Riparian corridors are among the most diverse of ecosystems, owing to a complex interplay of hydrologic, geologic, and ecological factors. A spatio-temporal model of disturbance and succession in forested riparian zones is presented here, derived from an extensive study of channel morphology, sedimentary regime, and vegetational ecology within the Lake Erie Gorges ecoregion of western New York State. Hydrologic impacts, sediment characteristics and sources, and channel and landform dynamics have been catalogued along eight middle-order rivers spanning the range of hydrogeomorphic attributes in the region. Study sites are largely free of anthropogenic alterations for a century or more, including 300+ year-old stands in the Zoar Valley Canyon of 6th-order Cattaraugus Creek. Composition of riparian forests was quantified within 10-m thru 30-m square survey quadrats, with stand ages estimated by increment coring. Landforms stable enough to develop forests are associated only with either coarse clastic (high mass) or fine clay/silt sediments (cohesive), representing a clear dichotomy in the geomorphic template. Coarse-sediment channels comprise mosaics of bar and island forms deposited laterally and episodically by high flow events. Fine-sediment rivers are more often incrementally meandering single-thread channels, with most bank accretion being vertical.
Results/Conclusions
Non-metric multidimensional scaling (NMDS) ordination of tree stratum composition on ~60 fluvial landforms indicates successional convergence among coarse-sediment stands, yielding oak, maple, beech, hemlock, etc., assemblages similar to mature upland forests of the region. Fine-sediment landforms instead support predominantly early-successional willow, cottonwood, sycamore, etc., stands through 100 years or more. These diverge markedly from coarse-sediment riparian forests in NMDS ordination. Hydrology can influence riparian ecology of either channel type directly, by flooding, or indirectly, by shaping the geomorphic template on which vegetation develops. Alteration of geomorphology must necessarily alter or remove associated vegetation. However, and conversely, vegetation can be disturbed by flooding while leaving underlying geomorphology unchanged. If hydrologic influence eventually becomes nil, a landform has effectively become a “terrestrial” surface. Forests on course-sediment landforms are often decoupled from direct hydrologic influence in as little as 20 years, and thus embark on endogenous successional trajectories. Riparian ecology on fine-sediment landforms, in contrast, can remain tied to the hydrologic regime via flood disturbance for many decades. On some fine sediment banks habitual and persistent inundation may not even be a “disturbance” in the strictest sense, but rather a predictable “condition” to which riparian stands are perennially adapted.