Vegetation change in a sensitive forested wetland in the Upper Midwest

The vegetation of Cedarburg Bog, a relatively undisturbed 1000-hectare forested wetland in southeastern Wisconsin, includes species common in northern Wisconsin and Canada. As part of a project to explore the ecological effects of climate and land use changes, we resampled bog herb, shrub and tree strata in 2006 using 165 plots established in 1991. Our initial research questions included determining how, if at all, wetland indicator species abundance and floristic quality have changed. Using the 125 most frequent plant species, we found the frequency of occurrence within four wetland affinity categories varied significantly (X²=20.24, df=3, p<0.0001), although the number of species within each group did not vary between sample years. Occurrences of obligate wetland species and those that usually occur in wetlands increased <5%, but occurrences of facultative species and those that usually occur outside wetlands both increased >30%. While the relative abundance of wetland species decreased, floristic quality declined only slightly. We assessed floristic quality using a conservation coefficient (CC) system developed for Wisconsin by regional ecologists—each species received an integer ranging from one to ten that indicates the likelihood of occurrence at sites undisturbed by humans. Current mean CC for the entire bog is 6.4, unchanged from 1991, but plot-level mean CC decreased by 0.1 on average (t=2.21, df=164, p=0.03). By exploring changes in the sensitive vegetation of Cedarburg Bog, we hope to better understand the effects of climate and land use changes on regionally rare communities that include species at the outer limits of their ranges.