When viewed across the entirety of the western United States, vegetation patterns can largely be explained by the periodicity of precipitation and the interaction between temperature and precipitation. However, at finer spatial scales, topography and forest community dynamics become increasingly important determinants of how vegetation is distributed. Quaking aspen (Populus tremuloides) is the most widespread tree species in North America, and in southwestern Idaho, aspen are partially dependent on seasonal snowdrifts that accumulate based on topographic effects and wind patterns. These snowdrifts melt as temperatures rise in the spring and summer, providing a soil moisture subsidy into the growing season. A year or two of extremely low snow accumulation and high temperatures can cause high levels of tree mortality and make affected stands more susceptible to future drought. In addition to climatic effects, conifer encroachment on aspen can increase competitive pressure on aspen regenerative success. We evaluated the long-term future consequences of changing climate and conifer competition on quaking aspen in Reynolds Creek, a small well-instrumented watershed in southwest Idaho. To address the influence of site-level climate, we modeled succession and competition over an 85 year future period. Six different RCP8.5 global circulation models were used to create an ensemble climate change scenario under which we examined aspen regeneration, conifer competition, and climate-related tree mortality.
We found that anticipated reductions in snowpack depth and related increases in climatic water deficit (CWD) had significant negative effects on aspen’s ability to persist at a specific site long-term. Overall, aspen were able to persist on 20% of currently occupied sites over the course of the 85-year model scenarios. Probability of establishment decreased by an ensemble average of ~80% on snowpack-dependent sites. Climate-linked mortality events increased in frequency over all climate scenarios, and under the most severe emissions scenario resulted in complete extirpation from many previously-occupied sites. On average, five consecutive years of critically high CWD was the threshold for clonal mortality and permanent site extirpation. Our research suggests that the length of individual drought periods is the most important determinant of long-term persistence of aspen. It is possible that future aspen mortality in this area may be a climate niche adjustment, i.e. a repayment of what Dullinger describes as the ‘extinction debt’ of trees that live on the margins of their currently suitable range.