Soil surface elevation dynamics in a mangrove-to-marsh ecotone characterized by vegetation shifts
Mangrove forest encroachment into coastal marsh habitats has been described in subtropical regions worldwide in recent decades. To better understand how soil processes may influence vegetation change, we studied soil surface elevation change, accretion rates, and soil subsurface change across a coastal salinity gradient in Florida, USA, an area with documented mangrove encroachment into saline marshes. Our aim was to identify if variations in the soil variables studied exist and to document any associated vegetation shifts. We used surface elevation tables and marker horizons to document the soil variables over 5 years in a mangrove-to-marsh transition zone or ecotone. Study sites were located in three marsh types (brackish, salt, and transition) and in riverine mangrove forests. Mangrove forest sites had significantly higher accretion rates than marsh sites and were the only locations where elevation gain occurred. Significant loss in surface elevation occurred at transition and salt marsh sites. Transition marshes, which had a significantly higher rate of shallow subsidence compared to other wetland types, appear to be most vulnerable to submergence or to a shift to mangrove forest. Submergence can result in herbaceous vegetation mortality and conversion to open water, with severe implications to the quantity and quality of wetland services provided.
KeywordsAccretion Coastal marsh Mangrove forest encroachment Sea-level rise Subsidence Vegetation change
Funding for this study was provided by the U.S. Fish and Wildlife Service (Intragovernmental Agreements 4500035235, 4500081468) and the U.S. Geological Survey Ecosystems Mission Area. We thank Kevin Godsea, Wade Gurley, and Mark Danaher, U.S. Fish and Wildlife Service, for logistical and technical support. Darren Johnson, Cherokee Nation Technologies, Wetland and Aquatic Research Center, provided data analyses. Comments provided by Donald Cahoon and anonymous reviewers helped to improve the manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. The data are available at https://doi.org/10.5066/P9XZYJ2X (Howard et al., 2019).
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