Does biogeomorphic feedback lead to abrupt shifts between alternative landscape states?: An empirical study on intertidal flats and marshes

Abstract

Low‐elevation bare intertidal flats and high‐lying vegetated marshes are the main components of intertidal areas of estuaries, deltas and coastal embayments. Large‐scale transitions between them have been reported worldwide. Because vegetated marshes provide significant services to coastal societies, predicting transitions between vegetated and unvegetated states is of widespread importance. Previous theoretical and modeling work highlighted the potential bistable nature of intertidal elevations, with low‐elevation bare flats and high‐elevation vegetated marshes being two alternative stable states. However, empirical evidence of this bistable condition is limited. In this study, we tested empirically the hypothesis that bare flats and vegetated marshes can be considered as alternative stable landscape states with the occurrence of rapid catastrophic shifts between them. We analyzed historical records of intertidal elevation surveys and aerial pictures from the macrotidal current‐dominated Western Scheldt estuary (SW Netherlands). We found (1) a bimodal distribution of intertidal elevations corresponding to either a completely bare state or a densely vegetated state. (2) The shift from bare to vegetated state is accompanied with a relatively rapid shift in elevation, i.e., the mean accretion rate during the shift is 2 to 8 times larger than during the equilibrium state. (3) A threshold elevation could be identified above which the shift from bare to vegetated state has a high chance to occur. Hence, our results demonstrate the abrupt nonlinear shift between low‐lying bare flats and high‐elevation vegetated marshes, suggesting that the occurrence of catastrophic shifts between alternative stable states is indeed a potential mechanism in intertidal systems.