Modelling salt-marsh vegetation dynamics through species dispersal and competition

Salt marshes are widespread morphological features in coastal and estuarine tidal landscapes, and are ecologically and economically important as they significantly contribute to coastal primary production, support high biodiversity, and provide a broad range of valuable ecosystem services.

The ability of salt marshes to counteract changes in external forcings depends on the complex dynamic interactions between physical and biological processes acting at different spatial and temporal scales. In particular, the evolution of tidal marshes in the vertical direction results from the balance and feedbacks between organic and inorganic deposition, erosion, and changes in relative sea level. For example, colonization of salt marsh platforms by halophytic vegetation enhances both organic and inorganic deposition due to increased flow resistance, reduced bottom shear stresses, capture of sediment particles by plant stems, and direct biomass accumulation. Moreover, halophytes control soil aeration, which feeds back into vegetation zonation and the related biogeomorphic interactions typically observed in tidal marshes.

In spite of their importance, however, modeling vegetation dynamics in intertidal marshes remains a major challenge both at the theoretical and practical/numerical level. Improving our current understandings of the mechanisms that drive the zonation of halophytic species is of critical importance to enhance projections of salt-marsh response to changes in climate and relative sea level.

Here we present a new bi-dimensional, spatially explicit ecological model aimed to simulate the spatial dynamics of halophytic vegetation in tidal saline wetlands. Vegetation dynamics are treated differently compared to previous models, which employed relatively simple deterministic or probabilistic mechanisms, dictated only by the ability of different species to adapt to different topographic elevations. In our model, in contrast, spatial vegetation dynamics depend not only on the local habitat quality, but also on spatially explicit mechanism of dispersal and competition among multiple, potentially interacting species. The temporal evolution of vegetation biomass at each site depends on death and colonization processes, both local and resulting from dispersal. These processes are modulated for each species by the habitat quality of the considered site. The latter is synthesized only through the local elevation relative to the mean sea level, and is mathematically modeled using a logistic function that represents the theoretical niche of each considered species.

Results indicate that such a relatively simple model, where species have elevation-dependent fitness and otherwise neutral traits, can predict realistic diversity and species-richness patterns. More importantly, the model is also able to effectively reproduce the outcome of classical ecological experiments, in which a species is transplanted to an area outside its optimal (realized) niche. A direct comparison clearly shows how previous models not accounting for dispersal and interspecific competitions are unable to reproduce such dynamics.