Reciprocal Vegetation-Flow Feedbacks Driving Early-Stage Landscape Evolution in a Restored Wet Meadow

Just as taxonomic classification has improved understanding in biology, ecogeomorphologists would benefit from a functional classification of biota based on the biophysical feedbacks that they engage in. Early stages of landscape development following disturbance provide a unique opportunity to delineate and understand these feedback processes, as the diversity in functional morphotypes (a.k.a. 'ecomorphs,' to expand on a concept from terrestrial ecology) is high and the potential for self-organization of landscape pattern strong. We used the opportunity of a stream restoration that reset its floodplain to 'initial conditions' to perform a suite of biophysical measurements designed to delineate the classes of feedback that influence landscape evolution in distinct ways. The Big Spring Run restoration (Lancaster, PA), completed in November 2011, involved removal of 15,000 t of legacy sediment from the valley bottom to expose a Holocene hydric layer and reestablish wet meadow hydrology and biota. By performing repeat biogeomorphic surveys within a study grid, we tested the hypothesis that distinct ecomorphs determine the persistence and location of channel and microtopographic features. The qualitatively distinct patch types surveyed included carpet-forming mat vegetation, tussock-forming vegetation, sparsely vegetated mudflats, benthic algal mats, mixed herbaceous communities, grasses, and clonal emergent vegetation. Within each sampling location, changes in vegetation community architecture, grain size distribution, critical shear stress for sediment entrainment, and topography were monitored over time, and flow resistance was measured. An overbank flow event that completely filled the floodplain provided an additional opportunity to measure vegetation-flow-sediment interactions. Once emergent vegetation was bent over by flow, vegetation had a negligible influence on flow velocity--in contrast to most other wetlands--but continued to shelter the bed from sediment entrainment and promote the deposition of fines. In the two years of the study, distinct biogeomophic succession trajectories associated with different ecomorphs emerged: 1) Stabilization of sediment by benthic algae, followed by occupation by mat-forming vegetation and ultimately by grasses 2) incipient channel formation around clonal emergents or trees, which induced topographic steering of flow, and 3) replacement of clonal emergents with grasses in areas not immediately adjacent to the channel or sustained by groundwater seeps. Principal component analyses allowed for rigorous quantitative identification of ecomorphs, and long-term feedbacks of those ecomorphs on landscape evolution are presently being evaluated by modeling. Overall, landscape dynamics in this restored wet meadow-floodplain are driven by deterministic succession trajectories but are influenced by stochastic elements: the spatial distribution of groundwater seeps and the initial colonization of patch types that remain upright during high flow. However, whereas hydrology might respond quickly to restoration change, biological, geological, and hydrological feedbacks might take a decade or more to stabilize.