Influences on Understory Plant Communities in Mesic Uplands of the Chequamegon National Forest, Wisconsin

Abstract

Recently, researchers have shown that understory communities in northern Wisconsin are converging in composition, a process termed homogenization. They have also identified possible mechanisms for homogenization including whitetail deer herbivory and exotic earthworm introductions. Homogenization can result from species losses, introductions, or both, and may contribute to changes in diversity over time. This study focused on documenting changes in understory plant communities in mesic uplands of the Chequamegon National Forest in northern Wisconsin since the early 1990’s, and identifying possible influences of such changes. Permanent plots, originally established by the U.S. Forest Service, were revisited in the summers of 2008 and 2009. Data were collected on understory vegetation, earthworm abundance, and deer browsing in both managed and unmanaged forests, which were compared to previous measurements from 1990-1992. Species richness and diversity (H’) were not statistically different from the early 1990s. Present diversity and richness did not differ between managed and control units. Exotic species were not common, averaging <1 per unit. Neither increasing earthworm biomass nor increasing deer browse pressure impacted richness or diversity. However, the relationship between earthworm abundance and richness was nearly significant. Management did not contribute to increased compositional similarity over the study period. However, there was a 22% convergence in relative abundance and composition over the study period. Forest managers should be aware of the observed increases in community similarity and monitor potential drivers of homogenization. Future monitoring efforts in the Chequamegon should take into account changes in plant assemblages, sensitive species and growth forms, in addition to measures of diversity.