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1. Large grazers modify vegetated ecosystems and are increasingly viewed as keystonespecies in trophic rewilding schemes. Yet, as their ecosystem influencesare context-dependent,a crucial challenge is identifying where grazers sustain,versus undermine, important ecosystem properties and their resilience.2. Previous work in diverse European saltmarshes found that, despite changingplant and invertebrate community structure, grazers do not suppress below-groundproperties, including soil organic carbon (SOC). We hypothesised that,in contrast, eastern US saltmarshes would be sensitive to large grazers as extensiveareas are dominated by a single grass, Spartina alterniflora. We predicted thatgrazers would reduce above-andbelow-groundSpartina biomass, suppress invertebratedensities, shift soil texture and ultimately reduce SOC concentration.3. We tested our hypotheses using a replicated 51-monthlarge grazer (horse) exclusionexperiment in Georgia, coupled with observations of 14 long-termgrazedsites, spanning ~1000 km of the eastern US coast.4. Grazer exclusion quickly led to increased Spartina height, cover and flowering,and increased snail density. Changes in vegetation structure were reflected inmodified soil texture (reduced sand, increased clay) and elevated root biomass,yet we found no response of SOC. Large grazer exclusion also reduced drought-associatedvegetation die-off.5. We also observed vegetation shifts in sites along the eastern US seaboard wheregrazing has occurred for hundreds of years. Unlike in the exclusion experiment,long-termgrazing was associated with reduced SOC. A structural equation modelimplicated grazing by revealing reduced stem height as a key driver of reduced soilorganic carbon.6. Synthesis: These results illustrate the context dependency of large grazer impactson ecosystem properties in coastal wetlands. In contrast to well-studiedEuropean marshes, eastern US marshes are dominated and structured by a singlefoundational grass species resulting in vegetation and soil properties being moresensitive to grazing. Coastal systems characterised by a single foundation speciesmight be inherently vulnerable to large grazers and lack resilience in the faceof other disturbances, underlining that frameworks to explain and predict largegrazer impacts must account for geographic variation in ecosystem structure.

Sharp, S., Davidson, K., Angelini, C., Fischman, H., Pennings, S.C., Fowler, M.S. and Griffin, J. 2024. Large grazers suppress a foundational plant and reduce soil carbon concentration in eastern US saltmarshes. Journal of Ecology. (DOI: 10.1111/1365-2745.14407)