Coastal areas are experiencing increased pressure from both environmental change and human use. Salt marshes are dynamic and sometimes collapse into mud-flats from disturbance, an alternative stable state from which recovery might be lengthy. Livestock grazing was long suspected to cause erosion by trampling, and by removing plants that attenuate waves and bind the soil. Yet, grazing could also stimulate erosion resistance by inducing belowground root growth and enhancing soil compaction. Hence, to clarify the influence of land use on salt marsh erosion, we sampled the plant community, soil chemistry and soil geotechnical properties along 2-5 creeks in 3 grazed and 4 ungrazed marshes in north Wales (United Kingdom). Along each of these creeks, we assessed erosion in two ways: (1) by counting the number of slump blocks and (2) by digging out 3 plant-soil cores that were then eroded in the lab using a wave flume. We used these response variables in stepwise regression models, with plant and soil features as predictors, and then constructed a structural equation model to assess the direct and indirect effects of land use on salt marsh erosion.
Results/Conclusions
The structural equation models found support for only indirect effects of grazing on salt marsh stability. Grazing positively affected soil compaction and soil geotechnical properties, which in turn negatively affected erosion rates from plant-soil cores. Thus, the net indirect effect of grazing was to reduce marsh erosion. Erosion rates from cores were also moderated by soil organic matter, but this latter variable was not influenced by grazing. The same pathway led to a positive indirect effect of grazing on the number of slump blocks in creeks. Again, grazing had a direct positive effect on shear stress, but in this case, creek slumping increased with shear stress. These findings indicate that grazing influences coastal salt marsh processes in opposing ways by directly increasing soil cohesion and compaction, leading to increased soil shear stress, which on the one hand reduces erosion rates, and on the other increases creek-edge slumping. We recommend that managers and scientists remain vigilant of both the obvious direct as well as the more nuanced indirect pathways that can influence marsh dynamics. The relative strength of each of the processes identified in this study will determine the resilience of these ecosystems to the pressures of both environmental and land use change.