Flashy, decoupled, or declining? Single theories each fail to explain the diversity of drought mortality signals in tree rings

Flashy, decoupled, or declining? Single theories each fail to explain the diversity of drought mortality signals in tree rings | Authorea try { document.documentElement.classList.add('js'); } catch (e) { } var _gaq = _gaq || []; _gaq.push(['_setAccount', 'G-8VDV14Y67G']); _gaq.push(['_trackPageview']); (function() { var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true; ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js'; var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s); })(); Skip to main content Preprints Collections Wiley Open Research IET Open Research Ecological Society of Japan All Collections About About Authorea FAQs Contact Us Quick Search anywhere Search for preprint articles, keywords, etc. Search Search ADVANCED SEARCH SCROLL This is a preprint and has not been peer reviewed. Data may be preliminary. 21 February 2025 V1 Latest version Share on Flashy, decoupled, or declining? Single theories each fail to explain the diversity of drought mortality signals in tree rings Authors : Alicia Formanack 0000-0002-2109-6967 [email protected] , Kiona Ogle 0000-0002-0652-8397 , and Drew Peltier Authors Info & Affiliations https://doi.org/10.22541/au.174014719.99192566/v1 272 views 150 downloads Contents Abstract ABSTRACT 1 | Introduction 4. Discussion Supplementary Material Information & Authors Metrics & Citations View Options References Figures Tables Media Share Abstract Increasing drought frequency and severity are driving global forest dieback. Growth recorded in tree rings may predict drought ‘winners’ and ‘losers,’ but past studies of growth in drought-killed trees have produced conflicting support for different theories about drought mortality. We found that clusters of growth behaviors computed from the rings of 2,934 drought-killed and drought-surviving trees from seven species were not consistent with any single theory. Drought-killed subalpine fir and Engelmann spruce trees exhibited “flashy” growthhighly variable climate-growth responses over timecompared to survivors. Drought-killed Scots pine and Norway ospruce trees showed stable, climate-insensitive growth compared to survivors, suggesting “decoupling” from climate. Finally, in red oak and subalpine fir, disturbances like fire, logging, and biotic agents possibly influenced declines in climate sensitivity in both drought-killed and surviving trees. Our consolidated conceptual framework may be useful for predicting future tree mortality, while providing enhanced ecological and physiological understanding. Flashy, decoupled, or declining? Single theories each fail to explain the diversity of drought mortality signals in tree rings Alicia Formanack 1 , Kiona Ogle 1 , and Drew Peltier 2 1 School of Informatics, Computing, and Cyber Systems, Northern Arizona University, Flagstaff AZ 2 School of Life Sciences, University of Nevada, Las Vegas, NV Abbreviated title: Drought mortality signals in tree rings Keywords: climate decoupling, climatic sensitivity, drought, flashy growth, forest decline, tree growth, tree mortality, tree rings Article type: Letter Abstract word count: 146/150 Main text count: 4742/5000 Words in each text box: 0 Number of references: 77 Number of figures, tables, and text boxes: 5 Corresponding author: Alicia Formanack, 1295 Knoles Dr, Flagstaff, AZ 86011, ph: (928) 523-9011, email: [email protected] Author contributions: A.F. and K.O. designed the study. A.F. analyzed data and generated the tables, figures, and maps with input from D.P. and K.O. A.F. wrote the first draft of the manuscript. All authors contributed substantially to revisions. Acknowledgements: This work was supported by the National Science Foundation under awards #2213599 and #1829075. We thank Rohan Boone for the initial processing of data. Data availability statement: The final dataset and code will be permanently archived and publicly available on GitHub (https://github.com/amf258/Formanacketal_2025) via Zenodo upon acceptance. ABSTRACT Increasing drought frequency and severity are driving global forest dieback. Growth recorded in tree rings may predict drought ‘winners’ and ‘losers,’ but past studies of growth in drought-killed trees have produced conflicting support for different theories about drought mortality. We found that clusters of growth behaviors computed from the rings of 2,934 drought-killed and drought-surviving trees from seven species were not consistent with any single theory. Drought-killed subalpine fir and Engelmann spruce trees exhibited “flashy” growthhighly variable climate-growth responses over timecompared to survivors. Drought-killed Scots pine and Norway ospruce trees showed stable, climate-insensitive growth compared to survivors, suggesting “decoupling” from climate. Finally, in red oak and subalpine fir, disturbances like fire, logging, and biotic agents possibly influenced declines in climate sensitivity in both drought-killed and surviving trees. Our consolidated conceptual framework may be useful for predicting future tree mortality, while providing enhanced ecological and physiological understanding. 1 | Introduction Forests play a dominant role in the terrestrial carbon cycle and are increasingly threatened by large-scale mortality events (Allen et al. 2015; Anderegg et al. 2020; Mantgem et al. 2009). With rising temperatures and intensification of the water cycle (Swain et al. 2025), droughts are affecting tree growth and mortality across all biomes (Allen et al. 2010; Hammond et al. 2022). In response to drought, tree species can exhibit various survival strategies that interact with climate variability, forest dynamics, and physiological traits (Baldi & La Porta 2022). Identifying intra- and inter-specific patterns in tree growth responses to drought is essential to understanding risks to future carbon storage (Chen et al. 2012; DeSoto et al. 2020; Gazol et al. 2018; Zeng et al. 2023). Moreover, understanding how tree growth responds to climate over time could point to intraspecific growth strategies that create drought ‘winners’ and ‘losers’ (Gazol et al. 2023). Tree rings provide temporally rich, annually resolved data on the relationship between tree growth and climate (Martinelli 2004; Schöngart et al. 2006; Tipton et al. 2016), making them ideal for exploring early warning signs of drought-induced mortality. Numerous studies of tree drought responses have been conducted (Bigler et al. 2004; Haavik et al. 2011; Ireland et al. 2014; Mäkinen et al. 2001), yet outside of model species, our ability to predict drought-related mortality remains poor (McDowell et al. 2016; Meir et al. 2015). Furthermore, growth responses to climate (i.e., climate sensitivity) are influenced by many factors including physiological traits, microsite conditions, competition, and genetic variation (Choat et al. 2018; Gessler et al. 2017; Trugman et al. 2021). The study of drought-killed trees can therefore be useful to identify drought response diversity and to understand regionally specific patterns in climate sensitivity (Cailleret et al. 2019; DeSoto et al. 2020). Drought responses may then represent trade-offs in tree growth strategies, ultimately influencing survival and mortality (Willi & Van Buskirk 2022). We identified three competing concepts of how drought-induced mortality may be reflected in tree-ring timeseries. For some species, highly variable climate-growth responses over time can be indicative of higher potential for mortality (Cailleret et al. 2017, 2019; Ogle et al. 2000; Smith et al. 2019). This “flashy” growth response to climate may be a drought strategy focused on recovery instead of avoidance but could also reflect more “acquisitive” traits (Maracahipes et al. 2018; Umaña et al. 2023; Wright et al. 2010) that entail greater risk during drought events. In contrast, both low climate sensitivity and low growth can indicate higher probability of future mortality (Bigler et al. 2006; DeSoto et al. 2020; Gillner et al. 2013). Often, these trees have ring widths that appear “decoupled” from climate, showing low growth with very little variation. Several other studies describe tree and forest mortality as “declining”, where reductions in growth or climate sensitivity, associated with an inciting event in the past (e.g., a severe drought), lead to eventual mortality (Amoroso et al. 2017; Minorsky 2003; Voelker et al. 2008). In this scenario, declining trees never recover pre-drought levels of growth or climate sensitivity and slowly die (Bigler et al. 2007; Cailleret et al. 2017; DeSoto et al. 2020). It remains unclear if any of these patterns are universal, as past studies employed different approaches, or if tree growth in dying trees proceeds differently in different species and regions of the world. We evaluated growth-climate relationships of drought-surviving trees and drought-killed trees using tree-ring data from seven species (five gymnosperms, two angiosperms) in the United States and western Europe. We sought to understand how climate-growth responses (climate “sensitivities”) differed between drought-surviving and drought-killed trees of the same species. We developed a Bayesian hierarchical model that estimated the effects of climate on ring width, including lagged climate effects (”memory”: Ogle et al. 2015). We computed indices of ring-width sensitivity to different seasonal climate variables and performed clustering of those sensitivity indices, allowing us to discover and evaluate three hypotheses related to the growth patterns of drought-killed trees: (H1) flashy growth, (H2) decoupling, and (H3) declining. We hypothesized that (H1) dying trees are more sensitive to climate, with higher variation in growth than surviving trees of the same species, reflecting a “flashy”, more acquisitive growth strategy. Alternatively, (H2) dying trees are less sensitive to seasonal climate (“decoupled”), reflecting a more conservative growth strategy. As a hypothesis, (H3) dying trees were declining (i.e., gradual weakening of climate-growth sensitivities over time), demonstrating their inability to recover from past damage incurred by stress. Our results highlight the complexity and diversity of tree responses to climate stress within and among species, and that these responses likely depend upon site history and species traits (e.g., acquisitive versus conservative) that influence physiology and lead to differing drought responses. 2 | Methods 2.1 | Study area and species We used a subset of records from a previously compiled dataset comprising 58 published and unpublished studies of tree-ring widths and mortality (Cailleret et al. 2017). At each site, live and dead trees of the same species were identified and cored; tree cores were cross-dated and annual ring widths measured using standard dendrochronology methods (Stokes 1996). From this dataset, we extracted 133 sites (23 species) where “drought” was reported as the predominant cause of tree mortality. We further subset the data, selecting species with at least 100 trees (living and dead trees combined), producing a dataset with 93 sites consisting of 12 different species. We incorporated data for another species ( Populus tremuloides , quaking aspen) representing 11 sites in the western US (Ireland et al. 2014). Some species were excluded following preliminary analyses due to poor model mixing and convergence issues, leaving seven species. These seven tree species ( Table 1 ) are widely studied, commercially and culturally important, and have distributions spanning North America and Europe ( Fig. 1 ). They represent both conifers and broadleaf species: Pinus sylvestris (Scots pine), Abies alba (silver fir), Quercus rubra (red oak), Picea engelmannii (Engelmann spruce), Picea abies (Norway spruce), Populus tremuloides (quaking aspen), and Abies lasiocarpa (subalpine fir). 2.2 | Climate data Monthly climate data (mean temperature and total precipitation) were obtained from the Climatic Research Unit gridded dataset (CRU, 0.5° resolution) for the 1901-2017 period (Harris et al. 2020). We anticipated that (1) growth sensitivities to climate (e.g., temperature and precipitation) depend on the seasonality of climate, where effects differ between cold (e.g., winter) and warm (e.g., summer) seasons, and (2) changes (trends) in such sensitivities might also depend on season. Thus, we aggregated climate data into “seasons” (i.e., winter and summer) and standardized the mean of the average monthly temperature (e.g., from 1905-2016) by site. Summer was defined as months with higher-than-average monthly temperatures (e.g., April 2000 to September 2000). The winter season follows a hydrological year definition and included months with lower-than-average monthly temperatures (e.g., October 1999 to March 2000). 2.3 | Model description and implementation To explore variation in climate sensitivity between drought-killed trees and drought-surviving trees, we implemented a Bayesian hierarchical regression model of tree growth (i.e., ring width) as a function of seasonal climate (e.g., winter and summer precipitation and temperature). Since studies have revealed effects of climate can last up to four years after an extreme climate event (Anderegg et al. 2015), we implemented a variant of the stochastic antecedent modeling (SAM) framework (Ogle et al. 2015; Peltier et al. 2018) to estimate the impact of current and past seasonal climate—up to four years prior to ring formation. Below we describe the model structure, which we implemented separately for drought-killed and drought-surviving trees, for each species. Distinct from previous SAM models applied to tree-ring data (e.g., Peltier et al., 2018), we allowed the effects of temperature and precipitation to vary by season, thus allowing for the possibility of season-specific influences of a given covariate (e.g., temperature). We define the likelihood of log-transformed ring width, RW = log( r + 1), where r is the original ring width (mm), and we add 1 to r since some cores had missing rings (i.e., r = 0). For each year y and core c , we assumed that RW y,c was normally distributed, with mean\(\mu\) r,c and variance \(\sigma^{2}\): The mean log-scale ring width, \(\mu_{y,c}\), is regressed on tree age ( Age , standardized) at the time of ring formation, the previous year’s growth index ( RW y-1 , c , standardized), and standardized antecedent seasonal precipitation ( P ant ), temperature ( T ant ), and their interactions ( P ant × T ant ). For simplicity, let X 1 , X 2 , X 3 , and X 4 denote winter P ant , summer P ant , winter T ant , and summer T ant , respectively. The mean model is as follows: The intercept (α 1 ), age effect (α 2 ), and autoregressive effect (α 3 ) all vary at the level of core, c . The main effects ( β 1 , …, β 4 ) of antecedent seasonal climate ( X j terms) all vary at the level of site, where s ( c ) denotes site s associated with core c . Similarly, the six different interaction effects (γ 1,2 , …, γ 3,4 ) associated with interactions between climate variables X j and X k also vary at the level of site, s . The antecedent climate variables, P ant and T ant (or, more generally, X 1 , X 2 , X 3 , and X 4 ), are calculated as the weighted averages of the yearly seasonal total precipitation or average temperature over the past four years up to and including the end of the concurrent growing season. For antecedent climate variable X j , year y , and site s : Note that x j,y,s denotes the original, yearly, standardized seasonal climate variable . The importance weights w j,t are unique to each climate variable for each species, but assumed invariant across sites within a species; the weights were estimated for each year t into the past up to four years ago relative to year y , for each variable X j . Weights were constrained to sum to 1 across all lags ( t = 0 [concurrent year], 1, 2, 3, 4) for a given variable and season, so they are interpreted as the relative importance of climate during a given year and season for each climate variable. Prior specifications for all stochastic parameters and associated model code are reported in Methods S1. The model described by Equations - was fit in JAGS 4.0.0 (Plummer 2003) via the R (R Core Team 2022) package rjags (Plummer 2008) and jagsUI (Kellner 2015) following standard methods. Posterior summary statistics were calculated, including the posterior mean, standard deviation, and 95% central credible interval (CI). Parameters were considered significantly different between status categories if the CI for one status category (i.e., surviving trees) did not contain the mean of the other category (i.e., drought-killed trees). Climate sensitivities were considered statistically significant when the CI associated with the relevant coefficient (i.e., β or γ terms) did not include zero. 2.4 | Realized climate sensitivities Given Equation includes two-way interactions among climate variables , we simplified model outputs using partial first derivatives of the expected (mean) log-scale ring width with respect to each climate variable. This calculated quantity (a “realized climate sensitivity”) summarizes the sensitivity of ring widths to seasonal climate by accounting for the main and interactive effects of different antecedent climate variables. We computed these realized climate sensitivities (RS) at the species- and status-level across years and sites. For example, the time-varying RS of log-scale ring width to antecedent winter precipitation, X 1 ,j in Equation , is computed as: The “^” notation for log-scale ring width, and for the climate variables, indicates site-level averages of these quantities for a given species and status group. Similarly, theand terms denote the global-level climate effects that are representative of the focal species-status group. The RS for a focal climate variable, varies annually, conditional on other climatic conditions occurring during each year, as governed by the interaction effects, e.g., and in Equation . Then, RS j,y , are used to assess mean climate sensitivity, variability in (standard deviation) climate sensitivity, and climate sensitivity trends (i.e., changes in sensitivity over time). We computed the temporally varying RS indices for each climate variable and species using 15,000 posterior samples (5,000 samples per chain; 3 chains) of the parameters (and terms, see Fig. S4-10) . We summarized the RS indices three different ways: we calculated the overall mean (across all years) and 95% credible interval (CI) for each climate variable’s sensitivity index (i.e., for each RS j , j = 1, 2, 3, 4 variables). We also summarized the temporal variation in each RS j (for each j ) using standard deviations (sd), and trends (slope from a linear regression) of the RS j indices across years, and computed posterior means and 95% CI of these 15,000 standard deviations and trends. 2.5 | Hierarchical clustering of realized climate sensitivities To describe differences in responses among species and status groups, we performed hierarchical clustering of the mean, standard deviation, and trend of RS for all climate variables across all species-status combinations. After standardizing the RS values, we calculated the distance matrix and preformed complete hierarchical clustering in R using the hclust function. This analysis produced a dendrogram for which we could evaluate grouping (clusters) of species and status groups. 3. Results 3.1 | Model performance The coefficient of determination (R 2 ) from regressions of observed versus predicted log-scale ring widths (RW) spanned 0.70-0.94 (Fig. S1, S2), with the highest R 2 occurring for surviving Engelmann spruce (R 2 = 0.94) and the lowest for surviving aspen (R 2 = 0.70). For most species, the R 2 of drought-killed trees was higher than surviving trees, except Engelmann spruce and subalpine fir. 3.2 | Summary of seasonal climate effects Across species and status groups, about 80% of the climate main effects were significant (i.e., 95% CI did not contain zero), and nearly 70% of the climate two-way interactions were significant. Some of the main effects differed between drought-killed and drought-surviving trees within a given species (details below). For gymnosperms, ring width sensitivity to summer precipitation differed significantly between surviving and dying trees of the same species (Figs. S3 and S4). Surviving Scots pine, subalpine fir, and Norway spruce grew more in response to increases in summer precipitation than their drought-killed counterparts. Conversely, drought-killed silver fir and Engelmann spruce trees grew more in response to summer precipitation relative to surviving trees. Surviving Engelmann spruce trees were the only group responding negatively to summer precipitation. Summer temperature was a significant driver of ring width across all species (Fig. S3 and S4). Silver fir and red oak growth was enhanced by warmer summer temperatures, whereas Scots pine and Norway spruce responded negatively to warmer summer temperatures. Summer temperature effects did not differ between surviving and dying trees for all four of these species. Conversely, the effect of winter temperature differed significantly between surviving and dying angiosperms; surviving aspen and red oak trees produced narrower rings with increasing winter temperatures. In contrast, drought-killed red oak did not respond to winter temperature, while dying aspen were less negatively affected than surviving aspen. Given the difficulty in interpreting the effects of individual climate drivers in the presence of interacting climate effects, we focus on the realized sensitivities next (RS, see equation 4, Fig. S5-S11). 3.4 | Single growth strategies fail to capture diverse climate mortality signals Realized sensitivity (RS) often differed between drought-killed and drought-surviving trees (Fig. S5-S11), but differences were not consistent across species; we highlight three examples. Drought-killed subalpine fir exhibited highly variable sensitivity to winter temperatures, illustrating “flashy” growth responses compared to drought-surviving trees (Fig. 2a). Drought-killed Scots pine showed reduced sensitivity to summer precipitation and overall lower variation compared to drought-surviving trees (Fig. 2b), providing an example of dying trees that are relatively “decoupled” from climate variation. Drought-killed silver-fir showed “declining” sensitivity to summer temperature, but so did surviving silver-fir (Fig. 2c). In short, we found evidence for each of the three hypotheses, with different species’ responses to seasonal climate aligning with different hypotheses. Summarizing patterns in growth responses across RS indices, climate variables, species, and status groups, hierarchical clustering identified four relatively distinct groups (Fig. 3). Critically, all four groups contained dying trees of one species and surviving species of another, indicating differences were not consistent across species or across status groups. The clustering algorithm placed drought-killed and drought-surviving trees in different groups for five species (Engelmann spruce, Norway spruce, Scots pine, subalpine fir, and aspen), indicating dying and surviving trees have different growth-climate sensitivities in these species (Fig. 3). Surviving and dying dead red oak and silver fir trees were placed in the same group (4), suggesting that drought-killed and drought-surviving trees exhibited similar growth-climate characteristics for these two species (Fig. 3). The arrangement of the dendrogram along with the height of the branches reveals how similar or different species-status RS indices are relative to each other, with greater branch height indicating greater differences. 3.5 | Evidence for flashy growth (H1) We categorized climate-growth sensitivity as flashy when drought-killed (or drought-survived) trees had a higher RS mean and/or standard deviation when compared to drought-surviving (or drought-killed) trees of the same species. Surviving Norway spruce and Scots pine, and dying Engelmann spruce (Fig. 3, group 1), had a high RS mean for summer precipitation; that is, increases in summer precipitation led to comparatively larger increases in ring width (Fig. 4a) across all three species in this group. Dying Engelmann spruce trees were almost twice as sensitive to summer precipitation (Fig. 4a) relative to surviving trees (Fig. 4a), consistent with H1. The dendrogram branching structure in group 1 (Fig. 3) suggests a subgroup consisting of surviving Norway spruce and Scots pine, which share high, positive sensitivity to winter temperature and high variability (standard deviation) in the effects of winter temperature over time (Figs. 4a, 4b). Group 3 (Fig. 3) is the smallest group, consisting of dying subalpine fir and surviving aspen, which share high variability in the (negative) effects of winter temperature (Figs. 5a, 5b). The RS mean for winter temperature for dying subalpine fir is 1.3 times higher than for surviving subalpine fir (Fig. 5a), consistent with H1. 3.6 | Evidence for decoupling (H2) Ring widths in drought-surviving Engelmann spruce and drought-killed aspen, Norway spruce, and Scots pine (Fig. 3, group 2) are relatively insensitive to (“decoupled” from) summer precipitation (Figs. 4a, 4b). Their growth is only one-fifth as sensitive to summer precipitation as their surviving counterparts, supporting H2. The length of the dendrogram terminal branches within group 2 (Fig. 3) suggests that dying Norway spruce and Scots pine have very similar growth characteristics, forming a subgroup of two. In addition to decoupling from summer precipitation, dying Norway spruce and Scots pine show little to no (positive) sensitivity to winter temperature, with very low variability (standard deviation) in the effects of winter temperature over time (Figs. 4a, 4b). Both drought-killed and drought-surviving silver fir, a subset of group 4 (Fig. 3), appear decoupled from winter precipitation (Fig. 5a). 3.7 | Evidence for decline (H3) Dendrogram branching patterns within group 4 (Fig. 3) suggest that surviving and dying trees share similar growth-climate relationships within their respective species groups, silver fir and red oak. However, the red oak growth patterns are distinct from those of silver fir. In this group, surviving subalpine fir is more similar to silver fir (both surviving and dying trees) than red oak (surviving and dying). Surviving and dying red oak exhibit similar negative sensitivity to winter precipitation and positive sensitivity to summer precipitation (Fig. 5a). However, negative trends in summer temperature sensitivity and positive trends in winter precipitation sensitivity suggest declining sensitivities (Fig. 5c). Surviving and dying silver fir and surviving subalpine fir show declining sensitivity to summer temperature (Fig. 5c), as positive effects diminished over time. Surviving Scots pine, surviving Norway spruce, and dying Engelmann spruce (Fig. 3, group 1) demonstrate declining sensitivity to summer temperature (Fig. 4c), indicating negative effects are weakening over time. Interestingly, drought-killed subalpine fir show declining climate sensitivity to most climate variables, except for winter precipitation, which shows increasingly negative trend in sensitivity, thus sensitivity to winter precipitation has been increasing over time (Figs. 5a, 5c). 4. Discussion Our study explored three hypotheses related to mechanisms underlying growth-climate responses of drought-killed trees relative to those that survive. No single framework could describe the growth-climate patterns of drought-killed trees, as dying trees exhibited varied growth patterns across species. Drought-killed subalpine fir and Engelmann spruce exhibited “flashy” growth in response to seasonal climate compared to conspecific trees that survived, supporting H1. Drought-killed Scots pine and Norway spruce showed more stable, less climate-sensitive growth than survivors, reflecting “decoupling” from climate, supporting H2. Silver fir and red oak had similar responses to climate across drought-surviving and drought-killed trees. Evidence of declining temperature and precipitation sensitivity was detected for surviving and dying silver fir and red oak, supporting H3. Drought-killed trees of many species also showed evidence of decline in climate sensitivities for at least one seasonal climate variable. Interestingly, drought-killed trees of some species had positive trends in climate sensitivity, the opposite of H3, indicating that these trees became more tightly coupled to climate over time. These differences can be explained by species traits, forest types, and region, where, for example, the time and severity of droughts have been very different between the US and Europe. Integrating these multiple frameworks together provides an opportunity to understand the varied drivers of drought mortality. 4.1 | Flashy growth (H1) is a risky, resource-acquisitive strategy In this study, Engelmann spruce and subalpine fir trees were generally sampled near the southern edge of their ranges, where the risks and intensity of drought are likely the strongest (Anderegg et al. 2019; Sánchez‐Salguero et al. 2017). This highly sensitive and variable (flashy) growth strategy might be beneficial if trees are able to quickly recover lost sapwood, leaf area, or fine roots after experiencing drought (Hesse et al. 2023; Tomasella et al. 2017). However, frequent and prolonged droughts may increase the risk of hydraulic failure (Li et al. 2020). Growth-climate responses that are highly flashy could lead to structural overshoot (Zhang et al. 2021), potentially making trees more vulnerable to drought impacts such as mortality. An increase in biomass under favorable conditions due to flashy growth responses may be challenging to sustain during drought conditions, potentially resulting in cavitation or shedding of branches and needles, which causes significant interannual variability in growth (Nadal-Sala et al. 2021). Surviving subalpine fir and Engelmann spruce trees displayed a more conservative strategy; these trees likely prioritized more energy-intensive carbon tasks, such as repair and defense, over rapid growth. While this conservative strategy may result in lower growth, it can also reduce the risk of insect or pathogen attack, particularly since drought conditions often coincide with biotic pressures (Brienen et al. 2020; Kolb et al. 2019; Ogle et al. 2000). 4.2 | Decoupling (H2) suggests persistent hydraulic failure and carbon starvation Unlike Engelmann spruce and subalpine fir, drought-killed Norway spruce and Scots pine trees exhibited a lower sensitivity to climate, following H2, suggesting a struggle to meet needs for even minimal growth (McDowell et al. 2022). This reduced growth and lower climate sensitivity could result from a chronic imbalance in their carbon status (e.g., associated with leaf dieback) and impaired hydraulic conductance, related to reduction in sapwood area over time (Cailleret et al. 2017; Dickman et al. 2015; Pellizzari et al. 2016; Trugman et al. 2018). Furthermore, these dying trees were less able to utilize summer precipitation than surviving trees (see Figs. 4 and 5), again perhaps reflecting hydraulic limitations associated with reduced leaf, sapwood, or root area, greater sensitivity of stomatal conductance to atmospheric demand, or differences in rooting depths (Jyske et al. 2010; Magnani et al. 2002; Martinez Del Castillo et al. 2024). Dying Norway spruce trees in this study were found on rockier and lower-quality sites compared to their surviving counterparts, which may have limited their rooting area and exposed their roots to drier conditions more frequently (Mäkinen et al. 2001). Additionally, compared to other species in this study, Norway spruce trees were sampled from a more central part of their geographic range (Aakala et al. 2011; Mäkinen et al. 2001), where drought events are typically less severe or less frequent. As a result, trees that occur in better microsite conditions and have higher climate sensitivity may recover from drought more effectively. The flashy responses to climate may be less risky and more rewarding for these surviving Norway spruce and Scots pine, as their locations had mean annual precipitation (MAP) that was 1.5 to 2.5 times higher than that of the sites sampled for Engelmann spruce and subalpine fir. 4.3 | Declining sensitivity (H3) is not consistently related to drought mortality While “decline” in vigor or climate sensitivity is an attractive theory for tree mortality, we found that both drought-surviving and drought-killed trees showed both decreasing (“declining”) and increasing (“enhanced”) trends in climate sensitivities for multiple seasonal climate variables. Dying subalpine fir showed declining sensitivity to summer precipitation, summer temperature, and winter precipitation, providing support for H3. Unlike other species in this study, the subalpine fir and Engelmann spruce sites represented high-elevation communities in the Colorado Rockies, US. Recent climate warming in these forest-alpine ecotones has resulted in interactive stressors, such as dramatic changes in snow amount and duration (Siirila-Woodburn et al. 2021), high biotic pressure (Lalande et al. 2020), and unprecedented wildfire events (Higuera et al. 2021), leading to decline (Perret et al. 2023). Consistent with some boreal studies, we found positive trends for the effects of temperature for some species (e.g., Scots pine, Norway spruce, surviving aspen), highlighting that trees in some sites may benefit from additional warming (Martinez Del Castillo et al. 2024; Wilmking et al. 2005). However, negative trends (“decline”) in sensitivity to summer temperature were found for dying and surviving trees (e.g., silver fir, red oak), perhaps suggesting climate change associated pressures or signaling that even surviving trees in these studies were at risk of future mortality. Tree responses to climate are also influenced by forest history and disturbances such as fire or thinning (Marqués et al. 2022; Rodman et al. 2024). In the Ozark and Ouachita Mountains in Arkansas, US, fire suppression has significantly contributed to a decline in climate sensitivity for red oak associated with increased competition (Haavik et al. 2011; Soucy et al. 2005). Our findings indicate that both drought-surviving and drought-killed red oak share similar climate sensitivities, steadily declining in their sensitivities to summer temperature and winter precipitation. The cessation of fires has led to a marked shift in species establishment and forest succession, which formerly favored oaks in early successional stages, but now shade-tolerant, fire-intolerant species are favored (Soucy et al. 2005). Oaks have a relatively high light requirement; therefore, as forests grew denser in the absence of fire, competition for crucial resources, especially water and light, intensified (Johnson et al. 2002). This increase in competition pressure, combined with the stress of drought conditions, likely resulted in a decline in growth and climate sensitivity of red oak. Likewise, surviving and dying silver fir trees had similarly declining sensitivities to summer temperature. In the Spanish Pyrenees, the observed decline of silver fir may be linked to historical logging practices and drought stress driven by rising temperatures (Camarero et al. 2011). Diameter-limit cutting was the dominant timber harvesting method in this area, affecting primarily fast-growing large trees and thereby allowing smaller, slower-growing trees to persist (Camarero et al. 2017). While the reduction of competition post-logging may have led to temporary increases in biomass, this surge in productivity ultimately proved unsustainable under drought conditions, compromising the carbon balance of these trees, leading to a decline in climate sensitivity (Linares et al. 2009). 4.4 | Conclusions Our study shows that climate sensitivity and its role in drought mortality or survival is highly nuanced and reflects species- and regionally specific responses to the environment. Integrating past studies, we propose a combined framework for how to assess differences in growth among dying and surviving trees, which may be useful for species-specific prediction of future mortality risk. Especially in areas such as the southwestern US, where aridity and drought conditions are extreme and mean annual precipitation is low, flashy or acquisitive growth strategies can be risky during drought. However, flashy responses to climate may be less risky and more rewarding in areas of northern Europe with relatively higher mean annual precipitation and lower frequency and intensity of droughts, where climate decoupling can lead to carbon starvation or persistent hydraulic failure. A major finding is that most species had non-stationarity in climate responses over time (e.g.,Peltier & Ogle 2020), but their growth-climate relationships were inconsistently related to drought-induced mortality. This may reflect ongoing risks of future mortality, even in trees that survived the studies synthesized here. References Aakala, T., Kuuluvainen, T., Wallenius, T. & Kauhanen, H. (2011). 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Supplementary Material File (summary_figures_ecoletters.docx) Download 1.56 MB Information & Authors Information Version history V1 Version 1 21 February 2025 Copyright This work is licensed under a Non Exclusive No Reuse License. Keywords climate decoupling climatic sensitivity drought forest decline tree growth tree mortality tree rings Authors Affiliations Alicia Formanack 0000-0002-2109-6967 [email protected] Northern Arizona University View all articles by this author Kiona Ogle 0000-0002-0652-8397 Northern Arizona University View all articles by this author Drew Peltier University of Nevada Las Vegas View all articles by this author Metrics & Citations Metrics Article Usage 272 views 150 downloads .FvxKWukQNSOunydq8rnd { width: 100px; } Citations Download citation Alicia Formanack, Kiona Ogle, Drew Peltier. Flashy, decoupled, or declining? Single theories each fail to explain the diversity of drought mortality signals in tree rings. Authorea . 21 February 2025. 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