PATTERNS AND DRIVERS OF RIPARIAN FOREST VEGETATION CHANGES OVER LAST HALF CENTURY IN SOUTHERN WISCONSIN

Floodplain and swamp forests have faced widespread destruction and alteration due in part to changes in landuse, invasion by pests and weedy plants, alteration of natural flow regimes, and climate change. Often, the changes imposed by these interacting forces go unnoticed due to a lack of long-term baseline data. Using a data set from the 1950s, we resurveyed 50 riparian forests throughout southern Wisconsin to document changes in diversity, composition and structure of canopy and understory vegetation. Using univariate and multivariate analyses, we evaluated these changes in the context of dam-regulated vs. unregulated rivers, tree species’ flooding tolerances, variations in edaphic and hydrologic variables, and changes in surrounding landscape variables.

In the canopy, we observed significant shifts in the relative abundances and sizes of notable lowland forest tree species, indicating impacts of disease and hydrologic regime changes. Ulmus spp. (elms) have declined in abundance and in size due to Dutch elm disease. These declines have contributed to significant increases in rapidly growing, flood-tolerant species such as Fraxinus spp. (ash) along unregulated rivers. Along dam-regulated rivers flood-intolerant and shade-tolerant species such as Carya cordiformis (bitternut hickory) have increased in abundance. Accompanying these increases in later-successional flood intolerant species are significant declines in early colonizing, flood-tolerant species and key species such as Quercus bicolor (swamp white oak). Acer saccharinum (silver maple) was the dominant species across many sites in the 1950s and has become even more dominant along rivers experiencing increased growing season flow (m³/s).

Sites adjacent to large rivers and surrounded by more contiguous forest showed the greatest increases in native understory diversity since the 1950s. In more fragmented and urbanizing landscapes where floods are rare, woody and exotic plants have increased in the understory, accompanied by losses of native species diversity. However, species diversity remains high at sites prone to frequent flooding even in fragmented landscapes. As natural flooding regimes appear to best maintain native species diversity in fragmented landscapes, managers should seek to recreate these whenever feasible.