Short-term evolution pattern in salt marsh landscapes: the importance of physical constraints

Context: Salt marsh landscapes at the land-sea interfaces exhibit contrasting spatiotemporal dynamics, resulting from varying physical constraints that limit new marsh establishment. The expansion of salt marsh landscapes towards the sea or their retreat towards the land is determined by patch-level changes, relying on the balance of power between the intrinsic biota traits and external physical disturbances. Objectives: Examine how marsh dynamics respond to physical constraints, and clarify the pathway from coupled physical processes involving hydrodynamic forces, sediment transport, and morphological changes to rapid patch evolution and landscape changes. Methods: We defined and distinguished four types of marsh changes based on patch proximities from five-month drone images in two typical marsh pioneer zones of the Yangtze Estuary, China: outlying expansion, edge expansion, infilling expansion, and retreat. Hydrodynamics and sediment transport were synchronously measured and compared near the two marsh edges, and morphological changes were generated by drone-derived digital elevation models (DEMs). Results: We identified distinct seasonal patterns of net marsh expansion at the accretion-prone site, that is: Net marsh expansion started from the outlying expansion in spring, followed by edge expansion in summer and infilling expansion in autumn. However, at the erosion-prone site that experienced high bed shear stress, low sediment availability and high seaward sediment transport, we only observed limited infilling and edge expansion in spring. This suggests that the potential for long-distance patch formation beyond the initial marsh edges is diminished in areas subjected to intensive physical disturbances. Conclusions: Patch evolution dynamics in response to site-specific physical constraints drive state differentiation of salt marsh landscape changes. Consequently, the heterogeneous evolution in salt marsh landscapes should be taken into account in restoration practice.