FOREST FRAGMENTATION AND ALIEN PLANT INVASION OF CENTRAL INDIANA OLD-GROWTH FORESTS

We assessed alien plant invasion of old-growth forest islands in rural central Indiana by means of paired sample sites along "warm" (south and west) edges and "cool" (north and east) edges in each of seven forest remnants. In general, the fragments appeared resistant to alien invasion; alien species richness and frequency dropped sharply inward from forest edges, and forest interiors were relatively free of aliens. However, aliens were more diverse and more frequent on warm edges than on cool ones. The main factor limiting invasion is probably low light availability in forest interiors, though limited dispersal and low disturbance levels may also be important. We suggest that invasion is discouraged by the edge response of the forests themselves: development of a dense wall of bordering vegetation that reduces interior light levels and wind speeds. The most successful invaders are mostly escaped ornamentals or other species not commonly found in adjoining fields. They can be expected to become more prominent in the future, but their spread at the landscape scale may be slowed by the "oceans" of cropland surrounding these forest islands.