Oak island dynamics in Southeastern Wisconsin

Abstract

The oak forests of southeastern Wisconsin appear relatively consistent in species composition and probably have shown little change in this regard since European settlement, perhaps because they were originally forest islands in a prairie matrix and remain as islands in an agricultural setting. Elimination of periodic fire permitted the oak savanna to develop into oak forest. Apparently the degree of separation between islands and the xeric nature of the sites restrict invasion by exotic species established as cultivars in nearby villages and farmsteads. Neither tree species richness nor tree species diversity could be predicted from island size, substantiating the belief that colonization and extinction dynamics differ appreciably between oceanic and terrestrial islands. Local seed sources are available for a limited number of adapted species and distance from seed source appears more important than island size. In the oak forest islands, the edge effect appears to extend further into the stand than in the beech-maple forests studied by Levenson (1976); apparently an oak island requires a minimum size of approximately seven to eight hectares to prevent domination by edge species.