The ability to understand why certain environmental factors affect tidal creek stability is important to predict how salt marsh communities will respond to climate change and restoration efforts. Salt marsh function is dependant on the landscape which is chiefly shaped by the interaction between the flow of water and the density of vegetation. However topography, sediment transport, nutrient availability and uptake, sediment accumulation, and groundwater flow are also important yet misunderstood contributing factors to the organization of the land. Salt Marshes have a number of landscape patterns that range from low lying sloughs with sparse vegetation to high marsh ridges with dense vegetation. Larsen and Harvey (2010) developed the Ridge and Slough Automata Landscape model (RASCL) to represent the slow evolution of salt marshes over millennia demonstrating the interaction between vegetation and water flow. Understanding this interaction allows researchers to predict how different disturbances affect the landscape. Appropriate mitigation and restoration efforts can be determined based on the arrangement and specific shape of vegetation patches in regards to the orientation and velocity of flowing water. —Acadia Tucker
Larsen, L., Harvey, J. 2010. Modeling of hydroecological feedbacks predicts distinct classes of landscape pattern, process, and restoration potential in shallow aquatic ecosystems. doi:10.1016/j.geomorph.2010.03.015.
RASCL, in particular, reproduced two wetland ecosystems in the Everglades of Florida because it represents both densely vegetated, flow-resistant ridges, and sparsely vegetated sloughs. This makes it applicable to most salt marsh ecosystems that resemble areas with dense vegetation interacting with low-lying, fast flowing creeks. Larson and Harvey used a range of abiotic and biotic parameters such as initial water level, suspended sediment load, soil porosity, and periods of high flow fluxes to account for wet seasons. The model function was based on two dominating feedback loops, signals that either accelerate or inhibit the process of evolution. Sediment-flow feedback is a positive signal that accelerates the build up of vegetation patches by the accumulation of peat and sediment, increasing the density of plants and decreasing the flow of water. Differential peat accretion feedback, a negative signal, prohibits the accumulation of peat and sediment in shallow and deep waters to regulate the coverage of high-density patches of vegetation.
RASCL was run 125 times to show the main geomorphic processes that create different landscape patterns under different parameters. In general, during the early stages of landscape development, patches of vegetation are not permanent and develop gradually. During this time developing patches of vegetation are vulnerable to erosion caused by fast flowing water and insufficient sediment transport. Patches of vegetation only become stable once a certain volume of vegetation (less than 1%) reaches a critical elevation above the surface of the water. As more plants colonize an area, their roots trap sediment and slowly build up a landmass beneath them (positive feedback). Newly formed patches begin to elongate and expand as the flow of water is redirected around them causing sediment to be redistributed on the downstream side of the patch. Peat accumulation and patch elongation stop once the flow of water decreases and the water level becomes too shallow among the patches of vegetation (negative feedback). This further redirects water away from the flow-resistant, dense patches of vegetation and intensifies the flow of water over the edges of sparsely vegetated areas. The increase of flow intensifies erosion and narrow creeks form along the areas of reduced flow resistance, interlaced throughout the patches of dense vegetation. Salt marsh ecosystems stabilize once the rate of erosion equalizes the expansion of new patches.
The model produced three major landscape patterns that stabilized at different points along the evolutionary process. The majority of salt marsh landscapes developed into an area dominated by ridged topography with limited water flow through the dense patches of plants (75%). Other simulations developed into a diverse landscape with ridges of dense vegetation interacting with flowing creeks and shallow sloughs (18%). However, under some of the parameters, the landscape pattern never stabilized and continued to shift as patches of vegetation were continually created and destroyed (7%).
By manipulating the input variables Larsen and Harvey determined that any alteration in the velocity or volume of water entering a salt marsh could severely transform the landscape pattern and thus significantly change the natural function of the ecosystem. If the flow of water is limited it could cause a rapid expansion of densely vegetated patches, while an increase in flow could drown the whole habitat. Larsen and Harvey found that by analyzing the shape and orientation of vegetation patches they could predict the particular disturbances active within a salt marsh and therefore take the corrective measures to restore the damaged ecosystem. For example, the rapid expansion of densely vegetated patches produced by a disruption of flow permanently alters the natural erosion rate, which can be detected by analyzing the shape change of certain vegetation patches. Restoring the normal flow of water cannot reverse this result alone; it can only be reversed by reducing the concentration of vegetation before restoring the natural flow of water.