Assessing vegetation change in coastal landscapes of the northern Gulf of Mexico

Abstract

Multiple factors have caused rapid changes in coastal landscapes in the last half century. Coastal natural areas have been set aside to mitigate some of these changes for habitat preservation, among other goals. However, areas set aside for conservation are not exempt from these rapid changes. A major concern for coastal wetlands is the potential for habitat loss resulting from external land-use changes and sea-level rise, which essentially threaten these natural areas from all sides. In order to quantify these trends, we determined the types and rates of land-use/land-cover conversion in differing coastal sites in the U.S. along the northern Gulf of Mexico from the 1950s to the 1990s using existing National Wetlands Inventory (NWI) habitat data and Geographic Information Systems (GIS). All sites were located in protected areas and contained an intact marsh-to-forest transition. A buffer zone of ∼2000 m around each site was also analyzed. Two sites, Mandalay National Wildlife Refuge (MNWR) and the Barataria Preserve Unit of the Jean Lafitte National Historical Park and Preserve (JLNHPP), were located on the Mississippi Deltaic Plain in Louisiana, while the other sites, Grand Bay National Estuarine Research Reserve (GBNERR) and Weeks Bay National Estuarine Research Reserve (WBNERR), were located on the Gulf Coastal Plain in Mississippi and Alabama, respectively. Results showed prevalent marsh loss across all sites in the study, although the rate and type of marsh conversion to other land-cover types varied between the Mississippi Delta sites and the Coastal Plain. In the Delta, marsh was converted to open water along shorelines and in internal patches, but the majority of marsh loss was attributed to scrub-shrub encroachment. In the Coastal Plain, marsh was lost more slowly overall, both along the shoreline and forest-marsh boundary. The main trend in the Coastal Plain was replacement of agricultural areas by forest. The buffers experienced an increase in anthropogenically-modified categories, except for a decrease in agricultural areas. Our study suggests that coastal transitions of the northern Gulf of Mexico have indeed experienced landward and seaward losses and that marsh areas are especially vulnerable. It appears that marshes are not keeping pace with the spatial shifts in the aquatic to terrestrial transition as sea level rises, although results in the Coastal Plain are less conclusive because major land-use changes dominate the trends.