Coastal wetlands have two dimensions of vulnerability to sea‐level rise (SLR), a vertical one, in cases where SLR outpaces their capacity to vertically accrete, and a lateral one, in cases where they are restricted from migrating inland by topography and land use. We conducted a meta‐analysis of accretion rates, standardized our analysis by using only137Cs based estimates, and used model intercomparison to generate a vertical resilience index, a function of local SLR, tidal range, and tidal elevation category for the tidal wetlands of the contiguous US. We paired the vertical resilience index with a lateral resilience index made up of elevation, water level, and land cover maps, then projected them both into the future using localized SLR scenarios. At the regional scale, the vertical resilience index predicts changes from marsh aggradation to submergence for the coastal US Mid‐Atlantic, Southeast, and portions of the Northeast by 2100. At the sub‐regional scale, there is a geographic tradeoff between vertical and lateral resilience with more northerly wetlands vulnerable to the lack of suitable proportional area to migrate into, and more southerly wetlands vulnerable to accretion deficit. We estimate between 43% and 48% of the existing contiguous US wetland area, almost entirely located in watersheds along the Gulf of Mexico and Mid‐Atlantic coasts, is subject to both vertical and lateral limitations. These vertical and lateral resilience indices could help direct future research, planning, and mitigation efforts at a national scale, as well as supplement more processed informed approaches by local planners and practitioners.
The lateral extent and vertical stability of salt marshes experiencing rising sea levels depend on interacting drivers and feedbacks with potential for nonlinear behaviors. A two‐dimensional transect model was developed to examine changes in marsh and upland forest lateral extent and to explore controls on marsh inland transgression. Model behavior demonstrates limited and abrupt forest retreat with long‐term upland boundary migration rates controlled by slope, sea‐level rise (SLR), high water events, and biotic‐abiotic interactions. For low to moderate upland slopes the landward marsh edge is controlled by the interaction of these inundation events and forest recovery resulting in punctuated transgressive events. As SLR rates increase, the importance of the timing and frequency of water‐level deviations diminishes, and migration rates revert back to a slope‐SLR‐dominated process.
more » « less- Award ID(s):
- 1832221
- NSF-PAR ID:
- 10373276
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- Geophysical Research Letters
- Volume:
- 47
- Issue:
- 17
- ISSN:
- 0094-8276
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract An accelerating global rate of sea level rise (SLR), coupled with direct human impacts to coastal watersheds and shorelines, threatens the continued survival of salt marshes. We developed a new landscape‐scale numerical model of salt marsh evolution and applied it to marshes in the Plum Island Estuary (Massachusetts, U.S.A.), a sediment‐deficient system bounded by steep uplands. To capture complexities of vertical accretion across the marsh platform, we employed a novel approach that incorporates spatially variable suspended sediment concentrations and biomass of multiple plant species as functions of elevation and distance from sediment sources. The model predicts a stable areal extent of Plum Island marshes for a variety of SLR scenarios through 2100, where limited marsh drowning is compensated by limited marsh migration into adjacent uplands. Nevertheless, the model predicts widespread conversion of high marsh vegetation to low marsh vegetation, and accretion deficits that indicate eventual marsh drowning. Although sediment‐deficient marshes bounded by steep uplands are considered extremely vulnerable to SLR, our results highlight that marshes with high elevation capital can maintain their areal extent for decades to centuries even under conditions in which they will inevitably drown.
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Abstract Low‐lying coastlines are vulnerable to sea‐level rise and storm surge salinization, threatening the sustainability of coastal farmland. Most crops are intolerant of salinity, and minimization of saltwater intrusion is critical to crop preservation. Coastal wetlands provide numerous ecosystem services, including attenuation of storm surges. However, most research studying coastal protection by marshes neglects consideration of subsurface salinization. Here, we use two‐dimensional, variable‐density, coupled surface‐subsurface hydrological models to explore how coastal wetlands affect surface and subsurface salinization due to storm surges. We evaluate how marsh width, surge height, and upland slope impact the magnitude of saltwater intrusion and the effect of marsh migration into farmland on crop yield. Results suggest that along topographically low coastlines subject to storm surges, marsh migration into agricultural fields prolongs the use of fields landward of the marsh while also protecting groundwater quality. Under a storm surge height of 3.0 m above mean sea level or higher and terrestrial slope of 0.1%, marsh migration of 200 and 400 m protects agricultural yield landward of the marsh‐farmland interface compared to scenarios without migration, despite the loss of arable land. Economic calculations show that the maintained yields with 200 m of marsh migration may benefit farmers financially. However, yields are not maintained with migration widths over 400 m or surge height under 3.0 m above mean sea level. Results highlight the environmental and economic benefits of marsh migration and the need for more robust compensation programs for landowners incorporating coastal wetland development as a management strategy.
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Abstract Ghost forests, consisting of dead trees adjacent to marshes, are a striking feature of low-lying coastal and estuarine landscapes, and they represent the migration of coastal ecosystems with relative sea-level rise (RSLR). Although ghost forests have been observed along many coastal margins, rates of ecosystem change and their dependence on RSLR remain poorly constrained. Here, we reconstructed forest retreat rates using sediment coring and historical imagery at five sites along the Mid-Atlantic coast of the United States, a hotspot for accelerated RSLR. We found that the elevation of the marsh-forest boundary generally increased with RSLR over the past 2000 yr, and that retreat accelerated concurrently with the late 19th century acceleration in global sea level. Lateral retreat rates increased through time for most sampling intervals over the past 150 yr, and modern lateral retreat rates are 2 to 14 times faster than pre-industrial rates at all sites. Substantial deviations between RSLR and forest response are consistent with previous observations that episodic disturbance facilitates the mortality of adult trees. Nevertheless, our work suggests that RSLR is the primary determinant of coastal forest extent, and that ghost forests represent a direct and prominent visual indicator of climate change.more » « less
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Abstract Ghost forests consisting of dead trees adjacent to marshes are striking indicators of climate change, and marsh migration into retreating coastal forests is a primary mechanism for marsh survival in the face of global sea‐level rise. Models of coastal transgression typically assume inundation of a static topography and instantaneous conversion of forest to marsh with rising seas. In contrast, here we use four decades of satellite observations to show that many low‐elevation forests along the US mid‐Atlantic coast have survived despite undergoing relative sea‐level rise rates (RSLRR) that are among the fastest on Earth. Lateral forest retreat rates were strongly mediated by topography and seawater salinity, but not directly explained by spatial variability in RSLRR, climate, or disturbance. The elevation of coastal tree lines shifted upslope at rates correlated with, but far less than, contemporary RSLRR. Together, these findings suggest a multi‐decadal lag between RSLRR and land conversion that implies coastal ecosystem resistance. Predictions based on instantaneous conversion of uplands to wetlands may therefore overestimate future land conversion in ways that challenge the timing of greenhouse gas fluxes and marsh creation, but also imply that the full effects of historical sea‐level rise have yet to be realized.
