A river runs through it: 55-year changes in floodplain forest herbs in Wisconsin

Background/Question/Methods

Riparian forests have experienced widespread alteration due 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 long-term baseline data.  Using legacy data from the Wisconsin Plant Ecology Laboratory, we documented changes in understory diversity and composition in southern Wisconsin’s floodplain forests by resurveying 40 stands surveyed 55+ years ago.  We used univariate and multivariate analyses to characterize changes in species diversity and composition and examine how these changes related to local site and regional landscape factors.  We also compared these patterns of change with those observed among upland forests in the same region.

Results/Conclusions

On average, site-level diversity has increased 18.5%.  However, across the landscape, these plant communities have increased 30% in average pairwise similarity, considerably faster than the rate of homogenization observed in nearby upland forests (7.5%).  Although exotic species have increased in abundance and occupy more sites, native species account for most of these changes in local and regional diversity. While homogenization in the uplands reflected declines in native plant diversity, these riparian forests appear to be responding to different processes.  Increases in landscape connectivity as riparian forests expand may contribute to increases in native plant colonization among these riparian forests.  Observed increases in diversity at some sites could also reflect moderation of river flows due to dams and levees and/or increases in wetland specialists as baseflows of nearby rivers increase.  Frequent disturbance and micro-site heterogeneity of floodplain systems may promote the co-existence of native and exotic species. It also is possible that rates of native species immigration exceed rates of extinction.   Future monitoring will reveal whether these changes in diversity and composition are transient or portend systematic increases or decreases in ecosystem functions.