Climatic Range Filling of North American Trees

Understanding the degree to which species distributions are controlled by climate is crucial for forecasting biodiversity responses to climate change. Climatic equilibrium, when species are found in all places which are climatically suitable, is a fundamental assumption of species distribution models, but there is evidence in support of climate disequilibria in species ranges. Long-lived, sessile organisms such as trees may be especially vulnerable to being outpaced by climate change, and thus prone to disequilibrium. In this dissertation, I tested the degree to which North American trees are in equilibrium with their potential climatic ranges using the ‘range filling’ metric, which is calculated by taking the proportion of modeled potential climatic range which is realized. In Chapter I, I demonstrated that most species are missing from the majority of their potential ranges (mean range filling 48%). Range filling was strongly positively correlated with realized range size. I found that small-ranged species have underfilled climatic ranges relative to a spatially randomized ecological null model, suggesting that climate is a poor predictor of their distributions. I further show small-ranged species tend to have range shapes more indicative of dispersal limitations than climatic filling. In Chapter II, I found that range filling correlates with species’ functional differences; I found low range filling among species with traits correlated with pioneer plant strategies, while “climax” species showed high range filling. An analysis of range filling through time from pollen-derived distributions (Chapter III) revealed that while range filling decreased during post-glacial climatic change events, much of the contemporary disequillibrium is the result of decreases in the last 5000 years. Megafauna-adapted species which are proposed to have dispersal limitations due relying on now-extinct Pleistocene megafauna mostly do not show the hypothesized consequences, possibly due to compensatory dispersers. Overall, these findings suggest that nonclimatic factors and dispersal limitations drive climatic disequlibrium in North American trees. Considering this in light of human induced landscape fragmentation and climate change which exacerbate dispersal potential and increase equilibrium, attention should be given towards transplanting narrowly distributed tree species