Density‐dependent linkage of scale‐dependent feedbacks: a flume study on the intertidal macrophyte Spartina anglica

Abstract

Spartina anglica is an autogenic ecosystem engineer. At a local (within-vegetation) scale, it improves plant growth by enhancing sediment accretion through attenuation of hydrodynamic energy with its shoots. This constitutes a short-range positive feedback. The vegetation also shows a long-distance negative feedback through formation of erosion troughs around the tussock, which are restricting lateral expansion. There is a growing recognition of the important role of such scale-dependent feedbacks for landscape structuring and ecosystem stability. By a series of flume studies, we provide direct experimental evidence that 1) both the local positive and the long-distance negative feedback are strongly densitydependent with clear thresholds, and 2) that both feedbacks are inherently linked via their density dependence, partly as the result of conservation laws. The observed thresholds occurred at vegetation densities commonly found in the field. We demonstrate that threshold densities are affected by external hydrodynamic forcing for the long-distance negative feedback. We expect the same to be true for the local positive feedback (sediment accretion), so that under mild hydrodynamic forcing, this local positive feedback may occur in the absence of a long-distance negative feedback (formation of erosion troughs). The density dependence of nearby positive and the long-distance negative feedback and how external conditions affect the linkage of both feedbacks are important factors shaping the vegetated salt-marsh landscape.

Facilitation shifts paradigms and can amplify coastal restoration efforts

“…Geomorphologic studies in this area (44,47,48) have shown that small outplants like we used here can die from erosion stress induced by high flow environments typical of these heavily channelized estuaries (47,48). These studies also revealed that larger-sized clumps experienced greater growth and survival, ostensibly because of reduced erosion stress relative to overall patch size (47,48). We saw a similar pattern in our restoration study where individual transplants in clumped treatments experienced less edge erosion stress and had increased survivorship and growth.…”

Formation of tussocks by sedges: effects of hydroperiod and nutrients

“…Likewise, frost heaving is an unlikely mechanism, as little topographic variation was evident in our mesocosms after the first winter. These processes could, however, be important in other systems where microtopography is plant derived (Fogel et al 2004, Biasi et al 2005, Bouma et al 2009). …”

Does vegetation prevent wave erosion of salt marsh edges?

“…A priori, it is critical to distinguish between lateral (marsh edge) and medial (marsh interior) erosion, because the interaction between waves and the substrate is very different in each case (direct impact force in the case of the former, and attenuated orbital wave currents at the bed in the latter). Existing evidence for the medial erosion process suggests that above-ground portions of a plant attenuate orbital wave currents within relatively dense vegetated salt marshes, but it has also been shown in steady-flow conditions that these aboveground portions can result in surface scouring in adjacent locations that are less dense (29,30). Moreover, it has been hypothesized that these above-ground portions can actually trigger the formation of the vertical cliff-like edge at the land-water interface (31,32), as a result of relative differences in landward versus seaward flow velocities and wave stresses on either side of the edge (33).…”