A conceptual model of forest succession and diversity in eastern riparian zones

Background/Question/Methods

Riparian zones are widely regarded as among the most dynamic of ecosystems. They also have been envisioned, although not universally accepted, as major centers of biological diversity. We will propose here, using data from Zoar Valley in western New York State (among other sites), a conceptual model of riparian forest succession and diversity that incorporates overlapping fluvial (landform erosion/deposition and vegetational colonization) and terrestrial (gap-phase canopy dynamics) processes. Ongoing research in the minimally disturbed Zoar Valley Canyon of 6^th-order Cattaraugus Creek has revealed a 400-year chronosequence of broadleaf forest on floodplain and terrace landforms. Historical and aerial photographs also provide a detailed history of the past century of fluvial dynamics. Riparian “events”, i.e. succession-altering fluvial changes, can be classified according to both their frequency and spatial extent, thus generating a two-axis model framework with four quadrants representing different ecological possibilities. Temporal and spatial scales must, of course, be appropriate for the species of interest – in this case, eastern trees of the northern hardwood and mesophytic ecotypes.

Results/Conclusions

At one extreme of high frequency and extent of riparian events, the successional clock will be constantly reset such that vegetation might only rarely progress beyond early seres. At low event frequency and extent (diagonally opposite from the first case), riparian influence would be so minimal that surrounding forest dynamics might be indistinguishable from an upland system. Alpha diversity is minimized at these two extremes. High event frequency but low extent can yield the narrowly demarcated terrestrial slope vs. floodplain zonation typifying constrained channels. Low event frequency but high extent represents the episodic channel shifts of unconstrained lowland reaches. Alpha diversity is likewise minimized in these two situations, but beta diversity will be high because successional stages will be sharply delineated. In contrast to the “corner” regions of this model, diversity tends to be maximized centrally where there is maximum overlap of riparian and terrestrial processes and of successional stages. Eastern riparian zones such as Zoar Valley differ from the well-characterized systems of the Northwest (e.g. Queets River) in several fundamental ways, including higher species richness, shorter species life spans, and a smaller role for conifers.