Tidal branching wetlands morphology and its role on water-carbon budget cycles

Coastal wetlands represent invaluable “carbon banks”, as they naturally capture and store atmospheric carbon under water-logged conditions, where dead plant material decomposes very slowly, and organic layers build up in the soil. However, when drained or perturbed, wetlands can switch from carbon sinks to major sources, making monitoring and restoring degraded wetlands a worldwide environmental priority. Water table level plays a key role in regulating carbon exchange, but uncontrolled rewetting works do not suffice in restoring their optimal status. The reason is that forecasting beneficial effects of wetlands restoration is often challenged by the complexity of coupled water-soil-vegetation dynamics that both regulate soil respiration rate and shape micro-scale morphological features in the short and long terms. As a result, a significant number of studies have reported unexpected and significant failure outcomes in restoration works. Existing modeling frameworks generally neglect the spatial heterogeneity of wetland morphology and rely on heavy implementations of empirical functions, which limits model predictions to be site-specific. In this work, we adapt a landscape evolution model to explicitly simulate spatial wetlands morphology, accounting for coupled water, sediment, and vegetation dynamics. Carbon fluxes are then evaluated in a spatially explicit manner accounting for the high-resolution simulated heterogeneity of water table level, sediment elevation, and vegetation density. The modelled surface morphology is first compared, through standard river network metrics, with satellite images of tidal wetlands that exhibit different levels of river channeling. The simulated spatially-distributed carbon fluxes suggest that highly branched morphologies promote optimal water distribution and enhance carbon sequestration. These trends are confirmed by comparing simulated ecosystem fluxes with flux-tower eddy covariance measurements in several tidal wetlands.