Modeling effects of intraspecific variation, disturbance, and competition on climate-driven range shifts in forest trees
Both range shifts and local adaptation are expected to be important responses to a shifting climate. The response of trees to climate change is of particular interest because of the role of forests in the global carbon cycle, and because long generation times and the strong competitive effect of adults on juveniles impede both range shifts and evolutionary responses. Disturbance may accelerate forests’ responses by creating opportunities for better-adapted species or genotypes to establish. While disturbance, interspecific competition, and intraspecific genetic variation are recognized as important for climate change responses, no model to date has examined how all three factors interact at range boundaries. We have developed a simple 2-species, 2-locus model to begin to investigate these interactions. In this model, the landscape is divided into 50 m patches along a climate gradient. The landscape is inhabited by two species (high- and low-elevation) differing in their optimal climate. Within each species range, WW individuals do best at the warm range edge, CC individuals at the cold range edge, and WC individuals in the middle. The gradient shifts at 50 m/yr for 80 years, gradually eliminating the coldest climate. Climate then stabilizes for 300 years.
The model can be run with or without patch-scale disturbances. As one might expect, the presence of a high-elevation competitor impedes the migration of the low-elevation species, increasing the suitable area that remains uncolonized. When mean seed dispersal is greater than the patch width, disturbance reduces both extinction and colonization lags, and allows the distribution of genotypes to more closely approach the new optimum. However, when mean dispersal is less than 50 m, the area occupied by both species declines because of low colonization and high extinction. When genetic diversity was eliminated in the high-elevation species, instead allowing it a single genotype with broad tolerances optimized to the middle of its initial range, the area occupied by the high-elevation species after climate change was reduced as the low-elevation species was able to colonize more territory. These results suggest that interactions between interspecific competition, adaptive potential, and disturbance frequency, together with dispersal ability, are likely to affect how tree species’ ranges shift in response to climate change. We are currently incorporating genetic variation into a more sophisticated forest gap model to further explore these interactions.