Transient dynamics during climate change induced range shifts

Background/Question/Methods: One of the greatest challenges ecologists face is forecasting how global climate change will affect species distributions. Range limits are partially determined by species physiological tolerances to abiotic factors like temperature, thus, global warming is likely to result in range shifts upslope and polewards. Understanding the magnitude and rate of these range shifts is critical for planning conservation and management responses to a future warmer world. Unfortunately, although modeling approaches exist that allow us to predict where climatically suitable habitats will move with climate change, how fast species’ will respond by shifting their distributions is generally unknown. Here, we combine extensive data with literature surveys for three conifers occurring on Mt. Rainier (WA, USA) to determine: 1) where climatically suitable habitats of focal species will move under climate change scenarios projected for a future warmer world; 2) whether range expansion rates of focal species are rapid enough to fill newly available habitat as it becomes available; and 3) whether mortality of focal species at the trailing edges of current distributions is high enough for range contractions to keep pace with climate change.

Results/Conclusions: Focal species distributions will have to shift > 5 kilometers to maintain populations in climatically suitable habitat on Mt. Rainier under climate change scenarios for the Pacific Northwest. The migration rates required to fill newly available habitat exceed most measures of migration capacity for trees, implying that upper altitudinal limits of focal species will not keep pace with climate change. Similarly, slow tree mortality at lower range limits suggest that range contraction rates will be slower than rates of climate change. In short, transient dynamics during climate-change induced range shifts seem likely for focal trees on Mt. Rainier. However, rare long distance dispersal events and large-scale disturbance mediated mortality events, about which we know very little, could lead to more rapid range expansion and contraction than predicted. In total, these results lead us to two conclusions. First, short-term range shifts in response to global warming may be difficult to predict without additional information on dispersal, mortality and disturbance. Second, because pristine mountainous regions with steep gradients in climate and unfragmented habitats provide a best-case scenario for organisms experiencing climate change, transient dynamics are even more likely in when considering latitudinal range shifts in response to climate change.