55-year changes in groundlayer and overstory plant communities in southern Wisconsin lowland forests

Riparian forests have faced widespread alteration due in part to regulation of natural flow regimes, changes in land use, invasion by pests and pathogens, and climate change. The complex changes imposed by these interacting forces often go unnoticed due to a lack of longterm baseline data. Using historical data from the 1950s, I documented 55-year changes in diversity and composition of plant communities in the overstory and groundlayers of riparian forests distributed across multiple rivers in southern Wisconsin. These forests now have more and smaller trees and there have been significant shifts in overstory composition. Ulmus spp. have significantly declined in abundance consistent with Dutch elm disease. Colonizing species have declined generally while later successional flood-tolerant species (especially Fraxinus ) have increased along unregulated rivers. Intermediate flood-tolerant species (especially Carya cordiformis) increased among all sites but especially along dam-regulated rivers where flood intolerants also increased. On average, site-level diversity of the groundlayer has increased 18.5%. However, across sites, these plant communities have increased 30% in similarity. This convergence in composition was mostly due to increases in a few common native species and wetland indicators among sites surrounded by more contiguous forest. These stands had more exotic plants in the 1950s yet native diversity has increased since then. These changes likely reflect increases in the frequency and duration of flooding at these sites and the extent and contiguity of forested corridors. Within more fragmented urbanizing landscapes that rarely flood, woody and exotic plants typical of upland forests have increased rapidly. This has diminished local species diversity but increased regional (beta) diversity. Local species diversity remained higher at frequently flooded sites in more fragmented landscapes. These results suggest that natural flooding regimes serve to maintain native species diversity and composition in otherwise fragmented landscapes. Future monitoring will reveal whether these changes in diversity and composition are transient or portend systematic increases or decreases in ecosystem functions.