Carbon Cycling in Tidal Wetlands in the Mississippi River Alluvial Plain: a Test Case for Dynamic Modeling of Wetlands across the USA.

To support a comprehensive national-scale assessment of carbon sequestration and greenhouse gas (GHG) fluxes, the USGS LandCarbon Program has developed a model of wetland carbon cycling, including potential responses to future land cover change scenarios. The objectives of this model are to 1) assess baseline carbon sequestration and GHG fluxes in coastal and inland wetlands, 2) project scenarios of climate and land use impacts on carbon sequestration and GHG fluxes in coastal and inland wetlands, and 3) assess the potential to increase carbon sequestration in coastal and inland wetlands through management applications. We adapted the Land Use and Carbon Scenario Simulator (LUCAS) model for use in wetland ecosystems. LUCAS combines a state-and-transition simulation model (STSM) to predict land-change with a stock and flow model to simulate carbon dynamics, within a scenario based framework to assess major controlling processes, characterize uncertainties, and develop future scenarios. LUCAS was used in past LandCarbon assessments of terrestrial ecosystems and will serve as a platform to integrate the wetland assessment with the next terrestrial assessment in 2023.

As a test case, we calibrated LUCAS for the tidal wetlands of the Mississippi River Alluvial Plain (USEPA Ecoregion 73) in a spatially-referenced framework, using measurements of carbon pools and fluxes across 24 sites in the ecoregion, covering a range of salinity. This ecoregion has experienced loss and degradation of coastal wetlands driven by relative sea level rise, storm surges and other erosion processes. We then assessed the ecoregion-wide carbon sequestration under current conditions with static landcover, future conditions without action (2400 ha yr ^-1 wetland loss) and with partial mitigation of such losses (1200 ha yr ^-1 wetland loss). We estimated 0.67 Tg C yr ^-1 sequestration under current conditions, 0.31 Tg C yr ^-1 loss with no future mitigation and 0.26 Tg C yr ^-1 sequestration with partial mitigation. We will further discuss how empirical distributions from calibration data can be used to estimate projection uncertainties using a stochastic Monte Carlo modeling approach, and how this modeling platform will be extended from the test case ecoregion to both inland and coastal wetlands of all ecoregions in the continental USA.