Resilience of U.S. coastal wetlands to accelerating sea level rise

Abstract

Coastal wetlands provide a wide array of ecosystem services, valued at trillions of dollars per year globally. Although accelerating sea level rise (SLR) poses the long-term threat of inundation to coastal areas, wetlands may be sustained in two ways: by positive net surface-elevation change (SEC) from sediment and organic matter buildup and by accumulation, or horizontal migration into refugia—low-lying, undeveloped upland areas that become inundated. Using a simple model together with high-resolution elevation data, we provide, across the contiguous United States, analysis of the local effects of SLR, maximum SEC rates, and coastal development on the long-term resilience of coastal wetlands. We find that protecting current refugia is a critical factor for retaining wetlands under accelerating SLR. If refugia are conserved under an optimistic scenario (a high universal maximum SEC rate of 8 mm/yr and low greenhouse gas emissions), wetlands may increase by 25.0% (29.4%–21.5%; 50th, 5th–95th percentiles of SLR) by the end of the century. However, if refugia are developed under a more pessimistic scenario (a moderate universal maximum SEC rate of 3 mm/yr, high greenhouse gas emissions, and projections incorporating high ice-sheet contributions to SLR), wetlands may decrease by −97.0% (−82.3%–99.9%). These median changes in wetland area could result in an annual gain of ∼$222 billion compared to an annual loss of ∼$732 billion in ecosystem services in the US alone. Focusing on key management options for sustaining wetlands, we highlight areas at risk of losing wetlands and identify the benefits possible from conserving refugia or managing SEC rates.

Biophysical Drivers of Coastal Treeline Elevation

“…Quantifying the drivers of shifting ecotones is a critical step for predicting the impacts of sea level rise on future land use change. For example, most projections of future marsh area rely on selecting a tidal datum that defines the current landward boundary of marsh extent (Holmquist et al., 2021; Mitchell et al., 2020), or selecting a tidal datum as the future boundary of marsh extent (Buchanan et al., 2022; Osland et al., 2022; Warnell et al., 2022). However, our results demonstrate that the elevation of transition (i.e., threshold elevation in m NAVD88) between marsh and forest varies substantially with salinity, slope, and local drivers.…”

“…Maintenance of tidal marsh ecosystems, and the habitat provision, carbon sequestration, and water quality services they provide (Brittain & Craft, 2012; Craft et al., 2009; A. J. Smith & Kirwan, 2021), will rely upon upland conversion to marsh at a global scale (Schuerch et al., 2018). However, most marsh migration projections assume that the marsh‐forest boundary occurs at an elevation that can be approximated by a tidal datum (e.g., mean higher high water) (Buchanan et al., 2022; Doyle et al., 2010; Holmquist et al., 2021; Mitchell et al., 2020; Osland et al., 2022; Warnell et al., 2022), despite the understanding that the lower limit of coastal forests is driven by a range of biophysical factors beyond tides. Here, we examine the elevation of independently delineated, high‐resolution marsh‐forest boundary points in the Chesapeake Bay, located along the U.S. mid‐Atlantic coast, alongside biological and physical data sets to assess key drivers of coastal treeline elevation at the watershed scale.…”

Upland forest retreat lags behind sea‐level rise in the mid‐Atlantic coast

“…Upland forest is generally considered to be highly vulnerable to sea‐level rise and saltwater intrusion (Doyle et al., 2010; Fagherazzi et al., 2019; McDowell et al., 2022). Previous estimates of coastal forest loss to sea‐level rise assume that the positional change of the coastal tree line is synchronous with rising sea level (Buchanan et al., 2022; Enwright et al., 2016; Haer et al., 2013; Molino et al., 2022; Osland et al., 2022; Warnell et al., 2022). For example, recent studies based on modeled tidal datums predict that a 1.0–1.5 m mean global sea‐level rise will translate into hundreds of thousands of hectares of upland forests replaced by salt marshes across the conterminous United States within this century (Osland et al., 2022; Warnell et al., 2022).…”

Challenges, successes, and recommendations for management of coastal sandplain grasslands as regional biodiversity hotspots in the northeastern United States

“…Because much high‐quality sandplain grassland exists in areas behind dunes along coasts in locations that are vulnerable to storm surge and sea level rise, strategies should be developed to “set the stage” for grasslands to move inland from these locations (Buchanan et al 2022). Such strategies might include combinations of land conservation and expanding grasslands into agricultural lands or woodlands ahead of the loss or transformation of existing grasslands.…”