Plant Community Dynamics in a Mangrove-to-Marsh Transition Zone

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Mangroves will compete with salt marsh plants in transitional areas, and recent studies have documented the expansion of mangroves into marsh habitats. To better understand the plant community dynamics in this transition zone, USGS scientists are tracking vegeation changes over time in south Florida. 

Mangrove-to-Marsh Transition Zone

Mangrove-to-marsh transition zone

The Science Issue and Relevance: The subtropical region of the northern Gulf of Mexico contains a diverse array of marsh and forest habitats. Because of their position in the landscape, these habitats are extremely vulnerable to impacts from projected climate change. Mangroves are typical of subtropical intertidal wetlands, and compete with salt marsh plants in transitional areas. Recent studies have documented the expansion of mangrove forests into marsh habitats in the region. This expansion has been attributed to various factors: major land-development projects that have drastically altered hydrology by decreasing freshwater overland sheet flow; increased salt-water intrusion from rising sea levels; and the absence of winter freezes that can cause mangrove mortality. In areas where infrastructure such as highways and housing developments prevent migration of marshes landward, the ecosystem may be displaced toward primarily forested habitat. Resource managers are, therefore, concerned about the long-term sustainability of wildlife habitats and marsh-dependent birds in areas where mangrove expansion is occurring. 

Sampling permanent vegetation transects

Sampling permanent vegetation transects 

Methodologies for Addressing the Issue: This research focuses on the marsh-to-mangrove transition zone, or ecotone, in the Ten Thousand Island region of south Florida. Permanent vegetation transects were established in 2007 at Ten Thousand Islands National Wildlife Refuge (TTI NWR), near Naples, Florida. Data on herbaceous plant species composition and cover collected at these transects on an annual or biennial basis will document vegetation shifts over time on the refuge. To document the rate of mangrove expansion at TTI NWR, the number and height of mangrove seedlings identified along each transect is also being recorded. The study was expanded in 2011 with the installation of a network of surface elevation tables (SET) that allow monitoring soil surface elevation changes and sedimentation rates. In addition to the possible effects on vegetation assemblages of increased winter temperature, altered precipitation patterns, and accelerated sea-level rise rates that are associated with climate change, the study is also documenting the impacts of a large hydrologic restoration project adjacent to the refuge. The Picayune Strand Restoration Project (PSRP) was initiated in 2006 by the U.S. Army Corps of Engineers with the goal of eventually restoring freshwater overland flow to areas south of the project site, including TTI NWR. Control transects outside of the PSRP impact area are being monitored at Big Cypress National Preserve.

Mangroves are typical of subtropical intertidal wetlands, and compete with salt marsh plants in transitional areas

Mangroves are typical of subtropical intertidal wetlands, and compete with salt marsh plants in transitional areas.

Future Steps: Staff at TTI NWR are developing a prescribed fire plan for the refuge. If instituted, the existing framework of transects and the SET network will allow documentation of fire effects on refuge vegetation assemblages, soil elevation, and sediment accretion rates.