Climate change and coastal wetlands: the importance of temperature and rainfall regimes

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This article is part of the Spring 2016 issue of the Earth Science Matters Newsletter

Coastal wetlands are highly-productive and valuable ecosystems. In addition to providing habitat for many fish and wildlife species, coastal wetlands store carbon, provide food, improve water quality, protect coastlines, support coastal fisheries, and provide recreational and tourism opportunities. However, due to their position at the land-sea interface, coastal wetlands are vulnerable to many of the impacts that accompany a changing climate. As a result, resource managers and other decision makers are increasingly challenged to prepare for future change in a way that ensures healthy coastal wetlands.

salt marsh and mangrove trees in coastal wetland

A freeze sensitive mangrove-marsh transition zone in Cedar Key, FL. 

(Credit: Michael J. Osland, USGS. Public domain.)

Across the globe and in all ecosystems, macroclimatic drivers such as temperature and rainfall regimes govern ecosystem structure and function. Macroclimatic drivers have been the primary focus of climate change-related threat evaluations for terrestrial ecosystems, but they have been largely ignored for coastal wetlands. Climate change vulnerability assessments for coastal wetlands have generally focused solely on sea-level rise impacts (e.g., inundation) without considering other important aspects of climate change.

In a recent publication, scientists funded by the USGS Climate Research & Development Program have demonstrated the importance of incorporating macroclimate drivers into climate change vulnerability assessments for coastal wetlands.

Results of the study show that the southeastern United States is an excellent place to illustrate the global importance of temperature and rainfall regimes upon coastal wetland ecosystems. Coastal wetlands are abundant in this region; approximately eighty percent of the coastal wetlands in the contiguous United States are located in the southeastern portion of the country. From a macroclimatic perspective, this region is especially sensitive because these abundant coastal wetlands continuously span two climatic gradients (i.e., a rainfall gradient and a winter severity gradient) that greatly influence coastal wetland ecosystem structure and function and the supply of ecosystem goods and services. In hot and wet climates, mangrove forests (woody plants) are dominant. However, mangroves can be killed or damaged by extreme freeze events. So, in cold and wet climates, salt marsh grasses, sedges, and rushes are dominant. In dry climates, conditions often are too salty for plants; so, coastal wetlands in arid climates are often unvegetated salt flats.

In the future, changes in the frequency and intensity of extreme freeze events could lead to the poleward migration of mangrove forests at the expense of salt marshes in Texas, Louisiana, and parts of Florida. In drier regions (that is, south and central Texas), changes in freshwater availability are expected to result in changes in the abundance of coastal wetland foundation plant species. Such changes will affect some of the goods and services provided by these ecosystems. The intent of this research is not to minimize the importance of sea-level rise. Rather, the overarching aim is to illustrate the need to also consider macroclimatic drivers within vulnerability assessments for coastal wetlands.

The paper, Beyond Just Sea-Level Rise: Considering Macroclimatic Drivers within Coastal Wetland Vulnerability Assessments to Climate Change, was published in Global Change Biology. It is available at: https://onlinelibrary.wiley.com/doi/full/10.1111/gcb.13084

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