Different biogeographic factors may influence the richness of lichen and tree assemblages found on islands within boreal lakes. To examine the relative importance of island area, insularity and habitat for shaping these richness patterns, we surveyed lichen and tree species on islands of the Boundary Waters Canoe Area Wilderness in northern Minnesota, USA. We hypothesized that dispersal would be a more important limiting factor for trees, whereas lichen richness would be limited primarily by habitat. Thirty islands were sampled during a one-week study in September-October 2009. For each island we measured island area, distance from lakeshore, and counted the number of habitats. A total of 51 lichen and 15 tree species were found. Lichens occurred on every island and ranged between 2 and 28 species per island. Trees were found on just 13 of the 30 islands and their richness ranged from 1-12 species. Regression analysis and structural equation modeling showed that habitat number and island size were the principle factors affecting the richness of both taxa. Insularity had a small effect on lichen richness and no effect on tree richness. When trees and lichens were examined separately, island area influenced tree richness more than did habitat, whereas lichen richness was most strongly influenced by tree richness.
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Vol. 83 • No. 4