Testing the Limits: Understanding How Climate and Competition Affect Species' Ranges in a Warming World

Abstract

What factors determine where species occur, known as their geographic range limits? This classic ecological question has fascinated scientists for centuries, and is even more relevant today, in the face of anthropogenic climate change. Unfortunately, despite decades of research, we still lack a full understanding of the ecological processes driving range limits. Of particular interest, given forecasted global warming, is the extent to which climate determines species' range limits. If climate is important in controlling species' range limits, then ranges will likely shift up in elevation, as temperatures rise. This research examined how climatic factors, including temperature and precipitation, interact with biotic factors (specifically, competition between plants) to determine the distributions of common conifers on Mt. Rainier. I conducted observational studies of adult and sapling tree rings, as well as an experiment, in which seeds and seedlings were transplanted across species' altitudinal ranges and beyond their range limits, into different competitive environments. Taken together, my studies suggest that climate limits growth and survival at high elevations, while competition is more important at low elevations. These results support a classic, but little tested, hypothesis: biotic factors, such as competition, are more important at lower range limits, while abiotic factors, such as temperature, control upper range limits. Furthermore, these results suggest that climate change will lead to increased tree growth and upward expansion of Mt. Rainier's forests, beyond current high elevation treeline. Climate change will likely have less dramatic effects at low elevations, where climate does not appear to strongly limit growth and survival of focal tree species, but competitive dynamics between plants do limit growth and survival. More experimental studies of other biotic interactions, diverse species, and widespread locations are necessary to better understand effects of climate change, and to prioritize conservation and natural resource management efforts.