Assessment of climatic warming using a model of forest species migration

Significant shifts in plant species ranges are anticipated next century if climate warms due to greenhouse gas emissions. The magnitude of the projected warming is considerable; the rate at which it is predicted to occur is unprecedented. There is genuine reason for concern that the extent of the range shifts will exceed the dispersal abilities of many plant species, especially in the context of extensive habitat fragmentation. This study attempts to assess explicitly the influence of two factors — mechanism of dispersal and land use configuration — on the ability of plant species to migrate in response to climatic warming. Computer models were developed to simulate dispersal at the time interval of a generation for wind-dispersed and bird-dispersed tree species. These models were applied to three study areas in the eastern United States, each consisting of two 1 : 250 000 USGS land use land cover quadrangles, which had been reclassified according to probabilities of successful colonization. The study areas reflected the continuum of human impact on the landscape, from areas in intensive agriculture to heavily forested areas. The fastest modeled migration rate observed was 81 m/yr for the wind-dispersed species and 136 m/yr for the bird-dispersed species. Average migration rates were significantly lower. The wind-dispersed species was especially sensitive to habitat isolation and fragmentation. Significant variations in average bird-dispersed migration rates occurred with modest differences in the land use pattern within a landscape; no single predictor of bird-dispersed migration success emerged. Model results indicate that many species may be unable to migrate as range limits shift with a climatic warming, resulting in long-term climatic disequilibrium.