Exposure of trees to drought‐induced die‐off is defined by a common climatic threshold across different vegetation types

Increases in drought and temperature stress in forest and woodland ecosystems are thought to be responsible for the rise in episodic mortality events observed globally. However, key climatic drivers common to mortality events and the impacts of future extreme droughts on tree survival have not been evaluated. Here, we characterize climatic drivers associated with documented tree die‐off events across Australia using standardized climatic indices to represent the key dimensions of drought stress for a range of vegetation types. We identify a common probabilistic threshold associated with an increased risk of die‐off across all the sites that we examined. We show that observed die‐off events occur when water deficits and maximum temperatures are high and exist outside 98% of the observed range in drought intensity; this threshold was evident at all sites regardless of vegetation type and climate. The observed die‐off events also coincided with at least one heat wave (three consecutive days above the 90th percentile for maximum temperature), emphasizing a pivotal role of heat stress in amplifying tree die‐off and mortality processes. The joint drought intensity and maximum temperature distributions were modeled for each site to describe the co‐occurrence of both hot and dry conditions and evaluate future shifts in climatic thresholds associated with the die‐off events. Under a relatively dry and moderate warming scenario, the frequency of droughts capable of inducing significant tree die‐off across Australia could increase from 1 in 24 years to 1 in 15 years by 2050, accompanied by a doubling in the occurrence of associated heat waves. By defining commonalities in drought conditions capable of inducing tree die‐off, we show a strong interactive effect of water and high temperature stress and provide a consistent approach for assessing changes in the exposure of ecosystems to extreme drought events.     

Vapor–pressure deficit and extreme climatic variables limit tree growth

Assessing the effect of global warming on forest growth requires a better understanding of species‐specific responses to climate change conditions. Norway spruce and European beech are among the dominant tree species in Europe and are largely used by the timber industry. Their sensitivity to changes in climate and extreme climatic events, however, endangers their future sustainability. Identifying the key climatic factors limiting their growth and survival is therefore crucial for assessing the responses of these two species to ongoing climate change. We studied the vulnerability of beech and spruce to warmer and drier conditions by transplanting saplings from the top to the bottom of an elevational gradient in the Jura Mountains in Switzerland. We (1) demonstrated that a longer growing season due to warming could not fully account for the positive growth responses, and the positive effect on sapling productivity was species‐dependent, (2) demonstrated that the contrasting growth responses of beech and spruce were mainly due to different sensitivities to elevated vapor–pressure deficits (VPD), (3) determined the species‐specific limits to VPD above which growth rate began to decline, and (4) demonstrated that models incorporating extreme climatic events could account for the response of growth to warming better than models using only average values. These results support that the sustainability of forest trees in the coming decades will depend on how extreme climatic events will change, irrespective of the overall warming trend.     

Historical and event‐based bioclimatic suitability predicts regional forest vulnerability to compound effects of severe drought and bark beetle infestation

Vulnerability to climate change, and particularly to climate extreme events, is expected to vary across species ranges. Thus, we need tools to standardize the variability in regional climatic legacy and extreme climate across populations and species. Extreme climate events (e.g., droughts) can erode populations close to the limits of species' climatic tolerance. Populations in climatic‐core locations may also become vulnerable because they have developed a greater demand for resources (i.e., water) that cannot be enough satisfied during the periods of scarcity. These mechanisms can become exacerbated in tree populations when combined with antagonistic biotic interactions, such as insect infestation. We used climatic suitability indices derived from Species Distribution Models (SDMs) to standardize the climatic conditions experienced across Pinus edulis populations in southwestern North America, during a historical period (1972–2000) and during an extreme event (2001–2007), when the compound effect of hot drought and bark beetle infestation caused widespread die‐off and mortality. Pinus edulis climatic suitability diminished dramatically during the die‐off period, with remarkable variation between years. P. edulis die‐off occurred mainly not just in sites that experienced lower climatic suitability during the drought but also where climatic suitability was higher during the historical period. The combined effect of historically high climatic suitability and a marked decrease in the climatic suitability during the drought best explained the range‐wide mortality. Lagged effects of climatic suitability loss in previous years and co‐occurrence of Juniperus monosperma also explained P. edulis die‐off in particular years. Overall, the study shows that past climatic legacy, likely determining acclimation, together with competitive interactions plays a major role in responses to extreme drought. It also provides a new approach to standardize the magnitude of climatic variability across populations using SDMs, improving our capacity to predict population's or species' vulnerability to climatic change.     

Molecular and ecological plant defense responses along an elevational gradient in a boreal ecosystem

Plants have the capacity to alter their phenotype in response to environmental factors, such as herbivory, a phenomenon called phenotypic plasticity. However, little is known on how plant responses to herbivory are modulated by environmental variation along ecological gradients. To investigate this question, we used bilberry (Vaccinium myrtillus L.) plants and an experimental treatment to induce plant defenses (i.e., application of methyl jasmonate; MeJA), to observe ecological responses and gene expression changes along an elevational gradient in a boreal system in western Norway. The gradient included optimal growing conditions for bilberry in this region (ca. 500 m a.s.l.), and the plant's range limits at high (ca. 900 m a.s.l.) and low (100 m a.s.l.) elevations. Across all altitudinal sites, MeJA‐treated plants allocated more resources to herbivory resistance while reducing growth and reproduction than control plants, but this response was more pronounced at the lowest elevation. High‐elevation plants growing under less herbivory pressure but more resource‐limiting conditions exhibited consistently high expression levels of defense genes in both MeJA‐treated and untreated plants at all times, suggesting a constant state of “alert.” These results suggest that plant defense responses at both the molecular and ecological levels are modulated by the combination of climate and herbivory pressure, such that plants under different environmental conditions differentially direct the resources available to specific antiherbivore strategies. Our findings are important for understanding the complex impact of future climate changes on plant–herbivore interactions, as this is a major driver of ecosystem functioning and biodiversity.     

Nitrogen deposition outweighs climatic variability in driving annual growth rate of canopy beech trees: Evidence from long‐term growth reconstruction across a geographic gradient

In this study, we investigated the role of climatic variability and atmospheric nitrogen deposition in driving long‐term tree growth in canopy beech trees along a geographic gradient in the montane belt of the Italian peninsula, from the Alps to the southern Apennines. We sampled dominant trees at different developmental stages (from young to mature tree cohorts, with tree ages spanning from 35 to 160 years) and used stem analysis to infer historic reconstruction of tree volume and dominant height. Annual growth volume (G_V) and height (G_H) variability were related to annual variability in model simulated atmospheric nitrogen deposition and site‐specific climatic variables, (i.e. mean annual temperature, total annual precipitation, mean growing period temperature, total growing period precipitation, and standard precipitation evapotranspiration index) and atmospheric CO₂ concentration, including tree cambial age among growth predictors. Generalized additive models (GAM), linear mixed‐effects models (LMM), and Bayesian regression models (BRM) were independently employed to assess explanatory variables. The main results from our study were as follows: (i) tree age was the main explanatory variable for long‐term growth variability; (ii) GAM, LMM, and BRM results consistently indicated climatic variables and CO₂ effects on G_V and G_H were weak, therefore evidence of recent climatic variability influence on beech annual growth rates was limited in the montane belt of the Italian peninsula; (iii) instead, significant positive nitrogen deposition (N_dep) effects were repeatedly observed in G_V and G_H; the positive effects of N_dep on canopy height growth rates, which tended to level off at N_dep values greater than approximately 1.0 g m^?2 y^?1, were interpreted as positive impacts on forest stand above‐ground net productivity at the selected study sites.     

Differential declines in Alaskan boreal forest vitality related to climate and competition

Rapid warming and changes in water availability at high latitudes alter resource abundance, tree competition, and disturbance regimes. While these changes are expected to disrupt the functioning of boreal forests, their ultimate implications for forest composition are uncertain. In particular, recent site‐level studies of the Alaskan boreal forest have reported both increases and decreases in productivity over the past few decades. Here, we test the idea that variations in Alaskan forest growth and mortality rates are contingent on species composition. Using forest inventory measurements and climate data from plots located throughout interior and south‐central Alaska, we show significant growth and mortality responses associated with competition, midsummer vapor pressure deficit, and increased growing season length. The governing climate and competition processes differed substantially across species. Surprisingly, the most dramatic climate response occurred in the drought tolerant angiosperm species, trembling aspen, and linked high midsummer vapor pressure deficits to decreased growth and increased insect‐related mortality. Given that species composition in the Alaskan and western Canadian boreal forests is projected to shift toward early‐successional angiosperm species due to fire regime, these results underscore the potential for a reduction in boreal productivity stemming from increases in midsummer evaporative demand.     

The combined effects of a long‐term experimental drought and an extreme drought on the use of plant‐water sources in a Mediterranean forest

Vegetation in water‐limited ecosystems relies strongly on access to deep water reserves to withstand dry periods. Most of these ecosystems have shallow soils over deep groundwater reserves. Understanding the functioning and functional plasticity of species‐specific root systems and the patterns of or differences in the use of water sources under more frequent or intense droughts is therefore necessary to properly predict the responses of seasonally dry ecosystems to future climate. We used stable isotopes to investigate the seasonal patterns of water uptake by a sclerophyll forest on sloped terrain with shallow soils. We assessed the effect of a long‐term experimental drought (12 years) and the added impact of an extreme natural drought that produced widespread tree mortality and crown defoliation. The dominant species, Quercus ilex, Arbutus unedo and Phillyrea latifolia, all have dimorphic root systems enabling them to access different water sources in space and time. The plants extracted water mainly from the soil in the cold and wet seasons but increased their use of groundwater during the summer drought. Interestingly, the plants subjected to the long‐term experimental drought shifted water uptake toward deeper (10–35 cm) soil layers during the wet season and reduced groundwater uptake in summer, indicating plasticity in the functional distribution of fine roots that dampened the effect of our experimental drought over the long term. An extreme drought in 2011, however, further reduced the contribution of deep soil layers and groundwater to transpiration, which resulted in greater crown defoliation in the drought‐affected plants. This study suggests that extreme droughts aggravate moderate but persistent drier conditions (simulated by our manipulation) and may lead to the depletion of water from groundwater reservoirs and weathered bedrock, threatening the preservation of these Mediterranean ecosystems in their current structures and compositions.     

Integrating ecophysiology and forest landscape models to improve projections of drought effects under climate change

Fundamental drivers of ecosystem processes such as temperature and precipitation are rapidly changing and creating novel environmental conditions. Forest landscape models (FLM) are used by managers and policy‐makers to make projections of future ecosystem dynamics under alternative management or policy options, but the links between the fundamental drivers and projected responses are weak and indirect, limiting their reliability for projecting the impacts of climate change. We developed and tested a relatively mechanistic method to simulate the effects of changing precipitation on species competition within the LANDIS‐II FLM. Using data from a field precipitation manipulation experiment in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) ecosystem in New Mexico (USA), we calibrated our model to measurements from ambient control plots and tested predictions under the drought and irrigation treatments against empirical measurements. The model successfully predicted behavior of physiological variables under the treatments. Discrepancies between model output and empirical data occurred when the monthly time step of the model failed to capture the short‐term dynamics of the ecosystem as recorded by instantaneous field measurements. We applied the model to heuristically assess the effect of alternative climate scenarios on the piñon–juniper ecosystem and found that warmer and drier climate reduced productivity and increased the risk of drought‐induced mortality, especially for piñon. We concluded that the direct links between fundamental drivers and growth rates in our model hold great promise to improve our understanding of ecosystem processes under climate change and improve management decisions because of its greater reliance on first principles.