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

Abstract

Climate change is transforming ecosystems and affecting ecosystem goods and services. Along the Gulf of Mexico and Atlantic coasts of the southeastern United States, the frequency and intensity of extreme freeze events greatly influence whether coastal wetlands are dominated by freeze‐sensitive woody plants (mangrove forests) or freeze‐tolerant grass‐like plants (salt marshes). In response to warming winters, mangroves have been expanding and displacing salt marshes at varying degrees of severity in parts of north Florida, Louisiana, and Texas. As winter warming accelerates, mangrove range expansion is expected to increasingly modify wetland ecosystem structure and function. Because there are differences in the ecological and societal benefits that salt marshes and mangroves provide, coastal environmental managers are challenged to anticipate the effects of mangrove expansion on critical wetland ecosystem services, including those related to carbon sequestration, wildlife habitat, storm protection, erosion reduction, water purification, fisheries support, and recreation. Mangrove range expansion may also affect wetland stability in the face of extreme climatic events and rising sea levels. Here, we review the current understanding of the effects of mangrove range expansion and displacement of salt marshes on wetland ecosystem services in the southeastern United States. We also identify critical knowledge gaps and emerging research needs regarding the ecological and societal implications of salt marsh displacement by expanding mangrove forests. One consistent theme throughout our review is that there are ecological trade‐offs for consideration by coastal managers. Mangrove expansion and marsh displacement can produce beneficial changes in some ecosystem services, while simultaneously producing detrimental changes in other services. Thus, there can be local‐scale differences in perceptions of the impacts of mangrove expansion into salt marshes. For very specific local reasons, some individuals may see mangrove expansion as a positive change to be embraced, while others may see mangrove expansion as a negative change to be constrained.

Modeling strategies and data needs for representing coastal wetland vegetation in land surface models

“…From a carbon sequestration perspective, mangrove encroachment increases aboveground biomass, but belowground biomass may not differ much relative to salt marshes, depending on site hydrology and geomorphology (Fig. 3; Osland et al ., 2022). Tidal restrictions through infrastructure can increase hydroperiod durations that kill off mangrove forests (Mbense et al ., 2016; Krauss et al ., 2018) and lead to replacement with unvegetated flats.…”

“…Total aboveground biomass N pools in mangroves are c. 4-12 times higher than graminoid salt marsh species (Macy et al, 2021). Belowground biomass of mangrove and graminoid salt marshes may have similar C and N pools, but this is an area requiring further research (Macy et al, 2021;Osland et al, 2022). Succulents produce lower total biomass than mangroves or graminoids and have lower C : N ratios than mangroves or graminoids (Radabaugh et al, 2017).…”

Addressing climate change and development pressures in an urban estuary through habitat restoration planning

“…Dual pressures from sea-level rise and changes in the length of the freezefree season have affected tidal wetlands, such that mangrove forests are outcompeting salt marshes and salt barrens for available niche space. Mangrove expansion as a result of climate change has been observed throughout the Gulf of Mexico (Comeaux et al, 2012;Osland et al, 2022). Anthropogenic water withdrawals have also reduced freshwater flows reaching tidal marshes, contributing to reductions in coverage of key species (e.g., Juncus roemerianus) that have favored mangroves (Raabe et al, 2012).…”

Dispersal and coastal geomorphology limit potential for mangrove range expansion under climate change

“…Research that can predict mangrove range expansions at regional and local scales is therefore needed to inform mitigation and conservation strategies for future decades (Osland et al, 2022).…”

“…Both mangroves and salt marshes are valued for their ecosystem services, particularly in relation to climate change mitigation and adaptation (Duarte et al, 2013; Murdiyarso et al, 2015; Serrano et al, 2019). Research that can predict mangrove range expansions at regional and local scales is therefore needed to inform mitigation and conservation strategies for future decades (Osland et al, 2022).…”