Quantifying within-species trait variation in space and time reveals limits to trait-mediated drought response

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.