Climate change is stressing many forests around the globe, yet some tree species may be able to persist through acclimation and adaptation to new environmental conditions. The ability of a tree to acclimate during its lifetime through changes in physiology and functional traits, defined here as its acclimation potential, is not well known. We investigated the acclimation potential of trembling aspen (Populus tremuloides) and ponderosa pine (Pinus ponderosa) trees by examining within-species variation in drought response functional traits across both space and time, and how trait variation influences drought-induced tree mortality. We measured xylem tension, morphological traits, and physiological traits on mature trees in southwestern Colorado, USA across a climate gradient that spanned the distribution limits of each species and three years with large differences in climate. Trembling aspen functional traits showed high within-species variation, and osmotic adjustment and carbon isotope discrimination were key determinants for increased drought tolerance in dry sites and in dry years. However, trembling aspen trees at low elevation were pushed past their drought tolerance limit during the severe 2018 drought year, as elevated mortality occurred. Higher specific leaf area during drought was correlated with higher percentages of canopy dieback the following year. Ponderosa pine functional traits showed less within-species variation, though osmotic adjustment was also a key mechanism for increased drought tolerance. Remarkably, almost all traits varied more year-to-year than across elevation in both species. Our results shed light on the scope and limits of intraspecific trait variation for mediating drought responses in key southwestern US tree species and will help improve our ability to model and predict forest responses to climate change. 

Data were collected during the summers of 2014, 2018, and 2019. Many of the functional traits were measured on samples processed in a laboratory.