Anthropogenic climate change is anticipated to have large consequences for forest ecosystems, particularly through elevated temperatures and shifts in precipitation regimes. Unfortunately, tree species that are unable to acclimate to these climatic changes will likely be poorly suited to the new environmental conditions which could lead to reductions in species’ fitness and survival. We investigated acclimation potential in in-situ mature quaking aspen (Populus tremuloides) forest stands in the San Juan National Forest in Colorado by quantifying physiological responses at the low (hot/dry), middle, and upper (cool/wet) elevation limits for this species, and across different climatic years: an average precipitation year (2014), a severe drought year (2018), and an above-average wet year (2019). We measured physiological responses by quantifying changes in predawn (ΨPD) and midday (ΨMD) water potential, sapwood specific hydraulic conductivity (Ks), hydraulic conductivity, percent loss of conductivity (PLC), and leaf area-to-sapwood area ratio (AL:AS). We compared both spatial and temporal changes in these responses by comparing measurements across elevation and between the different climatic years.
Results/Conclusions
We observed substantial spatial and temporal variation in physiology, and high intra annual variability at the low elevation limit (dry range margin) for these aspen stands. We also found evidence of acclimation to changes in climate, as well as temporal delays in some responses, suggesting a lag in physiological response for this species. Water potentials (both ΨPD and ΨMD) were most negative in low elevation aspen trees, and during the severe drought (2018) for all trees. AL:AS showed much more temporal variability than spatial variability, and was surprisingly higher (i.e. more leaf area) in all aspen trees during the severe drought. Hydraulic conductivity (Ks) was significantly lower during the severe drought than the average (2014) or wet (2019) years, but did not vary across elevation. PLC increased with elevation and was highest during severe drought and the following wet year. Conclusions from this study have broad implications for southwestern US forests, and will further our understanding of the climate change acclimation potential of quaking aspen which is not currently well known.