Changes in The Floodplain Forest Vegetation of The Lower Wisconsin River Over The Last Fifty Years

Abstract

Over the last five decades, the floodplain forests along the Lower Wisconsin River have experienced several events capable of shifting forest structure and composition, including river regulation, Dutch elm disease and increased levels of herbivory. This study analyzes changes in the structure and composition of the vegetation in the floodplain forests over the previous 50 y by comparing forest survey data collected in 2001 with data collected on the same sites by John Curtis in the 1950s. The results suggest that the forests are shifting to a previously unrecorded later successional stage, dominated increasingly by silver maple, hackberry and bitternut hickory, a possible result of river regulation. Early-successional stages, particularly those dominated by black willow and cottonwood, are disappearing. Disturbances, such as timber harvests, may be increasing the rate of this transition. Further, the forests have shown a large increase in the dominance of two thorny shrubs, prickly ash and European buckthorn, in the understory, suggesting browsing pressure may be affecting regeneration. Analysis of the ground layer parallels these findings, showing an increase in species more typical of higher elevations in the floodplain and mesic forests, as well as invasive species and dominance by several species less palatable to deer.