Canopy Heterogeneity and Environmental Variability Drive Annual Budgets of Net Ecosystem Carbon Exchange in a Tidal Marsh

Tidal salt marshes are important ecosystems in the global carbon cycle. Understanding their net carbon exchange with the atmosphere is required to accurately estimate their net ecosystem carbon budget (NECB). In this study, we present the interannual net ecosystem exchange (NEE) of CO2derived from eddy covariance (EC) for a Spartina alterniflorasalt marsh. We found interannual NEE could vary up to 3‐fold and range from −58.5 ± 11.3 to −222.9 ± 12.4 g C m−2year−1in 2016 and 2020, respectively. Further, we found that atmospheric CO2fluxes were spatially dependent and varied across short distances. High biomass regions along tidal creek and estuary edges had up to 2‐fold higher annual NEE than lower biomass marsh interiors. In addition to the spatial variation of NEE, regions of the marsh represented by distinct canopy zonation responded to environmental drivers differently. Low elevation edges (with taller canopies) had a higher correlation with river discharge (R2= 0.61), the main freshwater input into the system, while marsh interiors (with short canopies) were better correlated with in situ precipitation (R2= 0.53). Lastly, we extrapolated interannual NEE to the wider marsh system, demonstrating the potential underestimation of annual NEE when not considering spatially explicit rates of NEE. Our work provides a basis for further research to understand the temporal and spatial dynamics of productivity in coastal wetlands, ecosystems which are at the forefront of experiencing climate change induced variability in precipitation, temperature, and sea level rise that have the potential to alter ecosystem productivity. Salt marshes are dynamic coastal wetlands where frequent tidal flooding in conjunction with elevation gradients create plant zonation. In tidal marshes found in the southeastern United States, the species of marsh grass Spartina alternifloradominates much of the marsh area. This species' canopy height, density, and biomass vary along an elevation gradient and because of this, their productivity is spatially dependent. In this paper, we used measurements of carbon dioxide exchange between the marsh surface and the atmosphere to estimate the interannual ecosystem carbon budgets. We found that across years, ecosystem carbon fluxes could vary up to 3‐fold. This variability year‐to‐year could be explained by drought conditions, specifically temperature, precipitation, and river discharge. We also found that the magnitude of carbon fluxes and its response to environmental drivers were spatially dependent. Taller canopies with higher biomass found along tidal creeks had higher rates of carbon uptake and were more sensitive to river discharge. While shorter canopies with low biomass found in the marsh interiors were more sensitive to precipitation. Our findings suggest the atmospheric carbon dynamics in salt marshes are spatially dependent and scaling these estimates to larger areas requires careful consideration of habitat zones and local environmental drivers. Tidal marsh net ecosystem exchange can vary by 3‐fold annually and 2‐fold across a single species canopy gradientDrought, temperature, precipitation, and river discharge affect spatially explicit net ecosystem exchangeInclusion of within‐footprint habitat zones can reduce uncertainties in scaling up net ecosystem exchange Tidal marsh net ecosystem exchange can vary by 3‐fold annually and 2‐fold across a single species canopy gradient Drought, temperature, precipitation, and river discharge affect spatially explicit net ecosystem exchange Inclusion of within‐footprint habitat zones can reduce uncertainties in scaling up net ecosystem exchange