Salt marshes are dynamic systems that can suddenly expand or erode, affecting the delivery of important benefits such as flood protection and challenging coastal management. It is often not known what drives dramatic state shifts from marsh to mudflat or vice versa. Positive feedbacks between salt marsh vegetation and sedimentation are key to successful marsh growth. Spartina anglica has the capacity to improve marsh vertical growth by attenuating wave energy, leading to sediment capture. Such feedbacks are highly density dependent, but little is known about the interaction with wave forcing, which is thought to trigger marsh-mudflat transitions. This aim of the study was to investigate the effect of wave forcing on density-dependent feedbacks between marsh vegetation and sedimentation. Such interactions could have significant implications for landscape-scale marsh development, and affect resilience against state shifts. This study addressed these aims by examining the long-term development of planted out patches of Spartina anglica in the field. Patches (80x80cm) were planted in 3 treatments of vegetation density at each of 3 levels of hydrological forcing.
Results/Conclusions
The study found that density dependent feedbacks around salt marsh vegetation were influenced by differences in wave exposure. Switches from positive (i.e. sediment accretion inside the vegetation) to negative responses (i.e. erosion outside the vegetation) only occurred in dense vegetation under high-energy conditions. Under low forcing conditions, vegetation patch-density had a weaker effect on the surrounding sedimentary environment. In low energy environments, gully formation was markedly reduced around high density patches and there was overall sediment accretion around high and low density patches. The study also demonstrated that the mortality and growth rates of Spartina plants were associated with patch-density and exposure conditions. Mortality rates were greatest in low and medium density patches under the highest energy regimes. These findings may have important implications for salt marsh restoration management, because such interactions between the vegetation and the surrounding sediment can determine the success or failure of marsh establishment.