Populus tremuloides (Quaking Aspen) has the greatest range of any tree species in North America and is found throughout widely variable habitats. The occurrence of aspen in its habitats is partially driven by a suite of climatic characteristics whose means and extremes directly and indirectly affect the suitability of sites. These characteristics of climate under which aspen survive and thrive can be defined as its climatic niche. However, aspen populations in different regions may experience very different climates and have different phenotypes. Thus, for a species that is so widely distributed, how does one characterize its climatic niche? Do aspen inhabit one climatic niche, or several regional niches that could result in differences in genotype and phenotype?

This thesis explores the similarities and differences of climatic niche for this species and how regionally, climatic niche may be different. Eastern and western climatic niche groupings were discovered with differences in potential evapotranspiration (PET), growing degree days (GDD), and mean annual precipitation (MAP). Time series analysis uncovered trends of increasing temperature minimums resulting in increasing GDD for western aspen since 1990. Differences between the eastern and western aspens' climatic niches may relate to clone size and structure as well as favored reproductive strategies and biotic interactions such as competition with other tree species that are not treated here. This work is a first step in quantifying differences in climatic niche for aspen in the lower 48 states and can advise how aspen may respond differently, by region, to climate changes.