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Abstract Rising sea levels lead to the migration of salt marshes into coastal forests, thereby shifting both ecosystem composition and function. In this study, we investigate leaf litter decomposition, a critical component of forest carbon cycling, across the marsh-forest boundary with a focus on the potential influence of environmental gradients (i.e., temperature, light, moisture, salinity, and oxygen) on decomposition rates. To examine litter decomposition across these potentially competing co-occurring environmental gradients, we deployed litterbags within distinct forest health communities along the marsh-forest continuum and monitored decomposition rates over 6 months. Our results revealed that while the burial depth of litter enhanced decomposition within any individual forest zone by approximately 60% (decay rate = 0.272 ± 0.029 yr⁻¹(surface), 0.450 ± 0.039 yr⁻¹(buried)), we observed limited changes in decomposition rates across the marsh-forest boundary with only slightly enhanced decomposition in mid-forest soils that are being newly impacted by saltwater intrusion and shrub encroachment. The absence of linear changes in decomposition rates indicates non-linear interactions between the observed environmental gradients that maintain a consistent net rate of decomposition across the marsh-forest boundary. However, despite similar decomposition rates across the boundary, the accumulated soil litter layer disappears because leaf litter influx decreases from the absence of mature trees. Our finding that environmental gradients counteract expected decomposition trends could inform carbon-climate model projections and may be indicative of decomposition dynamics present in other transitioning ecosystem boundaries. 

Abstract Ecosystem connectivity tends to increase the resilience and function of ecosystems responding to stressors. Coastal ecosystems sequester disproportionately large amounts of carbon, but rapid exchange of water, nutrients, and sediment makes them vulnerable to sea level rise and coastal erosion. Individual components of the coastal landscape (i.e., marsh, forest, bay) have contrasting responses to sea level rise, making it difficult to forecast the response of the integrated coastal carbon sink. Here we couple a spatially-explicit geomorphic model with a point-based carbon accumulation model, and show that landscape connectivity, in-situ carbon accumulation rates, and the size of the landscape-scale coastal carbon stock all peak at intermediate sea level rise rates despite divergent responses of individual components. Progressive loss of forest biomass under increasing sea level rise leads to a shift from a system dominated by forest biomass carbon towards one dominated by marsh soil carbon that is maintained by substantial recycling of organic carbon between marshes and bays. These results suggest that climate change strengthens connectivity between adjacent coastal ecosystems, but with tradeoffs that include a shift towards more labile carbon, smaller marsh and forest extents, and the accumulation of carbon in portions of the landscape more vulnerable to sea level rise and erosion.