still hope the paper will be of interest for a wide audience of peatland ecologists.\u003c/p\u003e \u003cp\u003e \u003cb\u003eConclusion\u003c/b\u003e \u003c/p\u003e \u003cp\u003eAlthough vegetation structure on rewetted milled peatlands approaches this on reference sites with time, some plant functional types present in the undisturbed reference sites, e.g., shrubs, colonise these sites in the later development stages and hummock \u003cem\u003eSphagnum\u003c/em\u003e could be absent even decades after rewetting. Vegetation composition developing with time affects the carbon accumulation of rewetted sites. During the studied growing season, over a decade ago rewetted milled peatlands were carbon sinks similarly to the reference sites, whereas the most recently rewetted site was still a carbon source to the atmosphere. Although graminoids play an important role in the photosynthesis of rewetted sites, as they do in undisturbed reference bogs, the carbon accumulation of rewetted peatlands is related with development of the \u003cem\u003eSphagnum\u003c/em\u003e mat, which is present in the reference sites. A well-developed \u003cem\u003eSphagnum\u003c/em\u003e mat also reflects the development of other environmental variables, of a functioning acrotelm and the development of a C sink function. Thus, a well-developed \u003cem\u003eSphagnum\u003c/em\u003e lawn could be used as an indicator of successful restoration. However, general plant functional type composition can still differ from reference sites in some accounts even several decades after rewetting.\u003c/p\u003e \u003c/div\u003e "},{"header":"Conclusion","content":"\u003cp\u003eAlthough vegetation structure on rewetted milled peatlands approaches this on reference sites with time, some plant functional types present in the undisturbed reference sites, e.g., shrubs, colonise these sites in the later development stages and hummock \u003cem\u003eSphagnum\u003c/em\u003e could be absent even decades after rewetting. Vegetation composition developing with time affects the carbon accumulation of rewetted sites. During the studied growing season, over a decade ago rewetted milled peatlands were carbon sinks similarly to the reference sites, whereas the most recently rewetted site was still a carbon source to the atmosphere. Although graminoids play an important role in the photosynthesis of rewetted sites, as they do in undisturbed reference bogs, the carbon accumulation of rewetted peatlands is related with development of the \u003cem\u003eSphagnum\u003c/em\u003e mat, which is present in the reference sites. A well-developed \u003cem\u003eSphagnum\u003c/em\u003e mat also reflects the development of other environmental variables, of a functioning acrotelm and the development of a C sink function. Thus, a well-developed \u003cem\u003eSphagnum\u003c/em\u003e lawn could be used as an indicator of successful restoration. However, general plant functional type composition can still differ from reference sites in some accounts even several decades after rewetting.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e No funding was received for conducting this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflicts of interests:\u003c/strong\u003e The authors have no conflicts of interest to declare that are relevant to the content of this article.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material:\u003c/strong\u003e The datasets generated and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode availability:\u003c/strong\u003e The codes used for data modelling and analysis are available from the corresponding author upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions:\u003c/strong\u003e Both authors contributed to the study conception and design. Anna-Helena Purre conducted the material preparation, fieldwork, laboratory and data analysis, and she wrote the first draft of the manuscript. Mati Ilomets commented on previous versions of the manuscript. Both authors read and approved the final manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAnimal Research (Ethics):\u003c/strong\u003e Not applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Participate (Ethics):\u003c/strong\u003e Not applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to Publish (Ethics):\u003c/strong\u003e Not applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003ePlant Reproducibility:\u003c/strong\u003e Not applicable\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eClinical Trials Registration:\u003c/strong\u003e Not applicable\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eAndersen R, Pouliot R, Rochefort L (2013) Above-ground net primary production from vascular plants shifts the balance towards organic matter accumulation in restored \u003cem\u003eSphagnum\u003c/em\u003e Wetlands 33:811-821\u003c/li\u003e\n\u003cli\u003eBengtsson F, Rydin H, Baltzer JL, Bragazza L, Bu Z-J, Caporn SJ, Dorrepaal E, Flatberg KI, Galanina O, Gałka M, Ganeva A, Goia I, Goncharova N, H\u0026aacute;jek M, Haraguchi A, Harris LI, Humphreys E, Jirou\u0026scaron;ek M, Kajukało K, Karofeld E, Koronatova NG, Kosykh NP, Laine AM, Lamentowicz M, Lapshina E, Limpens J, Linkosalmi M, Ma J-Z, Mauritz M, Mitchell EAD, Munir TM, Natali SM, Natcheva R, Payne RJ, Philippov DA, Rice SK, Robinson S, Robroek BJM, Rochefort L, Singer D, Sten\u0026oslash;ien HK, Tuittila E-S, Vellak K, Waddington JM, Granath G (2020) Environmental drivers of \u003cem\u003eSphagnum\u003c/em\u003e growth in peatlands across the Holarctic region. 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Bot J Linn Soc 104:35-59\u003c/li\u003e\n\u003cli\u003eWaddington JM, Lucchese MC, Duval TP (2011) \u003cem\u003eSphagnum\u003c/em\u003e moss moisture retention following the re-vegetation of degraded peatlands. Ecohydrology 4:359-366\u003c/li\u003e\n\u003cli\u003eWaddington JM, Warner KD (2001) Atmospheric CO\u003csub\u003e2\u003c/sub\u003e sequestration in restored mined peatlands. Ecoscience 8:359-368\u003c/li\u003e\n\u003cli\u003eWilson D, Alm J, Riutta T, Laine J, Byrne KA, Farrell EP, Tuittila E-S (2007a) A high resolution green area index for modelling the seasonal dynamics of CO\u003csub\u003e2\u003c/sub\u003e exchange in peatland vascular plant communities. Plant Ecol 190:37-51\u003c/li\u003e\n\u003cli\u003eWilson D, Farrell CA, Fallon D, Moser G, M\u0026uuml;ller C, Renou-Wilson F (2016) Multiyear greenhouse gas balances at a rewetted temperate peatland. Glob Chang Biol 22:4080-4095\u003c/li\u003e\n\u003cli\u003eWilson D, Tuittila E-S, Alm J, Laine J, Farrell EP, Byrne KA (2007b) Carbon dynamics of a restored maritime peatland. Ecoscience 14:71-80\u003c/li\u003e\n\u003cli\u003eYu ZC (2012) Northern peatland carbon stocks and dynamics: a review. Biogeosciences 9:4071-4085\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"wetlands","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wela","sideBox":"Learn more about [Wetlands](https://www.springer.com/journal/13157)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/wela/default.aspx","title":"Wetlands","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Milled peatlands, above-ground biomass, rewetting, CO2 exchange, reference ecosystem, peatland restoration","lastPublishedDoi":"10.21203/rs.3.rs-323253/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-323253/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eRewetting is the most common restoration approach for milled peatlands in Europe, with the aim of creating suitable conditions for the development of peatland specific plant cover and carbon accumulation. Therefore, it is important to determine if time since rewetting is pivotal for milled peatlands to become functionally and structurally similar to their undisturbed counterparts. We investigate the temporal succession in rewetted peatlands in Estonia by a chronosequence of 4, 15, and 35 years before the measurements. Plant functional type (PFT) cover and biomass, bryophyte production and CO\u003csub\u003e2\u003c/sub\u003e fluxes were measured on two milled peatlands, as well as undisturbed bogs adjacent to milled peatlands. \u003c/p\u003e\u003cp\u003eDifferences in vegetation composition and CO\u003csub\u003e2\u003c/sub\u003e fluxes between the sites were greater for rewetted than undisturbed sites. The most recently rewetted site was mainly covered in bare peat and \u003cem\u003eEriophorum vaginatum\u003c/em\u003e and was a CO\u003csub\u003e2\u003c/sub\u003e source. On the rewetted site of 15 years, \u003cem\u003eSphagnum\u003c/em\u003e was present in addition to ombrotrophic sedges, and in the rewetted site of 35 years, lawn-hollow microtopography is starting to develop with various PFTs. Both of these sites were CO\u003csub\u003e2\u003c/sub\u003e sinks. Lawn \u003cem\u003eSphagnum\u003c/em\u003e was abundant on the two older rewetted sites, and was connected with CO\u003csub\u003e2\u003c/sub\u003e sink functioning in the rewetted sites. Still, hummock \u003cem\u003eSphagnum\u003c/em\u003e species, which were present in undisturbed bogs, were absent from all of the rewetted sites. With time, CO\u003csub\u003e2\u003c/sub\u003e fluxes, microtopography and vegetation develop after rewetting in the direction of undisturbed bogs, while vegetation composition still differs from the reference sites even 35 years after rewetting.\u003c/p\u003e","manuscriptTitle":"Time Since Rewetting Defines Vegetation Composition and Carbon Dioxide Fluxes on Former Milled Peatlands - Comparison With Undisturbed Bogs","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2021-03-25 13:52:40","doi":"10.21203/rs.3.rs-323253/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"reviewersInvited","content":"","date":"2021-06-22T00:00:00+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2021-03-23T00:00:00+00:00","index":0,"fulltext":""},{"type":"editorInvited","content":"Wetlands","date":"2021-03-18T00:00:00+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2021-03-16T00:00:00+00:00","index":"","fulltext":""},{"type":"submitted","content":"Wetlands","date":"2021-03-12T04:12:36+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"wetlands","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"wela","sideBox":"Learn more about [Wetlands](https://www.springer.com/journal/13157)","snPcode":"","submissionUrl":"https://www.editorialmanager.com/wela/default.aspx","title":"Wetlands","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"423b546a-f530-499a-b178-4948c813c9f1","owner":[],"postedDate":"March 25th, 2021","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":3216152,"name":"Environmental Policy"}],"tags":[],"updatedAt":"2021-11-22T03:10:45+00:00","versionOfRecord":[],"versionCreatedAt":"2021-03-25 13:52:40","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-323253","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-323253","identity":"rs-323253","version":["v1"]},"buildId":"7rjqhiLT3MXkJMwkYKINL","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}