The coastal wetland vegetation has been widely recognized for its ability in mediating the physical environments, through hydrodynamics, sediment dynamics and consequent morphology. Meanwhile, the biological processes within coastal wetland ecosystems depend on morphological and physical forcing. This two-way coupling of biological and physical interaction can be described using the term of ‘biomorphodynamics’ (Murray et al., 2008). The saltmarshes and mangroves in the southeast coast of China provide striking examples for biomorphodynamic studies.
In this work, we present a concrete example of biomorphodynamic studies which aims to examine the links between biological and physical processes, in association with the spatial competition between local mangroves and the exotic Spartina alterniflora. Yunxiao Mangrove National Natural Reserve, where the coexistence of mangroves and saltmarshes dominates the upper part of the tidal flat, was selected to carry out fieldwork. Both long-term remote sensing image analysis, and short-term in situ observations of hydrodynamics, sediment dynamics and morphology, as well as vegetation investigations and experiments were used in this study to explore the two-way coupling of bio-physical interactions.
Long-term remote sensing image analysis, reading back to 1988, revealed that over a long-term scale, the mangrove was not invaded by exotic Spartina alterniflora, but the expansion rate of the mangrove was retarded by the presence of Spartina alterniflora. Differently from the expansion of Spartina alterniflora which was mainly caused by the growth of patches, the mangrove spread either parallel to isobaths or along the tidal creeks.
The hydrodynamic and sediment dynamic observations found the significant difference between the mangrove and the saltmarsh, in terms of their influences on the benthic boundary layer conditions. The mangrove showed a better ability in reducing hydrodynamic forces than the saltmarsh, but the latter was found to be better at accumulating sediments. However, their effects on the physical environment were relevant to the water level changes. The mediation of the flow by different vegetation species, together with the energy dissipation at different water levels, was considered as the possible explanation for vegetation changes.
The field experiments of transplanting seedlings revealed that the seedlings could perform their best ability of altering hydrodynamics and sediment dynamics when they were planted into the sub-environments created by their own adults. This fact might explain why the mangrove was not invaded by the saltmarsh grass from a physical stress point of view.
In summary, our observations revealed that the mangrove trees and saltmarsh grass showed different ways of altering physical environments and in return, this difference affects their spatial competition. Therefore, the biomorphodynamics should be considered in the future coastal wetland restoration and conversation.
