Identifying Hydro-Geomorphological Conditions for State Shifts from Bare Tidal Flats to Vegetated Tidal Marshes

Chen Wang,Sven Smolders,Zhan Hu,Qingke Wen,David P Callaghan,Tjeerd J Bouma,Jim Van Belzen,Stijn Temmerman

doi:10.3390/rs12142316

Abstract

High-lying vegetated marshes and low-lying bare mudflats have been suggested to be two stable states in intertidal ecosystems. Being able to identify the conditions enabling the shifts between these two stable states is of great importance for ecosystem management in general and the restoration of tidal marsh ecosystems in particular. However, the number of studies investigating the conditions for state shifts from bare mudflats to vegetated marshes remains relatively low. We developed a GIS approach to identify the locations of expected shifts from bare intertidal flats to vegetated marshes along a large estuary (Western Scheldt estuary, SW Netherlands), by analyzing the interactions between spatial patterns of vegetation biomass, elevation, tidal currents, and wind waves. We analyzed false-color aerial images for locating marshes, LIDAR-based digital elevation models, and spatial model simulations of tidal currents and wind waves at the whole estuary scale (~326 km²). Our results demonstrate that: (1) Bimodality in vegetation biomass and intertidal elevation co-occur; (2) the tidal currents and wind waves change abruptly at the transitions between the low-elevation bare state and high-elevation vegetated state. These findings suggest that biogeomorphic feedback between vegetation growth, currents, waves, and sediment dynamics causes the state shifts from bare mudflats to vegetated marshes. Our findings are translated into a GIS approach (logistic regression) to identify the locations of shifts from bare to vegetated states during the studied period based on spatial patterns of elevation, current, and wave orbital velocities. This GIS approach can provide a scientific basis for the management and restoration of tidal marshes.

Highlights

Different stable states have been observed in many ecosystems and identified as an important constraint on ecological management and restoration [1,2,3]
We developed a GIS-based approach to identify the locations where suitable conditions occur that can favor state shifts between bare intertidal flats and vegetated marshes along an estuary
Our results demonstrate that stable states in vegetation biomass and in intertidal elevation co-occur, indicating the existence of two stable ecosystem states, either low-lying bare tidal flats or high-lying vegetated marshes, while intermediate elevations and vegetation biomass densities occur much less frequently

Summary

Introduction

Different stable states have been observed in many ecosystems and identified as an important constraint on ecological management and restoration [1,2,3]. Understanding the conditions under which shifts can be expected to happen from one ecosystem state to another, has been widely recognized as an important challenge. It is difficult to know whether a system is close to a catastrophic bifurcation unless the critical transition is already happening [2,4], and secondly, once the critical shift has occurred, it may be difficult to restore the ecosystem to its original state due to hysteresis effects [2,5]. Recent studies have investigated the conditions that lead to such shifts in different ecosystems [3,6]. Few studies aimed to identify the conditions for state shifts in biogeomorphic systems, which are governed by interactions between organisms and their geomorphologic environment

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