Long-Term Vegetation Census on St. George Island, Florida
Barrier islands occur across ~60% of the coastline of Florida and have well-established economic and ecological value. These islands maintain a number of important plant and animal species, protect seagrass and oyster beds critical for commercial and recreational fisheries, support tourism and recreation, and buffer inland areas from tropical storms. Barrier islands are created by a synergy between sand supply, winds and waves moving the sand, and plants stabilizing the sand to create dune fields. The role of dune vegetation in creating and stabilizing dunes has been studied world-wide, including some of the earliest studies of plant succession. In Florida, dune ecosystems also act as the “canary in the coal mine” for climate change, because they experience changes in storm intensity and frequency as well as effects of sea-level rise, before inland areas. However, there are almost no studies in the Gulf on how the islands and their vegetation are changing through time.
Vegetation Census since 1999
The Miller Lab from the Department of Biological Science at Florida State University has been conducting a long-term survey of plants and dunes on the eastern tip of St. George Island since 1999. The design includes 9 large grids (60 x 60 m), each of which contains a 7 x 7 grid of permanent stakes, creating 441 sampling points. There are three replicate grids each in the foredune, interdune, and backdune habitats. A 1 m2 quadrat is used at each sampling point each year to estimate cover for all plant species, and a rotating laser is used to determine elevation. Other soil characteristics, such as nutrients, moisture, and percent organic matter, have been quantified at irregular intervals. This study has resulted in 12 publications in peer-reviewed journals, with 4 others in manuscript form. This sort of long-term study is rare: the initial publication of the first 8 years of data has been cited over 250 times.
There are many interesting patterns in this now extensive data set, with ~80 species of plants, 441 plots across 9 plots and 3 habitats, and 26 years. Some of our main conclusions to date include:
- Perhaps obviously, hurricanes can dramatically reduce vegetation cover, while transforming the dune geomorphology, mostly due to storm surge. We have published studies on the significant variation among plant species in their resistance to and recovery from such storms.
- Less obvious, and requiring long-term data, is that most of the vegetation recovers within 2-3 years from tropical storms, leaving minimal evidence of long-term effects.
- We find that dunes on St. George Island have experienced a doubling of species number per 1 m2 over the last 26 years in all three habitats (foredunes, interdunes, and backdunes). Some of the species that were initially present are gone, but even more species have now taken their place. Some of these species have shifted from nearby habitats, while others are new to this part of the island. We did not detect this pattern when we published our analyses of the first eight years of the study – it required longer-term data.
- Important species have shown dramatic changes in abundance, including a 50% reduction in sea oats (Uniola paniculata) cover over the 26 years, and a sudden appearance and rapid dominance of wax myrtle (Myrica cerifera). These patterns would not have been identified without long-term data.
- We believe that these vegetation changes are associated in part with sea-level rise (SLR). SLR raises the freshwater lens that is critical for vegetation and increases soil moisture. Areas that used to have ephemeral water now have permanent wetlands, with a corresponding increase in wetland plant species. On other islands, SLR has been shown to lead ultimately to a reduction in the volume of the lens (increased evapotranspiration), as well as an increase in salt content in the lens. This dynamic would be critical to understand for the management of Florida barrier islands. We have installed a transect of piezometers across the dune habitats to follow the lens depth and salinity over time, funded by the Florida Department of Environmental Protection.
- Other factors, such as the effects of droughts and novel species (e.g., wax myrtle) are currently being analyzed. Preliminary results suggests that effects of droughts are a neglected factor that can affect vegetation as severely as hurricanes.