Aridity and soil properties drive the shrub-herb interactions along drought gradient in desert grassland in Inner Mongolia

Aridity and soil properties drive the shrub-herb interactions along drought gradient in desert grassland in Inner Mongolia | 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 Aridity and soil properties drive the shrub-herb interactions along drought gradient in desert grassland in Inner Mongolia Huiyang Hou, Yuzhen Zhang, Jianwei Zhou, Huijuan Liu, Yuanheng Li, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4491863/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Purpose Environmental conditions can control the structure and composition of plant communities by changing the direction and intensity of plant-plant interactions. However, how drought and soil properties drive the change of shrub-herb interaction in the shrub-encroachment desert steppe in Inner Mongolia remains unclear. Methods We explored the changes of shrub-herb interaction along the aridity gradient, and analyzed how the aridity index and soil properties affect the shrub-herb interaction. Field collection of plant samples and soils from northeast to southwest desert steppe in Inner Mongolia was undertaken and the samples were analyzed for biomass, diversity, soil PH, soil organic matter and other elements. Results The results show that the positive shrub-herb interaction (RII > 0) increased at first and then decreased in the range of aridity index 0.54 to 1.85 (drought degree decreased gradually). Aridity index (AI), soil organic matter (SOM) and soil total phosphorus (TP) were the main factors driving shrub-herb interaction. AI indirectly affected shrub-herb interaction through TP, and the facilitation of shrubs on herbs coverage and biomass increased with the increase of TP. The SOM can directly affect the shrub-herbs interaction, and the facilitation of shrubs on herb diversity decreases with the increase of SOM. Conclusion Overall, although shrub-herb interactions respond differently to environmental factors. This study underscores the positive effects of shrubs on vegetation restoration in desert steppe, and changing environmental conditions by increasing precipitation, increasing TP content, and reducing SOM content can enhance facilitation of shrub on herbs to accelerate the ecological restoration of degraded desert steppe. Aridity Caragana Microphylla Shrub-herb interaction Facilitation SGH soil properties Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Introduction Plant-plant interactions have been widely recognized as the core mechanisms shaping the distribution of species and the maintenance of biodiversity (Brooker et al. 2008 ; Arroyo et al. 2015 ). Several previous researches supposed that environmental conditions play important roles in determining the direction and intensity of plant-plant interactions (Cannone et al. 2022 ; Soliveres et al. 2015 ; Soliveres and Maestre, 2014 ). Despite this general consensus, how plant-plant interactions change driven by environmental conditions (stress levels, different environmental types) are still controversial. Therefore, understanding how the response of plant-plant interactions to environmental factors can accurately predict the direction of community succession. The stress gradient hypothesis (SGH) that increasing environmental severity results in the plant-plant interactions from facilitation to competition (Bertness and Callaway, 1994 ; Brooker and Callaghan, 1998 ). A great number of studies support SGH (Armas et al. 2011 ), however, in arid and semi-arid ecosystems where water is the major constraint, the consistency of SGH prediction results is deficient (Butterfield et al. 2010 ; Soliveres et al. 2011 ). This discrepancy of prediction results primarily exists at the near both extreme of stress gradients, either as predicted by SGH, or neutral interactions or even the transition from facilitation to competition with the increase of environmental stress (Maestre and Cortina, 2004 ; Michalet, 2006 ). These conclusions have attracted the attention of researchers and has been empirically supported in many studies, in particular, research on interaction between leguminous shrub and herbs (Armas et al. 2011 ; Cui et al. 2023 ; Gomaa et al. 2020 ). Despite decades of research, controversy has emerged as to whether shrub-herb interactions predictably change with increasing environmental stress, as assumed by the SGH (Soliveres et al. 2015 ; Armas et al. 2011 ; Q. He et al. 2013 ; Liancourt et al. 2017 ). Due to climatic change and irrational utilization of grassland, the abundance of shrubs in grassland ecosystems increase, result in the phenomenon of shrub encroachment exacerbated (Auken, 2000 ; D’Odoricoet al, 2012 ; Chen et al, 2015 ). The ecological consequences of shrub encroachment are significantly discrepancy in different regions affected by various environmental factors. Over a long period of time, shrub encroachment is considerable ecological manifestation of desertification (Hibbard et al. 2001 ). However, with the research has been conducted under different environmental conditions, a wave of results show that shrub encroachment increases the biodiversity of ecosystem, and support that shrub may be an important key component in reversing desertification (Eldridge et al. 2011 ; Maestre et al. 2009 ). The essence of shrub encroachment can be understood as the change of the direction and intensity of shrub-herb interaction, which leads to changes in community structure. Shrubs can regulate landscape and community structure through positive and/or negative effects on neighboring herbs (Knapp et al. 2008 ; O’Brien et al. 2017 ; Bai et al. 2020 ). Recently, some studies have explored how shrub-herb interactions affect community assembly and community succession. These researches found that shrub can change the spatial pattern of communities, which is more conducive to species coexistence, increase community diversity and productivity, thus accelerating the restoration of vegetation in degraded ecosystems (Ding et al. 2020 ; Fan et al. 2016 ; Li et al. 2021 ). This discovery is of great significance for the effective restoration of degraded semi-arid ecosystems. There are about 5.1 million hm 2 shrub-encroachment grasslands in Inner Mongolia, China, especially Caragana microphylla shrub-encroachment is relatively common (Chen et al. 2015 ; Zhou et al. 2019 ). Caragana microphylla is widely distributed in desert steppe, which has low precipitation, large evaporation and strong wind erosion (Knapp et al. 2017 ; Wang et al. 2021 ). Although the shrub encroachment phenomenon exists widely in the grasslands of northern China, its ecological consequences and related driving factors have not been well recognized on the regional scale. As a result of climatic changes, terrestrial ecosystems are expected to experience more severe droughts (Intergovernmental Panel on Climate Change (IPCC), 2023 ), which may promote changes in plant community composition in shrub-encroachment grasslands. In Inner Mongolia, shrub-herb interactions are affected by both precipitation and temperature (Yang et al. 2011 ). Therefore, the comprehensive consideration of precipitation and temperature will accurately predict the response of shrub-herb interaction to drought gradient, hence, we use aridity index (AI) to quantify regional hydrothermal conditions and drought degree (Quan et al. 2013 ). The changes of soil properties (such as soil pH, nutrients and other physical and chemical properties) and environmental wind speed will also affect the shrub-herb interaction (García-Cervigón et al. 2021 ; Zhou et al. 2023 ). For example, increasing soil nitrogen content increased shrub-herb competition (She et al. 2021 ). With the increase of soil phosphorus content, the competitive advantage of shrubs decreased, while the competitiveness of herbs increased, which significantly increased the herbaceous diversity plant communities (Rieger et al. 2019 ). In the Qinghai-Tibet Plateau, precipitation and temperature affected the facilitation of legume shrubs on herbs through soil nitrogen, and then changed the plant community composition (Cui et al. 2023 ; Ma et al. 2020 ). Shrub encroachment frequently occurs in arid and semi-arid grasslands worldwide, and it is significant to clarify the changes of shrub-herb interactions along drought gradients and soil properties, which can help to predict changes in species diversity, structure and community composition of shrub-encroachment grasslands affected by global change (Ploughe et al. 2019). At present, little is known about how the aridity index and soil properties determine the shrub-herb interaction on the drought gradient of shrub-encroachment desert steppe in Inner Mongolia. In this study, we taking Caragana microphylla shrub-encroachment desert steppe of Inner Mongolia as the research object, the changes of shrub-herb interaction along the aridity index gradient were analyzed through field experiments and laboratory tests. We explored the effects of aridity index, wind speed and soil properties on shrub-herb interaction. Based on the SGH and previous studies on shrub-herb interaction in shrub-encroachment, our study raised the following specific questions: (1) how does shrub-herb interaction change along the aridity index gradient in the experimental area? (2) which environmental factors are the key factors that cause these changes? What is the process? Material and methods Study Environment The experimental sites are located in a natural arid climate transects of 1000 km from northeast to southwest desert steppe area, Inner Mongolia, China (111°12′51″~115°06′54″, 41°30′52″~44°45′23″). The climate is temperate continental climate. The area has a temperature and precipitation change uniformly from northeast to southwest (Table 1 ). Mean annual precipitation in the past five years from 60 to 200 mm, and most falls between May and September (the main growing season of plants). The mean annual temperature ranges from 1 to 5℃(raining and hot during the same period). The compound gradient of precipitation and temperature is determined by aridity index (AI). The higher AI value represents wetter environment condition, on the contrary, the smaller AI value represents the drier environment condition (Quan et al. 2013 ). 20 sites were selected on natural Caragana microphylla shrub-encroachment grassland along the climatic gradient transect (see Fig. 1 ). Table 1 Location and environmental data of the study sites Number Site name Longitude (°E) Latitude (°N) Altitude (m) Precipitation (mm) Temperature (℃) Aridity index Wind speed (m/s) 1 Erlianhot 111°36′45″ 43°24′19″ 967 63.30 4.51 0.54 2.28 2 Sonid Left Banner 113°05′13″ 44°00′11″ 960 60.28 3.08 0.58 1.82 3 Sonid Left Banner 112°45′27″ 43°43′52″ 937 66.63 3.72 0.61 1.96 4 Erlianhot 112°12′10″ 43°19′30″ 1037 72.50 4.7 0.62 2.31 5 Sonid Left Banner 113°34′36″ 44°02′14″ 961 70.37 2.8 0.69 1.78 6 Siziwang Banner 111°31′01″ 42°21′54″ 1257 84.16 4.89 0.71 2.45 7 Sonid Right Banner 112°22′03″ 42°33′27″ 1233 96.94 4.94 0.81 3.02 8 Sonid Right Banner 113°25′31″ 43°28′32″ 1157 95.41 3.67 0.87 2.12 9 Siziwang Banner 111°46′33″ 42°02′10″ 1425 113.33 4.13 1.00 2.57 10 Siziwang Banner 112°15′51″ 42°16′58″ 1254 119.07 4.66 1.02 3.14 11 Siziwang Banner 111°12′51″ 41°51′01″ 1486 124.37 4.18 1.10 2.23 12 Sonid Left Banner 114°19′56″ 43°21′48″ 1067 122.02 3.74 1.11 2.28 13 Siziwang Banner 111°50′34″ 41°48′14″ 1419 125.87 4.07 1.12 2.45 14 Abaga Banner 115°06′54″ 44°45′23″ 1115 100.43 1.06 1.14 2.86 15 Sonid Right Banner 114°00′26″ 42°40′24″ 1097 134.95 4.52 1.16 2.57 16 Abaga Banner 114°49′17″ 43°58′15″ 1146 122.79 1.94 1.28 2.06 17 Siziwang Banner 112°16′09″ 42°00′17″ 1470 145.76 3.81 1.32 3.11 18 Abaga Banner 115°01′01″ 44°12′17″ 1254 122.67 1.31 1.35 2.35 19 Xiangbaiqi 114°53′52″ 42°24′29″ 1360 180.11 3.48 1.67 2.77 20 Chahar West Middle County 112°14′09″ 41°30′52″ 1652 189.48 2.78 1.85 2.77 Aridity index (AI) = Precipitation/(Temperature + 10) These climatic parameters were obtained from the WorldClim dataset ( http://www.worldclim.org/ ). The data layers used had 2.5 min spatial resolution. According to the climatic data in the transect, the aridity index of each plot was calculated: where p is the average precipitation (mm), T is the average temperature (℃). Vegetation dominated by the Caragana Microphylla (shrub), Stipa brevifloris , Leymus chinensis , Salsola collina , etc. Data Collection From July to September, 2023, we selected 20 plots of 50 m×50 m along the aridity index gradient. We randomly selected three individuals of Caragana microphylla with similar shape, and with growing well and unshaded for each plot. In order to minimize the potential interaction between individual shrubs, the distance between any two individuals is more than 10 m. Within each plot, we sampled three 1 m×1 m squares placed entirely beneath the canopy of shrubs and the same number of quadrats more than 5 m (try to avoid the influence of the shrub) away from the edge of the shrub canopy in open areas (Haas-Desmarais and Lortie, 2023 ). We recorded the plant species (species richness) and their cover in the quadrats. We measured the natural height of all individuals of each herbaceous plant, and calculated the average height of species. The aboveground parts (upward from the soil surface) of all individuals of each herb in the sample square were harvested separately into an envelope and brought back to the laboratory. We used the method of single diagonal sampling to collect three soil samples of 0 to 20 cm from each square with a soil drill of 2.5 cm in diameter, and put into aseptic self-sealing bag and numbered. All plant samples were washed with distilled water to remove dust particles, and then oven-dried at 65 ℃ for 72 h to determine their dry weights. The soil samples were air-dried, ground and screened to < 2 mm. Extracted 100g from three soil samples from each square, and finally mixed. Soil properties were determined by soil mixed samples. The PH of 1:5 soil water extract was determined by PH meter, the Soil Organic Matter (SOM) was determined by dichromate oxidation method, the Total Nitrogen (TN) was determined by semi-micro Kjeldahl nitrogen determination method, and the Total Phosphorus (TP) was determined by vanadium molybdate blue colorimetric method. Statistical Analyses We used relative interaction index (RⅡ) to evaluate the effect of shrubs on herbaceous plants (Armas et al. 2004 ). The RII calculation is as follows: $$RII=\frac{{X}_{shrub}-{X}_{open}}{{X}_{shrub}+{X}_{open}}$$ Where X is the herbs coverage, biomass, height and diversity in the quadrat beneath the shrub canopy (X shrub ) or in the open (X open ). The index ranges from − 1 to 1. RII > 0: shrubs show positive interaction to herbaceous plants; RII < 0: shrubs show negative interactions to herbaceous plants; RII = 0: shrub-herb interaction vector sum is 0. We used Levene test to check the normality and uniformity of residuals before statistical analysis. First, we conducted regression analyses to determine if the intensity of shrub-herb interactions varied in time with aridity in our study plots. Second, in order to evaluate the effects of climate factors and soil properties on shrub-herb interaction and their relationship, we conducted Pearson correlation analysis to exclude the collinear environmental factors. And redundancy analysis (RDA) was used to study the effect of environmental factors on shrub-herb interactions. Regression analysis (Regression) was used to test the changing trend of shrub-herb interaction along the various environmental factors. Finally, we used structural equation model (SEM) to determine the direct and indirect effects of environmental factors in regulating shrub-herb interaction. Pearson correlation analysis, RDA and SEM analyses were conducted in R Studio (2023.09.1). Other statistical analyses were performed using IBM SPSS Statistics 26 (IBM Corp, Armonk, NY, USA). Graphs were drawn using Origin 2021 (Origin Lab, Northampton, MA, USA), R Studio 2023.09.1 and Adobe Illustrator 2021. Results Effects of shrubs on herbs along aridity gradient Under the environmental gradient in our study area, shrubs showed facilitation on herbaceous plants continuously. The facilitation of shrub on neighbour-plants displaying a unimodal trend along the AI gradient (Fig. 3 ). At the community level, facilitation by shrub on herbaceous coverage (RII cover), biomass (RII biomass) and diversity (RII Shannon) increasing first and then decreasing along the AI gradient(p < 0.05, Fig. 3 acd). The maximum value was found in the range of AI from 1.0 to 1.2, indicating that facilitation by shrub on herbaceous biomass, coverage and diversity reached its maximum in this range. However, the effect of shrubs on herbaceous height (RII height) had no significant change along the AI gradient (Fig. 3 b). Effects of environmental factors on shrub-herb interactions We used Pearson correlation to eliminate multicollinearity between environmental factors. The results showed that the correlation between the environmental factors beneath the shrub canopy and in open areas along the AI gradient was small (Fig. 4 ), indicating that the collinearity is slightly and can guarantee the accuracy of RDA. We used RDA to evaluate the main environmental factors driving shrub-herb interaction, the results showed that AI and TP had greater effects on RII (biomass) and RII (cover), meanwhile, SOM and TN had greater effects on RII (height) and RII (Shannon). Furthermore, W had a greater influence on RII (height) and RII (Shannon), while soil PH had less influence on RII (biomass), RII (cover), RII (height) and RII (Shannon) (Fig. 5 ). AI, TP, SOM and TN were the main environmental factors driving changes of shrub-herb interaction, while PH and W had little effect on shrub-herb interaction. Based on the above analysis, we use regression analysis to explore changing trend of shrub-herb interaction along environmental factors (Fig. 6 ). The results showed that RII (biomass) and RII (cover) were positively correlated with TP. With the increase of TP, the facilitation of shrub on herbs biomass and coverage increased (R 2 = 0.1, p = 0.04;R 2 = 0.14, p = 0.02 Fig. 6 b). With the increase of TP, the facilitation of shrubs on herbs height increased at first and then decreased, showing a unimodal trend (R 2 = 0.23, p = 0.01 Fig. 6 b). There was a negative correlation between RII (Shannon) and SOM, and the facilitation of shrub on herbs diversity decreased with the increase of SOM (R 2 = 0.1, p < 0.05, Fig. 6 d). RII (height) was positively correlated with W, and the facilitation of shrub on herbs height increased with the increase of W. Relationship Between Aridity Index, Soil Properties and Shrub-herb Interactions The SEM results showed that the AI could directly affect RII (biomass), RII (cover), RII (height) and RII (Shannon) directly (Fig. 7 ), and among them, the path coefficient from AI to RII (biomass) and RII (cover) is higher, which is 0.88 and 0.94, respectively. AI can indirectly affect RII (biomass), RII (Shannon) and RII (cover) through TP, among which TP has the greatest indirect effect on RII (Shannon). W had a direct effect on RII (Shannon) and RII (height), and SOM had a direct effect on RII (Shannon) and RII (biomass). Discussion Shrub-herb interactions in desert steppe and its variation along aridity index gradient We explored the change of shrub-herb interactions along the natural aridity gradient. Our study revealed facilitation of shrub Caragana microphylla on herb under drought conditions, and the facilitation increased first and then decreased with the increase of drought degree. This is consistent with previous research results (Cavieres et al. 2016 ; Maestre et al. 2009 ; Ochoa-Hueso et al. 2018 ). The facilitation by shrub on herbaceous reached its maximum in the range of AI from 1.0 to 1.2(Fig. 3 ). This effect may arise from a greater availability of resources under the shrubs, such as nutrients and water (Pugnaire et al. 1996 ; Zhu et al. 2022 ). The weakening of facilitation along the fertility gradient appeared to be a consequence of improved abiotic conditions, as predicted by current theory (Bertness and Callaway, 1994 ; R. Brooker and Callaghan, 1998 ). The results of measuring competition and facilitation showed that the positive and negative effects of plant interactions along the gradient of habitat fertility in different ways seems to be more a consequence of a decline in facilitation than a change in competition (Pugnaire and Luque, 2001 ). Invasive shrubs can stimulate nutrient cycling and promote vegetation restoration by capturing resources under their canopies, thus reversing desertification in degraded systems (Zhao et al. 2007 ). During the past few years, the ecological consequences of shrub encroachment have been controversial. Shrub-encroachment are considered could decrease the richness, abundance and aboveground biomass of herbs, that has a negative impact on the structure and diversity of herbs plant communities (Zhang et al. 2022 ; Zhou et al. 2019 ). On the contrary, shrub in our study has a positive impact on the composition and structure of herbs communities in desert steppe. In desert steppe, infertile soil nutrients and water resources deficiency were the limiting condition of plant growth. Shrubs alleviate resource competition among plants to a certain extent by improving the microenvironment, which is beneficial to the growth of surrounding herbs (Li et al. 2021 ; Zhang et al. 2018 ). In addition, we found with drought intensified, the positive interaction of shrubs on herbs increasing first and then decreasing, displaying a unimodal trend. The results are consistent with the recent refined SGH, that is, near the extreme position of stress gradient, the positive interaction will be weakened or transformed into negative interaction. The possible reasons for the deviation from SGH prediction are: (1) the competition between shrubs and herbaceous increases with the increase of aridity index, which offsets its promoting effect (Tielborger and Kadmon, 2000 ). (2) With the increase of aridity index, herbaceous plant species were replaced by more stress-tolerant species, and these species were less likely to be promoted by shrubs (Le Bagousse-Pinguet et al. 2012 ; Liancourt et al. 2017 ). Zhang et al. found that with the aggravation of aridity stress, the shrub-herb interaction in the Badain Jaran Desert of China changed from positive to neutral (Zhang et al. 2018 ). O’Brien’s research showed that the positive interaction of shrubs over species richness and community productivity during the wetter growing season, but shifted to competitive dominance interaction in the drier season (O’Brien et al. 2017 ). The results of this study are basically consistent with the above research results. However, the study of Dohn et al (2013) showed that the positive shrub-herb interaction did not change significantly along the aridity gradient, which may be due to the relatively limited data on soil nitrogen concentration. In our study, considering the facilitation of shrubs on herbaceous plants, under certain drought conditions, shrub-encroachment has a restoration effect on desert steppe. Effects of environmental factors on shrub-herb interaction We show that AI, SOM and TP were the main factors driving the change of shrub-herb interaction to affect the community structure in our study area. Several researches indicated that climatic factors and soil properties such as precipitation and soil phosphorus are important factors influencing community structure (Liu et al. 2023 ; Xie et al. 2023 ). The SEM results showed directly effect of AI and SOM on shrub-herb interactions, and SOM determines the effect of shrubs on herbs diversity (Fig. 7 ). A possible explanation for this might be that the increase of competition between shrubs and herbs (Lal 2020 ). Soil organic matter have the greatest effect on shrub coverage among all soil factors (Fu et al. 2004 ), this may also be the reason for affecting the effect of shrubs on herbs diversity. During the dry period when the vegetation degradation and plant growth negatively affected, SOM is the only soil properties in relation with diversity index (Dölarslan et al. 2017 ). In recent years, scholars have carried out a large number of simulation studies on the internal relationship between drought and shrub-herb interaction and the impact of the former on the latter. In line with our results, precipitation affects community species composition and diversity by controlling shrub-herb interaction (He et al. 2022 ). Similarly, precipitation intensity pushes soil water deeper and increases the growth of shrubs, which is not conducive to the growth of herbaceous on the grassland (Kulmatiski and Beard, 2013 ). Therefore, the shrub-herb interaction can be affected by changing the exogenous environmental conditions, so as to promote the vegetation restoration of degraded desert steppe. For example, artificial precipitation enhancement and nutrient addition can indirectly improve the stability of grassland ecosystem by increasing facilitation by shrub on herb (He et al. 2022 ). Wind speed is often neglected in the study of environmental factors driving shrub-herb interaction. In our study, the increase of wind speed is beneficial to facilitation of shrubs on herbaceous plants. Zhang et al. showed that shrubs can effectively reduce wind erosion and improve microenvironment (Zhang et al. 2021). In addition, Zhi et al. found that shrubs had a significant reduction effect on wind speed, and its effect was related to shrub height and vegetation coverage (Zhi et al. 2024). This indirectly supports the conclusion that shrubs enhance the facilitation of shrubs on herbaceous plants during the increase of wind erosion, and also reminds us that the influence of shrub individual characteristics on shrub-herbs interaction is nonnegligible. Effect of soil phosphorus on shrub-herb interaction Our analysis found AI could indirectly affect shrub-herb interaction through TP (Fig. 7 ), which is consistent with previous studies (Jiao et al. 2016 ). Increasing precipitation indirectly enhanced the facilitation of shrubs on herbs by increasing soil phosphorus(P) content, soil phosphorus decreased with the increase of drought, showing a negative correlation (Jiao et al. 2016 ). This can be attributed to that transformation of soil P takes longer in arid areas with low leaching intensity than in humid areas, resulting in lower soil P content in arid areas (Walker and Syers, 1976 ). However, studies by Delgado-Baquerizo et al. showed that total phosphorus was not affected by drought degree (Delgado-Baquerizo et al. 2013 ). This difference may be due to different drought gradient scales, and in the study of Baquerizo et al, the soil total phosphorus content did not change significantly on a larger geographical scale. Therefore, the response of soil nutrients, especially phosphorus content, to drought degree on a global scale may not be applied to the regional scale. Sardans et al ' s meta-analysis showed that changes in soil phosphorus content caused by environmental factors may drive the structure of ecosystems by affecting plant-plant interaction, supporting the results of this study (Sardans et al. 2012 ). We found that soil P determines the effect of shrubs on herb productivity (Fig. 6 ). Previous studies have indicated that plant biomass were positively correlated with soil P in grasslands (Fayiah et al. 2019 ), our findings are in accord with that. It is expected that some parts of the world will become drier in the next few decades, and the results of our study are helpful to predict the vegetation change in these areas. Conclusions Overall, our study found shrub Caragana Microphylla had a positive impact on herb community, which is beneficial to the positive succession of shrub-encroachment desert steppe, shrubs may play an important role in desert steppe ecological restoration. The change of the facilitation along the aridity gradient showed a unimodal trend, which was consistent with the refined SGH. Aridity index, soil organic matter and soil total phosphorus were the main environmental factors driving changes of shrub-herb interaction. We can use the method of controlling exogenous water conditions (artificial rainfall, etc.), soil organic matter content (regular cleaning of plant residues, etc.) and soil phosphorus content (application of phosphorus fertilizer, etc.) to enhance the facilitation of shrub on herbs, which is beneficial to the ecological restoration of degraded desert steppe. Soil organic matter determines the effect of shrubs on herbs diversity, and soil phosphorus determines the effect of shrubs on herbs productivity. Aridity index indirectly affects the shrubs-herbs interaction through soil phosphorus. The increase of precipitation indirectly enhanced the facilitation of shrub on herb by increasing TP content, which was conducive to the restoration of degraded desert steppe. Declarations Competing interests The authors declare no competing financial interests. Author contributions SG and JZ designed the study. HH conducted the experiments, performed statistical analyses, and wrote the first manuscript. All authors contributed critically to the drafts and gave final approval for publication. Acknowledgements This research was funded by the Inner Mongolia Autonomous Region Natural Science Foundation (2020BS03046, 2023QN03007), and the Special Fund of Chinese Central Government for Basic Scientific Research Operations in Commonweal Research Institutes (1610332020015), and the Yinshanbeilu Grassland Eco-hydrology National Observation and Research Station, China Institute of Water Resources and Hydropower Research, Beijing 100038, China, Grant (YSS2022013), and the Basic scientific research business expenses of CAAS (1610332022012). Data availability The datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request. References Armas C, Ordiales R, Pugnaire FI (2004) Measuring plant interactions: a new comparative index. 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Front Plant Sci 14:1184618. https://doi.org/10.3389/fpls.2023.1184618 Zhou L, Shen H, Chen L, Li H, Zhang P, Zhao X, Liu T, Liu S, Xing A, Hu H, Fang J (2019) Ecological consequences of shrub encroachment in the grasslands of northern China. Landscape Ecol 34(1):119–130. https://doi.org/10.1007/s10980-018-0749-2 Zhu Y, Shen H, Akinyemi DS, Zhang P, Feng Y, Zhao M, Kang J, Zhao X, Hu H, Fang J (2022) Increased precipitation attenuates shrub encroachment by facilitating herbaceous growth in a Mongolian grassland. Funct Ecol 36(9):2356–2366. https://doi.org/10.1111/1365-2435.14100 Supplementary Files siteinformation.xlsx Cite Share Download PDF Status: Posted Version 1 posted 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. 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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-4491863","acceptedTermsAndConditions":true,"allowDirectSubmit":true,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":311096838,"identity":"a638d6a5-e2b3-441c-a60f-bb26cd23c9b4","order_by":0,"name":"Huiyang Hou","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1ElEQVRIiWNgGAWjYBACAwYGNjAJBIwPEipqSNPCbPDgzDFitUAAm+TDFmbCWszZe8we8xQcljfnX36tIrGBjYG/vTsBrxbLnjPmxjwGhw13znhTdiNxhwyDxJmzG/A77EaOmTRQC+OGG2fSbiSeYWMwkMglTos9SEtBYhsz8VoSN5xvP8ZAnJYzx8ok5xikJ2+4wcMskXDmGA9hvxxv3ibx5o+17Ybzxx9+/FFRI8ff3otfCxQ0MzBI5IAjlIcY5SBQx8DAf/wBsapHwSgYBaNghAEAUWpNSLev/E4AAAAASUVORK5CYII=","orcid":"","institution":"Chinese Academy of Agricultural Sciences Grassland Research Institute","correspondingAuthor":true,"prefix":"","firstName":"Huiyang","middleName":"","lastName":"Hou","suffix":""},{"id":311096839,"identity":"f4036b53-de31-428a-8410-28d51cb80dfa","order_by":1,"name":"Yuzhen Zhang","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yuzhen","middleName":"","lastName":"Zhang","suffix":""},{"id":311096840,"identity":"24c2acc8-2af8-4755-96ac-08591612bafb","order_by":2,"name":"Jianwei Zhou","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Jianwei","middleName":"","lastName":"Zhou","suffix":""},{"id":311096841,"identity":"4fb17fb9-b780-4231-8687-dc2ec574e639","order_by":3,"name":"Huijuan Liu","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Huijuan","middleName":"","lastName":"Liu","suffix":""},{"id":311096842,"identity":"5b2ead55-31c3-4cf7-afbd-983e0cec78c5","order_by":4,"name":"Yuanheng Li","email":"","orcid":"","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Yuanheng","middleName":"","lastName":"Li","suffix":""},{"id":311096843,"identity":"1a5b2ae2-580a-44d6-b3bc-f5c4530095ba","order_by":5,"name":"Shaobo Gao","email":"","orcid":"https://orcid.org/0000-0002-0473-6782","institution":"","correspondingAuthor":false,"prefix":"","firstName":"Shaobo","middleName":"","lastName":"Gao","suffix":""}],"badges":[],"createdAt":"2024-05-28 15:10:40","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4491863/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4491863/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":58626906,"identity":"246b23f0-6006-4562-a2ab-bccd4314ba1f","added_by":"auto","created_at":"2024-06-19 04:43:50","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":34103,"visible":true,"origin":"","legend":"\u003cp\u003eMap of the experimental settings, the sampled sites (triangles).\u003c/p\u003e","description":"","filename":"image1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/2be6c28b6fdd592476c9b6c7.jpeg"},{"id":58626909,"identity":"c44b2ab3-53f6-4067-b6d1-c37a621e8bc9","added_by":"auto","created_at":"2024-06-19 04:43:50","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":767271,"visible":true,"origin":"","legend":"\u003cp\u003eSchematic diagram of shrub-grass species pairs sampling in Inner Mongolian Shrub-encroached desert grassland.\u003c/p\u003e","description":"","filename":"image2.png","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/527961e65b060858d5ba894e.png"},{"id":58626472,"identity":"6df69987-a09e-4f5c-acc0-6f528cbd35a0","added_by":"auto","created_at":"2024-06-19 04:35:50","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":41914,"visible":true,"origin":"","legend":"\u003cp\u003eRelative Interaction Index (RII) for four characteristics of herbaceous community along the aridity gradient. RII values are above or below zero, indicating a positive or negative shrub effect on herbs. AI, aridity index.\u003c/p\u003e","description":"","filename":"image3.png","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/06e277a61ca4df3f17a25411.png"},{"id":58626470,"identity":"6862aca4-4c7e-41ca-882a-74dffa1a9e42","added_by":"auto","created_at":"2024-06-19 04:35:50","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":21852,"visible":true,"origin":"","legend":"\u003cp\u003eHeat map of environmental factors Pearson correlation at 0 m from the canopy and 5 m from the edge of the canopy, PH, soil pH; TP, soil total phosphorus content; TN, soil total nitrogen content; SOM, soil organic matter content; AI, aridity index; W, wind speed.\u003c/p\u003e","description":"","filename":"image4.png","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/2f8ee064f1de83b1e235c119.png"},{"id":58627601,"identity":"e01e8296-e4d1-4431-819e-56f79c1f40af","added_by":"auto","created_at":"2024-06-19 04:51:50","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":33925,"visible":true,"origin":"","legend":"\u003cp\u003eResult of redundancy analysis of environment properties and relative interaction index (at community level). Ordination diagrams presented RII scores (blue) and environmental factor scores (red) in the redundancy analysis. AI, aridity index; W, ambient wind speed; pH, soil pH; TN, soil total nitrogen content; TP, soil total phosphorus content; SOM, soil organic matter content.\u003c/p\u003e","description":"","filename":"image5.png","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/a24510516b28340111fe6e66.png"},{"id":58626476,"identity":"296b37e5-71f5-4e6e-9c5c-a40c4fdb525a","added_by":"auto","created_at":"2024-06-19 04:35:50","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":49424,"visible":true,"origin":"","legend":"\u003cp\u003eRegression analysis, (a) showing the relationship of soil pH with RII, (b) total phosphorus content (TP) with RII, (c) soil nitrogen content (TN) with RII, (d) soil organic matter content (SOM) with RII, (e) environmental wind speed(W) with RII.\u003c/p\u003e","description":"","filename":"image6.png","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/fb1c7fb1b7c7f766a1874300.png"},{"id":58626475,"identity":"1e17f393-3ace-40d8-a2fc-c65e0579d88c","added_by":"auto","created_at":"2024-06-19 04:35:50","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":57584,"visible":true,"origin":"","legend":"\u003cp\u003eStructural equation model (SEM) to illustrate the direct and indirect effects of aridity index (AI) and environment properties on the shrub-herb interaction (RII). Green and red arrows indicate positive and negative path coefficients respectively. Dotted arrows represent non-significant paths (p\u0026gt;0.05). Path widths are scaled proportionally to the path coefficient. Numbers adjacent to arrows are significant standardized path coefficients.\u003c/p\u003e","description":"","filename":"image7.png","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/8992cb47369a0029004799a1.png"},{"id":59532706,"identity":"acbaf995-c3f1-4535-b013-10297a3ef6f5","added_by":"auto","created_at":"2024-07-03 00:49:56","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1566910,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/ba32e8ec-5ad2-454a-a3e8-3b146d605598.pdf"},{"id":58626908,"identity":"378e067b-94c4-42b4-961c-92eb61c8702d","added_by":"auto","created_at":"2024-06-19 04:43:50","extension":"xlsx","order_by":5,"title":"","display":"","copyAsset":false,"role":"supplement","size":11685,"visible":true,"origin":"","legend":"","description":"","filename":"siteinformation.xlsx","url":"https://assets-eu.researchsquare.com/files/rs-4491863/v1/f7788c0f10b92e0a545beb43.xlsx"}],"financialInterests":"","formattedTitle":"Aridity and soil properties drive the shrub-herb interactions along drought gradient in desert grassland in Inner Mongolia","fulltext":[{"header":"Introduction","content":"\u003cp\u003ePlant-plant interactions have been widely recognized as the core mechanisms shaping the distribution of species and the maintenance of biodiversity (Brooker et al. \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; Arroyo et al. \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). Several previous researches supposed that environmental conditions play important roles in determining the direction and intensity of plant-plant interactions (Cannone et al. \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Soliveres et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Soliveres and Maestre, \u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e2014\u003c/span\u003e). Despite this general consensus, how plant-plant interactions change driven by environmental conditions (stress levels, different environmental types) are still controversial. Therefore, understanding how the response of plant-plant interactions to environmental factors can accurately predict the direction of community succession. The stress gradient hypothesis (SGH) that increasing environmental severity results in the plant-plant interactions from facilitation to competition (Bertness and Callaway, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; Brooker and Callaghan, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). A great number of studies support SGH (Armas et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e), however, in arid and semi-arid ecosystems where water is the major constraint, the consistency of SGH prediction results is deficient (Butterfield et al. \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e2010\u003c/span\u003e; Soliveres et al. \u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). This discrepancy of prediction results primarily exists at the near both extreme of stress gradients, either as predicted by SGH, or neutral interactions or even the transition from facilitation to competition with the increase of environmental stress (Maestre and Cortina, \u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e2004\u003c/span\u003e; Michalet, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e2006\u003c/span\u003e). These conclusions have attracted the attention of researchers and has been empirically supported in many studies, in particular, research on interaction between leguminous shrub and herbs (Armas et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Cui et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Gomaa et al. \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Despite decades of research, controversy has emerged as to whether shrub-herb interactions predictably change with increasing environmental stress, as assumed by the SGH (Soliveres et al. \u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Armas et al. \u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Q. He et al. \u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e2013\u003c/span\u003e; Liancourt et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2017\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eDue to climatic change and irrational utilization of grassland, the abundance of shrubs in grassland ecosystems increase, result in the phenomenon of shrub encroachment exacerbated (Auken, \u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e2000\u003c/span\u003e; D\u0026rsquo;Odoricoet al, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Chen et al, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2015\u003c/span\u003e). The ecological consequences of shrub encroachment are significantly discrepancy in different regions affected by various environmental factors. Over a long period of time, shrub encroachment is considerable ecological manifestation of desertification (Hibbard et al. \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). However, with the research has been conducted under different environmental conditions, a wave of results show that shrub encroachment increases the biodiversity of ecosystem, and support that shrub may be an important key component in reversing desertification (Eldridge et al. \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e2011\u003c/span\u003e; Maestre et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2009\u003c/span\u003e). The essence of shrub encroachment can be understood as the change of the direction and intensity of shrub-herb interaction, which leads to changes in community structure. Shrubs can regulate landscape and community structure through positive and/or negative effects on neighboring herbs (Knapp et al. \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e2008\u003c/span\u003e; O\u0026rsquo;Brien et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Bai et al. \u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Recently, some studies have explored how shrub-herb interactions affect community assembly and community succession. These researches found that shrub can change the spatial pattern of communities, which is more conducive to species coexistence, increase community diversity and productivity, thus accelerating the restoration of vegetation in degraded ecosystems (Ding et al. \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e2020\u003c/span\u003e; Fan et al. \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Li et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). This discovery is of great significance for the effective restoration of degraded semi-arid ecosystems. There are about 5.1\u0026nbsp;million hm\u003csup\u003e2\u003c/sup\u003e shrub-encroachment grasslands in Inner Mongolia, China, especially \u003cem\u003eCaragana microphylla\u003c/em\u003e shrub-encroachment is relatively common (Chen et al. \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e2015\u003c/span\u003e; Zhou et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). \u003cem\u003eCaragana microphylla\u003c/em\u003e is widely distributed in desert steppe, which has low precipitation, large evaporation and strong wind erosion (Knapp et al. \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e2017\u003c/span\u003e; Wang et al. \u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). Although the shrub encroachment phenomenon exists widely in the grasslands of northern China, its ecological consequences and related driving factors have not been well recognized on the regional scale. As a result of climatic changes, terrestrial ecosystems are expected to experience more severe droughts (Intergovernmental Panel on Climate Change (IPCC), \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e2023\u003c/span\u003e), which may promote changes in plant community composition in shrub-encroachment grasslands. In Inner Mongolia, shrub-herb interactions are affected by both precipitation and temperature (Yang et al. \u003cspan citationid=\"CR58\" class=\"CitationRef\"\u003e2011\u003c/span\u003e). Therefore, the comprehensive consideration of precipitation and temperature will accurately predict the response of shrub-herb interaction to drought gradient, hence, we use aridity index (AI) to quantify regional hydrothermal conditions and drought degree (Quan et al. \u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e2013\u003c/span\u003e).\u003c/p\u003e \u003cp\u003eThe changes of soil properties (such as soil pH, nutrients and other physical and chemical properties) and environmental wind speed will also affect the shrub-herb interaction (Garc\u0026iacute;a-Cervig\u0026oacute;n et al. \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zhou et al. \u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). For example, increasing soil nitrogen content increased shrub-herb competition (She et al. \u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e2021\u003c/span\u003e). With the increase of soil phosphorus content, the competitive advantage of shrubs decreased, while the competitiveness of herbs increased, which significantly increased the herbaceous diversity plant communities (Rieger et al. \u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). In the Qinghai-Tibet Plateau, precipitation and temperature affected the facilitation of legume shrubs on herbs through soil nitrogen, and then changed the plant community composition (Cui et al. \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Ma et al. \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Shrub encroachment frequently occurs in arid and semi-arid grasslands worldwide, and it is significant to clarify the changes of shrub-herb interactions along drought gradients and soil properties, which can help to predict changes in species diversity, structure and community composition of shrub-encroachment grasslands affected by global change (Ploughe et al. 2019). At present, little is known about how the aridity index and soil properties determine the shrub-herb interaction on the drought gradient of shrub-encroachment desert steppe in Inner Mongolia.\u003c/p\u003e \u003cp\u003eIn this study, we taking \u003cem\u003eCaragana microphylla\u003c/em\u003e shrub-encroachment desert steppe of Inner Mongolia as the research object, the changes of shrub-herb interaction along the aridity index gradient were analyzed through field experiments and laboratory tests. We explored the effects of aridity index, wind speed and soil properties on shrub-herb interaction. Based on the SGH and previous studies on shrub-herb interaction in shrub-encroachment, our study raised the following specific questions: (1) how does shrub-herb interaction change along the aridity index gradient in the experimental area? (2) which environmental factors are the key factors that cause these changes? What is the process?\u003c/p\u003e"},{"header":"Material and methods","content":"\u003cp\u003eStudy Environment\u003c/p\u003e\n\u003cp\u003eThe experimental sites are located in a natural arid climate transects of 1000 km from northeast to southwest desert steppe area, Inner Mongolia, China (111\u0026deg;12\u0026prime;51\u0026Prime;~115\u0026deg;06\u0026prime;54\u0026Prime;, 41\u0026deg;30\u0026prime;52\u0026Prime;~44\u0026deg;45\u0026prime;23\u0026Prime;). The climate is temperate continental climate. The area has a temperature and precipitation change uniformly from northeast to southwest (Table\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e). Mean annual precipitation in the past five years from 60 to 200 mm, and most falls between May and September (the main growing season of plants). The mean annual temperature ranges from 1 to 5℃(raining and hot during the same period). The compound gradient of precipitation and temperature is determined by aridity index (AI). The higher AI value represents wetter environment condition, on the contrary, the smaller AI value represents the drier environment condition (Quan et al. \u003cspan class=\"CitationRef\"\u003e2013\u003c/span\u003e). 20 sites were selected on natural \u003cem\u003eCaragana microphylla\u003c/em\u003e shrub-encroachment grassland along the climatic gradient transect (see Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003e).\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cdiv class=\"gridtable\"\u003e\n\u003cdiv class=\"colspec\" align=\"char\"\u003e\u0026nbsp;\u003c/div\u003e\n\u003ctable id=\"Tab1\" style=\"width: 1003px;\" border=\"1\"\u003e\u003ccaption\u003e\n\u003cdiv class=\"CaptionNumber\"\u003eTable 1\u003c/div\u003e\n\u003cdiv class=\"CaptionContent\"\u003e\n\u003cp\u003eLocation and environmental data of the study sites\u003c/p\u003e\n\u003c/div\u003e\n\u003c/caption\u003e\n\u003cthead\u003e\n\u003ctr style=\"height: 48px;\"\u003e\n\u003cth style=\"width: 63.5394px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eNumber\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 149.461px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eSite name\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 105px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eLongitude (\u0026deg;E)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 94px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eLatitude (\u0026deg;N)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 85px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eAltitude (m)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 133px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003ePrecipitation (mm)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 125px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eTemperature (℃)\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 84px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eAridity index\u003c/p\u003e\n\u003c/th\u003e\n\u003cth style=\"width: 111px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eWind speed (m/s)\u003c/p\u003e\n\u003c/th\u003e\n\u003c/tr\u003e\n\u003c/thead\u003e\n\u003ctbody\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e1\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eErlianhot\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e111\u0026deg;36\u0026prime;45\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e43\u0026deg;24\u0026prime;19\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e967\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e63.30\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; 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height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e112\u0026deg;12\u0026prime;10\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e43\u0026deg;19\u0026prime;30\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1037\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e72.50\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.62\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.31\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e5\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSonid Left Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e113\u0026deg;34\u0026prime;36\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e44\u0026deg;02\u0026prime;14\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e961\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e70.37\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.69\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.78\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e6\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSiziwang Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e111\u0026deg;31\u0026prime;01\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e42\u0026deg;21\u0026prime;54\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1257\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e84.16\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.89\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.71\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.45\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e7\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSonid Right Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e112\u0026deg;22\u0026prime;03\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e42\u0026deg;33\u0026prime;27\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1233\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e96.94\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.94\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.81\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.02\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e8\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSonid Right Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e113\u0026deg;25\u0026prime;31\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e43\u0026deg;28\u0026prime;32\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1157\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e95.41\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.67\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e0.87\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.12\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e9\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSiziwang Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e111\u0026deg;46\u0026prime;33\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e42\u0026deg;02\u0026prime;10\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1425\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e113.33\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.13\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.00\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.57\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e10\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSiziwang Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e112\u0026deg;15\u0026prime;51\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e42\u0026deg;16\u0026prime;58\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1254\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e119.07\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.66\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.02\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.14\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSiziwang Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e111\u0026deg;12\u0026prime;51\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e41\u0026deg;51\u0026prime;01\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1486\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e124.37\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.18\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.10\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.23\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSonid Left Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e114\u0026deg;19\u0026prime;56\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e43\u0026deg;21\u0026prime;48\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1067\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e122.02\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.74\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.28\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e13\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSiziwang Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e111\u0026deg;50\u0026prime;34\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e41\u0026deg;48\u0026prime;14\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1419\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e125.87\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.07\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.12\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.45\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e14\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eAbaga Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e115\u0026deg;06\u0026prime;54\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e44\u0026deg;45\u0026prime;23\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1115\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e100.43\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.06\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.14\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.86\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e15\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSonid Right Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e114\u0026deg;00\u0026prime;26\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e42\u0026deg;40\u0026prime;24\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1097\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e134.95\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e4.52\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.16\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.57\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e16\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eAbaga Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e114\u0026deg;49\u0026prime;17\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e43\u0026deg;58\u0026prime;15\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1146\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e122.79\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.94\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.28\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.06\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e17\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eSiziwang Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e112\u0026deg;16\u0026prime;09\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e42\u0026deg;00\u0026prime;17\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1470\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e145.76\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.81\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.32\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.11\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e18\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eAbaga Banner\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e115\u0026deg;01\u0026prime;01\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e44\u0026deg;12\u0026prime;17\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1254\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e122.67\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.31\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.35\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.35\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 35px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e19\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003eXiangbaiqi\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e114\u0026deg;53\u0026prime;52\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 35px;\" align=\"left\"\u003e\n\u003cp\u003e42\u0026deg;24\u0026prime;29\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1360\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e180.11\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e3.48\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.67\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 35px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.77\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003ctr style=\"height: 48px;\"\u003e\n\u003ctd style=\"width: 63.5394px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003e20\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 149.461px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003eChahar West Middle County\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 105px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003e112\u0026deg;14\u0026prime;09\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 94px; height: 48px;\" align=\"left\"\u003e\n\u003cp\u003e41\u0026deg;30\u0026prime;52\u0026Prime;\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 85px; height: 48px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1652\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 133px; height: 48px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e189.48\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 125px; height: 48px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.78\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 84px; height: 48px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e1.85\u003c/p\u003e\n\u003c/td\u003e\n\u003ctd style=\"width: 111px; height: 48px;\" align=\"char\" char=\".\"\u003e\n\u003cp\u003e2.77\u003c/p\u003e\n\u003c/td\u003e\n\u003c/tr\u003e\n\u003c/tbody\u003e\n\u003c/table\u003e\n\u003c/div\u003e\n\u003cp\u003eAridity index (AI)\u0026thinsp;=\u0026thinsp;Precipitation/(Temperature\u0026thinsp;+\u0026thinsp;10)\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThese climatic parameters were obtained from the WorldClim dataset (\u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttp://www.worldclim.org/\u003c/span\u003e\u003c/span\u003e). The data layers used had 2.5 min spatial resolution. According to the climatic data in the transect, the aridity index of each plot was calculated:\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cimg src=\"data:image/png;base64,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\"\u003e\u003c/p\u003e\n\u003cp\u003e\u003cbr\u003e\u003c/p\u003e\n\u003cp\u003ewhere p is the average precipitation (mm), T is the average temperature (℃). Vegetation dominated by the \u003cem\u003eCaragana Microphylla\u003c/em\u003e (shrub), \u003cem\u003eStipa brevifloris\u003c/em\u003e, \u003cem\u003eLeymus chinensis\u003c/em\u003e, \u003cem\u003eSalsola collina\u003c/em\u003e, etc.\u003c/p\u003e\n\u003cp\u003eData Collection\u003c/p\u003e\n\u003cp\u003eFrom July to September, 2023, we selected 20 plots of 50 m\u0026times;50 m along the aridity index gradient. We randomly selected three individuals of \u003cem\u003eCaragana microphylla\u003c/em\u003e with similar shape, and with growing well and unshaded for each plot. In order to minimize the potential interaction between individual shrubs, the distance between any two individuals is more than 10 m. Within each plot, we sampled three 1 m\u0026times;1 m squares placed entirely beneath the canopy of shrubs and the same number of quadrats more than 5 m (try to avoid the influence of the shrub) away from the edge of the shrub canopy in open areas (Haas-Desmarais and Lortie, \u003cspan class=\"CitationRef\"\u003e2023\u003c/span\u003e). We recorded the plant species (species richness) and their cover in the quadrats. We measured the natural height of all individuals of each herbaceous plant, and calculated the average height of species. The aboveground parts (upward from the soil surface) of all individuals of each herb in the sample square were harvested separately into an envelope and brought back to the laboratory. We used the method of single diagonal sampling to collect three soil samples of 0 to 20 cm from each square with a soil drill of 2.5 cm in diameter, and put into aseptic self-sealing bag and numbered.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eAll plant samples were washed with distilled water to remove dust particles, and then oven-dried at 65 ℃ for 72 h to determine their dry weights. The soil samples were air-dried, ground and screened to \u0026lt;\u0026thinsp;2 mm. Extracted 100g from three soil samples from each square, and finally mixed. Soil properties were determined by soil mixed samples. The PH of 1:5 soil water extract was determined by PH meter, the Soil Organic Matter (SOM) was determined by dichromate oxidation method, the Total Nitrogen (TN) was determined by semi-micro Kjeldahl nitrogen determination method, and the Total Phosphorus (TP) was determined by vanadium molybdate blue colorimetric method.\u003c/p\u003e\n\u003cp\u003eStatistical Analyses\u003c/p\u003e\n\u003cp\u003eWe used relative interaction index (RⅡ) to evaluate the effect of shrubs on herbaceous plants (Armas et al. \u003cspan class=\"CitationRef\"\u003e2004\u003c/span\u003e). The RII calculation is as follows:\u003c/p\u003e\n\u003cdiv id=\"Equa\" class=\"Equation\"\u003e\n\u003cdiv id=\"FileID_Equa\" class=\"mathdisplay\"\u003e$$RII=\\frac{{X}_{shrub}-{X}_{open}}{{X}_{shrub}+{X}_{open}}$$\u003c/div\u003e\n\u003c/div\u003e\n\u003cp\u003eWhere X is the herbs coverage, biomass, height and diversity in the quadrat beneath the shrub canopy (X\u003csub\u003eshrub\u003c/sub\u003e) or in the open (X\u003csub\u003eopen\u003c/sub\u003e). The index ranges from \u0026minus;\u0026thinsp;1 to 1. RII\u0026thinsp;\u0026gt;\u0026thinsp;0: shrubs show positive interaction to herbaceous plants; RII\u0026thinsp;\u0026lt;\u0026thinsp;0: shrubs show negative interactions to herbaceous plants; RII\u0026thinsp;=\u0026thinsp;0: shrub-herb interaction vector sum is 0.\u003c/p\u003e\n\u003cp\u003eWe used Levene test to check the normality and uniformity of residuals before statistical analysis. First, we conducted regression analyses to determine if the intensity of shrub-herb interactions varied in time with aridity in our study plots. Second, in order to evaluate the effects of climate factors and soil properties on shrub-herb interaction and their relationship, we conducted Pearson correlation analysis to exclude the collinear environmental factors. And redundancy analysis (RDA) was used to study the effect of environmental factors on shrub-herb interactions. Regression analysis (Regression) was used to test the changing trend of shrub-herb interaction along the various environmental factors. Finally, we used structural equation model (SEM) to determine the direct and indirect effects of environmental factors in regulating shrub-herb interaction. Pearson correlation analysis, RDA and SEM analyses were conducted in R Studio (2023.09.1). Other statistical analyses were performed using IBM SPSS Statistics 26 (IBM Corp, Armonk, NY, USA). Graphs were drawn using Origin 2021 (Origin Lab, Northampton, MA, USA), R Studio 2023.09.1 and Adobe Illustrator 2021.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003eEffects of shrubs on herbs along aridity gradient\u003c/p\u003e \u003cp\u003eUnder the environmental gradient in our study area, shrubs showed facilitation on herbaceous plants continuously. The facilitation of shrub on neighbour-plants displaying a unimodal trend along the AI gradient (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). At the community level, facilitation by shrub on herbaceous coverage (RII cover), biomass (RII biomass) and diversity (RII Shannon) increasing first and then decreasing along the AI gradient(p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eacd). The maximum value was found in the range of AI from 1.0 to 1.2, indicating that facilitation by shrub on herbaceous biomass, coverage and diversity reached its maximum in this range. However, the effect of shrubs on herbaceous height (RII height) had no significant change along the AI gradient (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eEffects of environmental factors on shrub-herb interactions\u003c/p\u003e \u003cp\u003eWe used Pearson correlation to eliminate multicollinearity between environmental factors. The results showed that the correlation between the environmental factors beneath the shrub canopy and in open areas along the AI gradient was small (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e), indicating that the collinearity is slightly and can guarantee the accuracy of RDA. We used RDA to evaluate the main environmental factors driving shrub-herb interaction, the results showed that AI and TP had greater effects on RII (biomass) and RII (cover), meanwhile, SOM and TN had greater effects on RII (height) and RII (Shannon). Furthermore, W had a greater influence on RII (height) and RII (Shannon), while soil PH had less influence on RII (biomass), RII (cover), RII (height) and RII (Shannon) (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). AI, TP, SOM and TN were the main environmental factors driving changes of shrub-herb interaction, while PH and W had little effect on shrub-herb interaction.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eBased on the above analysis, we use regression analysis to explore changing trend of shrub-herb interaction along environmental factors (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). The results showed that RII (biomass) and RII (cover) were positively correlated with TP. With the increase of TP, the facilitation of shrub on herbs biomass and coverage increased (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.1, p\u0026thinsp;=\u0026thinsp;0.04;R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.14, p\u0026thinsp;=\u0026thinsp;0.02 Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). With the increase of TP, the facilitation of shrubs on herbs height increased at first and then decreased, showing a unimodal trend (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.23, p\u0026thinsp;=\u0026thinsp;0.01 Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003eb). There was a negative correlation between RII (Shannon) and SOM, and the facilitation of shrub on herbs diversity decreased with the increase of SOM (R\u003csup\u003e2\u003c/sup\u003e\u0026thinsp;=\u0026thinsp;0.1, p\u0026thinsp;\u0026lt;\u0026thinsp;0.05, Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003ed). RII (height) was positively correlated with W, and the facilitation of shrub on herbs height increased with the increase of W.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eRelationship Between Aridity Index, Soil Properties and Shrub-herb Interactions\u003c/p\u003e \u003cp\u003eThe SEM results showed that the AI could directly affect RII (biomass), RII (cover), RII (height) and RII (Shannon) directly (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e), and among them, the path coefficient from AI to RII (biomass) and RII (cover) is higher, which is 0.88 and 0.94, respectively. AI can indirectly affect RII (biomass), RII (Shannon) and RII (cover) through TP, among which TP has the greatest indirect effect on RII (Shannon). W had a direct effect on RII (Shannon) and RII (height), and SOM had a direct effect on RII (Shannon) and RII (biomass).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eShrub-herb interactions in desert steppe and its variation along aridity index gradient\u003c/p\u003e \u003cp\u003eWe explored the change of shrub-herb interactions along the natural aridity gradient. Our study revealed facilitation of shrub \u003cem\u003eCaragana microphylla\u003c/em\u003e on herb under drought conditions, and the facilitation increased first and then decreased with the increase of drought degree. This is consistent with previous research results (Cavieres et al. \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e2016\u003c/span\u003e; Maestre et al. \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e2009\u003c/span\u003e; Ochoa-Hueso et al. \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). The facilitation by shrub on herbaceous reached its maximum in the range of AI from 1.0 to 1.2(Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e). This effect may arise from a greater availability of resources under the shrubs, such as nutrients and water (Pugnaire et al. \u003cspan citationid=\"CR45\" class=\"CitationRef\"\u003e1996\u003c/span\u003e; Zhu et al. \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). The weakening of facilitation along the fertility gradient appeared to be a consequence of improved abiotic conditions, as predicted by current theory (Bertness and Callaway, \u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e1994\u003c/span\u003e; R. Brooker and Callaghan, \u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e1998\u003c/span\u003e). The results of measuring competition and facilitation showed that the positive and negative effects of plant interactions along the gradient of habitat fertility in different ways seems to be more a consequence of a decline in facilitation than a change in competition (Pugnaire and Luque, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e2001\u003c/span\u003e). Invasive shrubs can stimulate nutrient cycling and promote vegetation restoration by capturing resources under their canopies, thus reversing desertification in degraded systems (Zhao et al. \u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e2007\u003c/span\u003e). During the past few years, the ecological consequences of shrub encroachment have been controversial. Shrub-encroachment are considered could decrease the richness, abundance and aboveground biomass of herbs, that has a negative impact on the structure and diversity of herbs plant communities (Zhang et al. \u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e2022\u003c/span\u003e; Zhou et al. \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e2019\u003c/span\u003e). On the contrary, shrub in our study has a positive impact on the composition and structure of herbs communities in desert steppe. In desert steppe, infertile soil nutrients and water resources deficiency were the limiting condition of plant growth. Shrubs alleviate resource competition among plants to a certain extent by improving the microenvironment, which is beneficial to the growth of surrounding herbs (Li et al. \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e2021\u003c/span\u003e; Zhang et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). In addition, we found with drought intensified, the positive interaction of shrubs on herbs increasing first and then decreasing, displaying a unimodal trend. The results are consistent with the recent refined SGH, that is, near the extreme position of stress gradient, the positive interaction will be weakened or transformed into negative interaction. The possible reasons for the deviation from SGH prediction are: (1) the competition between shrubs and herbaceous increases with the increase of aridity index, which offsets its promoting effect (Tielborger and Kadmon, \u003cspan citationid=\"CR54\" class=\"CitationRef\"\u003e2000\u003c/span\u003e). (2) With the increase of aridity index, herbaceous plant species were replaced by more stress-tolerant species, and these species were less likely to be promoted by shrubs (Le Bagousse-Pinguet et al. \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e2012\u003c/span\u003e; Liancourt et al. \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). Zhang et al. found that with the aggravation of aridity stress, the shrub-herb interaction in the Badain Jaran Desert of China changed from positive to neutral (Zhang et al. \u003cspan citationid=\"CR59\" class=\"CitationRef\"\u003e2018\u003c/span\u003e). O\u0026rsquo;Brien\u0026rsquo;s research showed that the positive interaction of shrubs over species richness and community productivity during the wetter growing season, but shifted to competitive dominance interaction in the drier season (O\u0026rsquo;Brien et al. \u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). The results of this study are basically consistent with the above research results. However, the study of Dohn et al (2013) showed that the positive shrub-herb interaction did not change significantly along the aridity gradient, which may be due to the relatively limited data on soil nitrogen concentration. In our study, considering the facilitation of shrubs on herbaceous plants, under certain drought conditions, shrub-encroachment has a restoration effect on desert steppe.\u003c/p\u003e \u003cp\u003eEffects of environmental factors on shrub-herb interaction\u003c/p\u003e \u003cp\u003eWe show that AI, SOM and TP were the main factors driving the change of shrub-herb interaction to affect the community structure in our study area. Several researches indicated that climatic factors and soil properties such as precipitation and soil phosphorus are important factors influencing community structure (Liu et al. \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e2023\u003c/span\u003e; Xie et al. \u003cspan citationid=\"CR57\" class=\"CitationRef\"\u003e2023\u003c/span\u003e). The SEM results showed directly effect of AI and SOM on shrub-herb interactions, and SOM determines the effect of shrubs on herbs diversity (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e). A possible explanation for this might be that the increase of competition between shrubs and herbs (Lal \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e2020\u003c/span\u003e). Soil organic matter have the greatest effect on shrub coverage among all soil factors (Fu et al. \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e2004\u003c/span\u003e), this may also be the reason for affecting the effect of shrubs on herbs diversity. During the dry period when the vegetation degradation and plant growth negatively affected, SOM is the only soil properties in relation with diversity index (D\u0026ouml;larslan et al. \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e2017\u003c/span\u003e). In recent years, scholars have carried out a large number of simulation studies on the internal relationship between drought and shrub-herb interaction and the impact of the former on the latter. In line with our results, precipitation affects community species composition and diversity by controlling shrub-herb interaction (He et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Similarly, precipitation intensity pushes soil water deeper and increases the growth of shrubs, which is not conducive to the growth of herbaceous on the grassland (Kulmatiski and Beard, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). Therefore, the shrub-herb interaction can be affected by changing the exogenous environmental conditions, so as to promote the vegetation restoration of degraded desert steppe. For example, artificial precipitation enhancement and nutrient addition can indirectly improve the stability of grassland ecosystem by increasing facilitation by shrub on herb (He et al. \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e2022\u003c/span\u003e). Wind speed is often neglected in the study of environmental factors driving shrub-herb interaction. In our study, the increase of wind speed is beneficial to facilitation of shrubs on herbaceous plants. Zhang et al. showed that shrubs can effectively reduce wind erosion and improve microenvironment (Zhang et al. 2021). In addition, Zhi et al. found that shrubs had a significant reduction effect on wind speed, and its effect was related to shrub height and vegetation coverage (Zhi et al. 2024). This indirectly supports the conclusion that shrubs enhance the facilitation of shrubs on herbaceous plants during the increase of wind erosion, and also reminds us that the influence of shrub individual characteristics on shrub-herbs interaction is nonnegligible.\u003c/p\u003e \u003cp\u003eEffect of soil phosphorus on shrub-herb interaction\u003c/p\u003e \u003cp\u003eOur analysis found AI could indirectly affect shrub-herb interaction through TP (Fig.\u0026nbsp;\u003cspan refid=\"Fig7\" class=\"InternalRef\"\u003e7\u003c/span\u003e), which is consistent with previous studies (Jiao et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). Increasing precipitation indirectly enhanced the facilitation of shrubs on herbs by increasing soil phosphorus(P) content, soil phosphorus decreased with the increase of drought, showing a negative correlation (Jiao et al. \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e2016\u003c/span\u003e). This can be attributed to that transformation of soil P takes longer in arid areas with low leaching intensity than in humid areas, resulting in lower soil P content in arid areas (Walker and Syers, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e1976\u003c/span\u003e). However, studies by Delgado-Baquerizo \u003cem\u003eet al.\u003c/em\u003e showed that total phosphorus was not affected by drought degree (Delgado-Baquerizo et al. \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e2013\u003c/span\u003e). This difference may be due to different drought gradient scales, and in the study of Baquerizo et al, the soil total phosphorus content did not change significantly on a larger geographical scale. Therefore, the response of soil nutrients, especially phosphorus content, to drought degree on a global scale may not be applied to the regional scale. Sardans et al ' s meta-analysis showed that changes in soil phosphorus content caused by environmental factors may drive the structure of ecosystems by affecting plant-plant interaction, supporting the results of this study (Sardans et al. \u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e2012\u003c/span\u003e). We found that soil P determines the effect of shrubs on herb productivity (Fig.\u0026nbsp;\u003cspan refid=\"Fig6\" class=\"InternalRef\"\u003e6\u003c/span\u003e). Previous studies have indicated that plant biomass were positively correlated with soil P in grasslands (Fayiah et al. \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e2019\u003c/span\u003e), our findings are in accord with that. It is expected that some parts of the world will become drier in the next few decades, and the results of our study are helpful to predict the vegetation change in these areas.\u003c/p\u003e"},{"header":"Conclusions","content":"\u003cp\u003eOverall, our study found shrub \u003cem\u003eCaragana Microphylla\u003c/em\u003e had a positive impact on herb community, which is beneficial to the positive succession of shrub-encroachment desert steppe, shrubs may play an important role in desert steppe ecological restoration. The change of the facilitation along the aridity gradient showed a unimodal trend, which was consistent with the refined SGH. Aridity index, soil organic matter and soil total phosphorus were the main environmental factors driving changes of shrub-herb interaction. We can use the method of controlling exogenous water conditions (artificial rainfall, etc.), soil organic matter content (regular cleaning of plant residues, etc.) and soil phosphorus content (application of phosphorus fertilizer, etc.) to enhance the facilitation of shrub on herbs, which is beneficial to the ecological restoration of degraded desert steppe. Soil organic matter determines the effect of shrubs on herbs diversity, and soil phosphorus determines the effect of shrubs on herbs productivity. Aridity index indirectly affects the shrubs-herbs interaction through soil phosphorus. The increase of precipitation indirectly enhanced the facilitation of shrub on herb by increasing TP content, which was conducive to the restoration of degraded desert steppe.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eThe authors declare no competing financial interests.\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eSG and JZ designed the study. HH conducted the experiments, performed statistical analyses, and wrote the first manuscript. All authors contributed critically to the drafts and gave final approval for publication.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis research was funded by the Inner Mongolia Autonomous Region Natural Science Foundation (2020BS03046, 2023QN03007), and the Special Fund of Chinese Central Government for Basic Scientific Research Operations in Commonweal Research Institutes (1610332020015), and the Yinshanbeilu Grassland Eco-hydrology National Observation and Research Station, China Institute of Water Resources and Hydropower Research, Beijing 100038, China, Grant (YSS2022013), and the Basic scientific research business expenses of CAAS (1610332022012).\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe datasets generated during and/or analysed during the current study are available from the corresponding author on reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eArmas C, Ordiales R, Pugnaire FI (2004) Measuring plant interactions: a new comparative index. 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Funct Ecol 36(9):2356\u0026ndash;2366. \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1111/1365-2435.14100\u003c/span\u003e\u003cspan address=\"10.1111/1365-2435.14100\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Aridity, Caragana Microphylla, Shrub-herb interaction, Facilitation, SGH, soil properties","lastPublishedDoi":"10.21203/rs.3.rs-4491863/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4491863/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003ch2\u003ePurpose\u003c/h2\u003e \u003cp\u003eEnvironmental conditions can control the structure and composition of plant communities by changing the direction and intensity of plant-plant interactions. However, how drought and soil properties drive the change of shrub-herb interaction in the shrub-encroachment desert steppe in Inner Mongolia remains unclear.\u003c/p\u003e\u003ch2\u003eMethods\u003c/h2\u003e \u003cp\u003eWe explored the changes of shrub-herb interaction along the aridity gradient, and analyzed how the aridity index and soil properties affect the shrub-herb interaction. Field collection of plant samples and soils from northeast to southwest desert steppe in Inner Mongolia was undertaken and the samples were analyzed for biomass, diversity, soil PH, soil organic matter and other elements.\u003c/p\u003e\u003ch2\u003eResults\u003c/h2\u003e \u003cp\u003eThe results show that the positive shrub-herb interaction (RII\u0026thinsp;\u0026gt;\u0026thinsp;0) increased at first and then decreased in the range of aridity index 0.54 to 1.85 (drought degree decreased gradually). Aridity index (AI), soil organic matter (SOM) and soil total phosphorus (TP) were the main factors driving shrub-herb interaction. AI indirectly affected shrub-herb interaction through TP, and the facilitation of shrubs on herbs coverage and biomass increased with the increase of TP. The SOM can directly affect the shrub-herbs interaction, and the facilitation of shrubs on herb diversity decreases with the increase of SOM.\u003c/p\u003e\u003ch2\u003eConclusion\u003c/h2\u003e \u003cp\u003eOverall, although shrub-herb interactions respond differently to environmental factors. This study underscores the positive effects of shrubs on vegetation restoration in desert steppe, and changing environmental conditions by increasing precipitation, increasing TP content, and reducing SOM content can enhance facilitation of shrub on herbs to accelerate the ecological restoration of degraded desert steppe.\u003c/p\u003e","manuscriptTitle":"Aridity and soil properties drive the shrub-herb interactions along drought gradient in desert grassland in Inner Mongolia","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-19 04:35:45","doi":"10.21203/rs.3.rs-4491863/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"ad9d759e-af48-40d5-9ddc-7588fba094d7","owner":[],"postedDate":"June 19th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2024-07-03T00:41:48+00:00","versionOfRecord":[],"versionCreatedAt":"2024-06-19 04:35:45","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-4491863","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4491863","identity":"rs-4491863","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}