|Biogeomorphic regime shifts between vegetated and bare states in tidal wetlands|
Wang, C. (2014). Biogeomorphic regime shifts between vegetated and bare states in tidal wetlands. PhD Thesis. University of Antwerp. Ecosystem Management Research Group (ECOBE): Antwerp. viii, 185 pp.
Catastrophic shifts between alternative stable states have been recognized in many complex dynamic systems, ranging from climate systems to medical and economic systems. In ecology, it is also considered as an important mechanism, because extensive human activities and changing environmental conditions are inducing catastrophic shifts to unwanted states in many different ecosystems worldwide. These ecosystem state shifts are difficult to predict and restore. In coastal and estuarine zones, large scale transitions from vegetated marshes to bare flats or open water, or vice versa, have been observed in many places in the world, due to the combined effect of several factors including sea-level rise and changes in sediment input. Understanding and prediction of these conversions are of significant importance for ecosystem management and restoration, especially considering the valuable ecosystem services provided by coastal marsh vegetation. The underlying mechanism for these catastrophic shifts is suggested to be alternative stable state behaviour, controlled by biogeomorphic feedbacks between marsh vegetation and sediment surface elevation. However, research is still in relative infancy. Empirical studies are particularly scarce, which is a similar challenge as found in other ecosystems.
This study aims to empirically test and predict the biogeomorphic shifts between bare and vegetated states in intertidal ecosystems. Multiple techniques were employed including experimental floodplains simulated in a flume, as well as GIS analyses of aerial photographs, LIDAR data and spatial models of tidal currents and waves. Three marsh sites were studied including the Scheldt Estuary (Netherlands), Venice lagoon (Italy) and Blackwater Marsh (Maryland, USA).
Our results show empirical evidences, provide prediction tools to explore the underlying mechanism and generate a conceptual theoretical model for the abrupt shifts between the high-lying vegetated state and low-lying bare state in floodplain biogeomorphic systems. To the best of our knowledge, several empirical evidences are presented for the first time in this study, including: (1) the bimodal distribution in surface elevation (Chapters 2, 3, and 5) and vegetation biomass (Chapters 3 and 4), which co-exist (Chapter 3), suggesting high-lying vegetated states and low-lying bare states as alternative attractors with an unstable state at intermediate elevation and biomass levels; (2) relative rapid shifts in elevation (Chapter 2) and vegetation biomass (Chapter 3) appear in the intermediate unstable state; and (3) the presence of threshold behavior in intertidal elevation associated with the shift (Chapters 2 and 3), and in the establishment of seedlings (i.e., threshold in root length) to survive the experimental flooding disturbances (i.e., threshold in flooding discharge) (Chapter 4). In addition, the spatial distribution of the shift from a low-lying bare state to a high-lying vegetated state is successfully predicted in an estuary based on the threshold elevation (Chapter 2). Furthermore, a GIS tool is developed to predict the vegetation establishment based on logistic regression models using not only elevation maps, but also spatial data of tidal currents and wind waves, which goes beyond earlier studies (Chapter 3). Moreover, the mechanism causing alternative stable states is also detected empirically, (i.e. biogeomorphic feedbacks between plant growth, hydrodynamic forces and sediment surface elevation) (Chapters 2, 3 and 4). Finally, a conceptual theoretical model is generated explaining the mechanism of catastrophic shifts in floodplain biogeomorphic systems (Chapter 6).
In summary, this thesis suggests that high-elevated vegetated and low-elevated bare states in tidal wetlands are alternative stable states with abrupt biogeomorphic shifts. However, the study in this thesis is mainly focused on the shift from the low-lying bare state to the high-lying vegetated state. Further research is needed to demonstrate the hysteresis effect, in which different thresholds are present for the shifts in the two directions between the alternative stable states.