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

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 density-dependent 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.