Monitoring Fifty Years of Plant Species Invasions in Wisconsin Forest Understory Communities

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Long-term ecological change is often hard to document without reliable baseline data. We obtained baseline data for and resurveyed 80 forest understory communities in Wisconsin. These sites were quantitatively surveyed between 1942-1956. Sixty-two sites were in the relatively unfragmented northern Wisconsin, and 18 were in highly fragmented southern Wisconsin. We examine percent changes in native species, the fraction of exotics in the flora, and the relationship between the abundance of exotic plants and native species decline. Since the initial surveys, native species richness declined 19%. In the 1950s, 1.6% of northern and 16.7% of southern sites contained exotics. By 2001, 71% of northern sites and 94% of southern sites had exotics. In 2001, the average ratio of exotic to native species per site was 1 exotic to 24.0 natives in the north, and 1 to 4.8 in the south. The presence and abundance of exotic species was a poor predictor of native species diversity and native species losses since 1950. However, specific exotic species and specific weedy native species are linked to such losses. These losses depended on the spatial scale examined. Both native and exotic species can exhibit invasive traits and alter plant community structure.


Presented at the 7th International Conference on the Ecology and Management of Alien Plant Invasions, Ft. Lauderdale, FL.

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