Changes in landscape factors, disturbance regimes, high deer abundance, and invasion of non-native plants have all contributed to the alteration of many of Wisconsin’s plant communities. We resampled 66 northern forest sites across a range of edaphic and climate conditions in order to assess forest canopy changes in relation to these variables, including impacts on understory plant composition. Stands were initially sampled in the 1950s and relocated in the early 2000s; each plot was sampled for both overstory diversity and basal area and understory diversity and abundance. We used Ward’s clustering method to identify groups of stands based on overstory composition, and we used similarity indices, ordination, and diversity indices to evaluate changes in species abundance and overall community structure.
Results/Conclusions
Sites clustered into three overstory groups: mesic sites dominated by northern hardwoods, dry sites with a significant pine inclusion in the canopy, and highly diverse 'transitional' sites. Collectively, forests gained maple, ash, and cherry species while losing pines, birches, and red oaks, supporting a general mesification of Wisconsin forests. Only 13 sites (20%) remained pine-dominated, all on soil with high sand content, while the rest demonstrated accelerated mesification. The driest sites additionally retained understory diversity. Fire suppression is postulated to have allowed invasion of mesic plants into historically drier sites: first (before 1950 and over the past 50 years) into sites with relatively high soil nutrient content and moisture-holding capacities, and now potentially into drier sites on glacial outwash sands.
