Patterns of Understory Herbaceous Plant Species Assemblages in the Wabikon Lake Forest Dynamics Plot in Northeastern Wisconsin

Abstract

Understory herbaceous plant species outnumber woody species over threefold in the 25.2 ha Wabikon Forest Dynamics Plot in northern Wisconsin, a permanent ecological research site where every tree 1 cm diameter and larger has been identified, measured, and mapped. The most abundant and widespread understory herbs are spring ephemeral wildflowers, which take advantage of light availability early in the growing season before deciduous trees have produced maximal leaf canopy cover. This study explores community composition and microhabitat variables associated with 108 herbaceous plant species in 316 1 m x 2 m plots distributed in a stratified random pattern within the Wabikon site. Multivariate analysis identified 10 consistent species assemblages (groups) of herbaceous understory plants, although species composition overlapped considerably among these groups. Species groups with the highest mean % cover tended to have the least distinctive species composition. In general, microhabitat variables (topographic variables, soil type, and tree species composition) were not strongly associated with these assemblages. However, high elevation (upland) plots tended to have higher percent cover of understory herbs but lower species richness compared with lower elevation (lowland) plots. I also found a statistically significant negative relationship between canopy species richness and understory species richness. Environmental and biotic variables were associated with the distributions of individual species, but these relationships are not strong or consistent enough to produce highly predictable, spatially structured understory plant assemblages at the Wabikon plot. Comparisons with historical studies in the vicinity of the Wabikon site suggest that the species composition of understory herbs has not changed dramatically during the past 60 years. Invasive species are still virtually absent at Wabikon today. Nevertheless, some notable differences were observed, including a lower abundance of Dryopteris intermedia and much higher abundance of Trillium grandiflorum at Wabikon compared with historical studies in similar forests in this region.