Differences in Evolutionary History and Ploidy Type Shape the Interactions of Populus tremuloides Michx. with Climate

Quaking aspen (Populus tremuloides Michx) is a species with high phenotypic plasticity and a broad distribution that, in the last decade, has experienced climate stress-induced mortality called Sudden Aspen Decline (SAD). In order to help mitigate the effects of SAD in the future, we need a better understanding of aspen’s climate adaptation. We also need to learn more about how aspen structure and function relate to climate. To that end, this dissertation seeks to improve our understanding of aspen biogeography by exploring multi-scale genetic-climate interactions. First, we examined aspen climate adaptation, asking if genetic relatedness corresponded to any similarities or differences in climate niche across aspen’s range. We found that two aspen sub-populations, with genetic differences, had distinctly different climate niches and we concluded that aspen species distribution models should include genetic relatedness to predict species range more accurately. We also studied how polyploidy in aspen affected the structure and function of leaves, branches, and ramets. We found that diploid and triploid aspen had differences in leaf traits that ultimately drove greater triploid maximum photosynthetic rates, stomatal conductance, and intrinsic water-use efficiency (iWUE). However, despite greater iWUE in triploids, we found that they were actually more prone to climate-induced stress because of higher stomatal conductance rates and less stomatal sensitivity and control than diploids. Finally, we measured growth and iWUE in tree rings from diploid and triploid aspen. We learned that diploid growth had statistically significant positive correlations with total precipitation and the last day with snow on the ground. We also found that triploid growth had a statistically significant positive correlation with total precipitation, and a negative correlation with vapor pressure deficit. In addition, we found that while iWUE in both diploids and triploids increased during years with less available water, iWUE was nearly 4% greater in triploid aspen in every year. To maintain this ~4% greater iWUE when there was less available water, it is likely that triploid aspen did not reduce photosynthetic rates and stomatal conductance as much as diploids. Again, this suggests that triploids may be more prone than diploids to climate-induced stress because triploids require more water than diploids when there is less water available. Overall, our findings improve our understanding of macroscale and local scale interactions between aspen and climate