Temperature induced shifts in leaf water relations and growth efficiency indicate climate change may limit aspen growth in the Colorado Rockies

Abstract

Higher temperatures and evaporative demand forecasted for Colorado forests by the end of the century suggest that soil water limitation increasingly will negatively impact whole plant performance. At the same time, upslope or poleward migration of plant ranges in response to warming may result in species experiencing cooler overnight temperatures, particularly if extremes in climate increase. In 2014 we established three experimental gardens along a temperature/elevation gradient to test the implications of shifting temperatures on tree function. Quaking aspen (Populus tremuloides) seedlings from a mid-elevation population were established in all three gardens and leaves were sampled monthly during the 2017 growing season. From these, we quantified the magnitude and timing of osmotic regulation, the relationships between leaf osmotic potential (Ψosm), midday leaf water potential (Ψmid), and soil moisture conditions under different temperature regimes, and growth efficiency (change in basal area/growing day). We observed a strong relationship between Ψosm and soil moisture, and a strong seasonal decline in Ψosm at the warmest and intermediate sites while the coldest site experienced a later increase in osmolytes associated with the highest degree of freeze tolerance. Growth efficiency was highest at the intermediate-temperature site – closest in elevation to the seed source location – but declined asymmetrically with warming or cooling. The novel abiotic conditions at both non-local sites resulted in declines in growth efficiency, suggesting that aspen will experience suboptimal conditions whether it stays in areas experiencing warming temperatures or if it migrates upslope to areas with colder overnight temperatures.