Linking management planning for coastal wetlands to potential future wave attenuation under a range of relative sea-level rise scenarios

Abstract

Understanding changes in wave attenuation by emergent vegetation as wetlands degrade or accrete over time is crucial for incorporation of wetlands into holistic coastal risk management. Linked SLAMM and XBeach models were used to investigate potential future changes in wave attenuation over a 50-year period in a degrading, subtropical wetland and a prograding, temperate wetland. These contrasting systems also have differing management contexts and were contrasted to demonstrate how the linked models can provide management-relevant insights. Morphological development of wetlands for different scenarios of sea-level rise and accretion was simulated with SLAMM and then coupled with different vegetation characteristics to predict the influence on future wave attenuation using XBeach. The geomorphological context, subsidence, and accretion resulted in large predicted reductions in the extent of vegetated land (e.g., wetland) and changes in wave height reduction potential across the wetland. These were exacerbated by increases in sea-level from +0.217 m to +0.386 m over a 50-year period, especially at the lowest accretion rates in the degrading wetland. Mangrove vegetation increased wave attenuation within the degrading, subtropical, saline wetland, while grazing reduced wave attenuation in the temperate, prograding wetland. Coastal management decisions and actions, related to coastal vegetation type and structure, have the potential to change future wave attenuation at a spatial scale relevant to coastal protection planning. Therefore, a coastal management approach that includes disaster risk reduction, biodiversity, and climate change, can be informed by coastal modeling tools, such as those demonstrated here for two contrasting case studies.

Frequency of extreme freeze events controls the distribution and structure of black mangroves (Avicennia germinans) near their northern range limit in coastal Louisiana

“…Historically, wetland restoration efforts in Louisiana have targeted plant communities dominated by marsh grasses, sedges or rushes. However, due to the recent increases in A. germinans abundance and height since the last major freeze event in 1989, mangrove‐dominated plant communities are increasingly viewed as potential vegetation targets for tidal saline and/or brackish wetland restoration efforts (Hijuelos et al., 2019; Mack et al., 2014; USDA‐NRCS, 2017). Our findings provide the first state‐level maps of the risk of mangrove freeze damage in Louisiana.…”

“…Mangrove range limits in the region are expected to continue to expand farther north in response to warming winters (Osland et al., 2018). In the face of climate change, mangroves are increasingly seen as potential vegetation targets for coastal wetland restoration and planning efforts (Hijuelos et al., 2019; Mack et al., 2014; USDA‐NRCS, 2017). For example, mangrove seedlings are sometimes planted at restoration sites, and mangrove propagules have also been occasionally collected and dispersed into salt marshes.…”

The impacts of mangrove range expansion on wetland ecosystem services in the southeastern United States: Current understanding, knowledge gaps, and emerging research needs

“…In a model‐based assessment that was parametrized with local field‐based measurements of vegetation structure along the Atlantic coast of Florida, Doughty et al (2017) found that wave attenuation and erosion prevention were greater in mangroves than graminoid‐dominated salt marshes, which was attributed to the larger canopy heights and stem diameters of mangrove trees. Similarly, in a model‐based comparison of mangroves and graminoid‐dominated marshes in Louisiana, Hijuelos et al (2019) noted that wave attenuation was greater in mangroves than graminoid‐dominated marshes. All three of these studies indicate that there may be a gain in coastal protection services associated with mangrove expansion, which is especially important given that climate change is increasing the frequency of major hurricanes (Kossin et al, 2017).…”

“…Fish and Wildlife Service) and Micheal Allen (University of Florida), oral communication, May 19, 2021]. Despite the potential gains in coastal protection supported by mangrove expansion (Doughty et al, 2017;Hijuelos et al, 2019;Pennings et al, 2021), the loss of salt marsh views is a critical and overarching concern for many Cedar Key residents, and some residents are interested in managing the expanding mangrove forests to regain the previously present salt marsh Beyond just research gaps in key thematic areas, there is a need for methodological refinements that expand upon findings from local scale inter-habitat comparisons. Most of the studies in this review have employed inter-habitat comparisons or comparative measurements made across mangrove-marsh ecotones, which are valuable approaches that require careful consideration of potentially confounding abiotic gradients (e.g., inundation and salinity gradients).…”

“…Fish and Wildlife Service) and Micheal Allen (University of Florida), oral communication, May 19, 2021]. Despite the potential gains in coastal protection supported by mangrove expansion (Doughty et al, 2017; Hijuelos et al, 2019; Pennings et al, 2021), the loss of salt marsh views is a critical and overarching concern for many Cedar Key residents, and some residents are interested in managing the expanding mangrove forests to regain the previously present salt marsh views and access [Andrew Gude (U.S. Fish and Wildlife Service) and Micheal Allen (University of Florida), oral communication, May 19, 2021].…”