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

Abstract

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.

Plant Hydraulic Stress Explained Tree Mortality and Tree Size Explained Beetle Attack in a Mixed Conifer Forest

“…Surviving trees retained their green canopy, making it difficult to detect based on color changes from satellite imagery or aerial photos. This might partially explain the lack of studies that separated drought stress and beetle attack (Lloret & Kitzberger, ). Combining ground surveys of both tree mortality and beetle attack status thus offered a novel way to separately evaluate the two processes, which have been largely compounded (Figure 8).…”

Widespread drought‐induced tree mortality at dry range edges indicates that climate stress exceeds species' compensating mechanisms

“…This mortality may be due to drier populations crossing beyond the species' “fundamental niche” and existing outside where the species can physiologically survive, either temporarily (representing die‐off with recovery) or permanently (representing local extirpation/geographic range contraction). In other species, drought impacts have been more severe at “drought naïve” populations in the range core or wetter populations that do not often experience drought (Isaac‐Renton et al, ; Lloret & Kitzberger, ; Zuleta, Duque, Cardenas, Muller‐Landau, & Davies, ), hinting that these non‐drought adapted or acclimated populations may be more vulnerable to extreme water deficits due to some combination of physiological vulnerability and “structural overshoots” at the ecosystem level (Jump et al, ). In these scenarios, compensating mechanisms such as within‐species drought response trait variation, for instance more embolism resistant xylem in drier populations (Anderegg, ; López et al, ), and/or lower ecosystem‐level tree density or leaf area, may have allowed drier populations to suffer less mortality than wetter populations during the same drought.…”

“…This mortality may be due to drier populations crossing beyond the species' "fundamental niche" and existing outside where the species can physiologically survive, either temporarily (representing die-off with recovery) or permanently (representing local extirpation/geographic range contraction). In other species, drought impacts have been more severe at "drought naïve" populations in the range core or wetter populations that do not often experience drought (Isaac-Renton et al, 2018;Lloret & Kitzberger, 2018;Zuleta, Duque, Cardenas, Muller-Landau, & Davies, 2017), hinting that these non-drought adapted or acclimated populations may be more vulnerable to extreme water deficits due to some combination of physiological vulnerability and "structural overshoots" at the ecosystem level .…”

Recent decadal drought reverts warming‐triggered growth enhancement in contrasting climates in the southern Andes tree line

“…Although climate change effects and impacts on plant distribution and phenology are nowadays seen in several biomes, their severity on tree performance may vary among and within species. As a consequence, population‐level effects of global warming are initially expected to be more readily detected in boundary areas of species distributions, where species may respond faster and more dramatically to climate change (Messaoud, Bergeron, & Leduc, ; but see Lloret & Kitzberger, ). The upper limit of trees’ elevational distribution, the alpine tree line, is a natural ecotone that has been shown to be physiologically controlled by low temperature on a global scale (Körner, ), and hence considered one of the most responsive ecological monitors to global warming (Jobbágy & Jackson, ; Körner, ).…”