Influence of dam regulation on 55-year canopy shifts in riparian forests

Floodplain and swamp forests are undergoing extensive changes due to altered flow regimes, invasive species, logging, and various land use changes. These changes often go unnoticed due to the absence of adequate baseline data and monitoring. Using a data set from 55 years ago, we resampled 50 lowland forest stands in southern Wisconsin to assess changes in forest overstory composition, structure, diversity, and dominant species abundances. We also applied univariate and multivariate analyses to determine whether these changes varied between dam-regulated and unregulated rivers, tree species with different flooding tolerances, the presence of logging, and variations in edaphic and hydrologic variables. Although these forests display various types of resilience, their forest canopies are substantially different from 55 years ago, reflecting shifts in hydrology and the impacts of disease. On average, these forests have retained the same local (alpha) diversity but have converged in species composition (declined in beta diversity). They are now composed of more and smaller trees. Along the unregulated rivers, colonizing species have declined while later successional flood-tolerant species have increased. In the aftermath of Dutch elm disease, Ulmus spp. have greatly declined in abundance and size. Species with less flooding tolerance have generally increased across sites, especially along dam-regulated rivers. Because they are subject to chronic disturbances that reset succession, floodplain forests may respond more readily to shifts in disturbances regimes. Such forests may therefore serve as sentinels for forecasting the types of change that we can expect to unfold more gradually in upland forests.