Vegetation Development in a Modified Riparian Environment: Human Imprints on an Allegheny River Wilderness

Pristine floodplain forests are virtually nonexistent in the eastern United States, requiring that preservation efforts focus on relatively intact representatives of these unique ecosystems, many situated where hydrologic modifications are the norm. This article examines the vegetation dynamics for one such natural area, a wilderness island in northwestern Pennsylvania, to assess how the ecological processes of a riparian preserve are affected by changes to the surrounding environment. Ordination of a vegetation sample identifies several landscape patches on the island; the structure and historical development of these communities are analyzed using tree ring patterns, aerial photography, and the flood regime characteristics preceding and following construction of a large dam upstream. Research on natural riparian sites has emphasized the role of floods as a disturbance that generates early successional habitat. Here, however, moderation of the hydrologic regime has shifted the impact of floods from disturbance to stressor. Peak flows are no longer sufficient to open sites for colonization, while the duration of flooding has increased. Without flood disturbance, later stages of succession become more widely represented, and species regeneration occurs in the context of competitive—rather than open—sites. The altered disturbance regime thus favors species with life history characteristics atypical of the pre-dam environment, including nonnative species, resulting in altered composition and vegetation dynamics. Managerial expectations that natural successional processes will eventually restore degraded riparian habitats in these modified settings are therefore unlikely to be fulfilled.