Understanding the degree to which species distributions are controlled by climate is crucial for forecasting biodiversity responses to climate change. Climatic equilibrium (i.e., that species are found in all places which are climatically suitable), is a fundamental assumption of species distribution models, but there is growing evidence for climatic disequilibria in past and present ranges. Long-lived, sessile organisms such as trees may be especially vulnerable to being outpaced by climate change, and may be particularly prone to disequilibrium. We tested the degree to which North American trees are in equilibrium with their potential climatic ranges using the ‘range filling’ approach, which is the proportion of the realized and modeled potential climatic ranges. We analyzed the ranges of 447 modern trees and compared our results to a geometric, non-ecological null model to determine range filling. Additionally, we analyzed range filling at 1000-year intervals for 24 tree pollen taxa from the Neotoma Database to reconstruct range filling dynamics in response to 21,000 years of climate change.
Results/Conclusions
We found that most North American tree species are missing from the majority of their potential ranges (with a mean range filling of 48%). Range filling was strongly positively correlated with realized range size, and this relationship was scale-dependent; 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. Further analysis of range filling through time from pollen-derived distributions revealed that while range filling decreased temporarily during abrupt post-glacial climate change events, contemporary disequilibrium emerged only in the last 5000 years, when climates were comparatively more stable. Intriguingly, trees with megafaunal dispersal syndrome do not show lower-than-average range filling, possibly due to compensatory dispersers (including humans). Overall, our findings suggest that non-climatic factors and dispersal limitations drive contemporary climatic disequilibrium in North American trees. Given that growing landscape fragmentation and climate change will exacerbate dispersal limitations and disequilibrium in the future, this approach could identify candidate range syndromes for managed relocation.