That intermediate disturbance can promote ecological diversity has been revisited numerous times – sometimes yielding support, others not. Riparian forests are frequently subjected to spatially and temporally varying hydrologic processes, often yielding a disturbance gradient potentially reflected in vegetational diversity. However, their ecology can also be driven by processes of landform establishment and stabilization, after which forest succession may proceed effectively independent of the flood regime. The main objective of this study within the Lake Erie Gorges ecoregion of New York and Pennsylvania was to more fully explore the interplay between disturbance and succession in species-rich middle order (4th-6th) riparian forests of the Northeast. Hydrologic regime, sediment characteristics and likely source, and channel and landform temporal dynamics were catalogued in eight river corridors spanning the range of hydrogeomorphic attributes in the ecoregion. Composition and structure of riparian forests were quantified within 10-m thru 30-m square survey quadrats, with stand ages estimated by increment coring. Study sites have been largely free of anthropogenic alterations for a century or more, which typically represents the most dynamic time frame for eastern deciduous riparian ecology.
Results/Conclusions
Coarse sediment corridors (outwash and local shale) are characterized by punctuated depositional bar forms and islands that support discrete vegetational mosaics. In contrast, fine sediment corridors (lake plain silt/clay) support meandering single channels harboring arcuate cohorts of forest parallel to the river. Nascent willow/cottonwood/sycamore woodlands of all hydrogeomorphic types are not separated in NMDS ordination. However, coarse sediment/landform mosaic stands diversify rapidly through the first 50 – 100 years of stand age. Fine sediment/meander bank stands do not. Maximum coarse-sediment forest diversity occurs in stem exclusion stands no longer subject to flood disturbance but where gap dynamics have not yet emerged. Thus, a diverse mix of early, mid, and late seral species is associated not with intermediate but with a near absence of disturbance. Although arcuate vegetation bands on fine sediment banks suggest flood influence, they also appear to track the incremental geomorphic progression of the meanders. Patterns in both alpha (landform) and gamma (whole corridor) diversity in coarse-sediment systems strongly suggest successional overlap and successional mosaic patterns, largely independent of direct disturbance. More limited evidence for flood influence in fine sediment systems thus far has not suggested a diversity maximum at intermediate disturbance levels.