Ecology of Barrier Island Plant Communities

Long-term monitoring of vegetation dynamics on St. George Island, FL

Since 1992, the Miller Lab has been working on various aspects of the vegetation on the eastern tip of St. George Island, Florida. St. George is a typical microtidal barrier island located on the Gulf Coast of the Florida panhandle. Its 48 km length and narrow width (0.5 to 2 km) are sculpted by a low tidal range and frequent overwash by storms. Habitat zones are determined by the island's characteristic topography, with a high fronting line of foredunes on the ocean side, behind which there is an overwash interdune, with older, more stable backdunes on the bayside. This dune structure is common to many barrier islands worldwide and is thought to be due, in part, to interactions with vegetation. Dunes and troughs create a heterogeneous habitat for a variety of plants, while the plants act to stabilize the geomorphology.

We originally worked on the effects of competition and herbivory on Sebatia stellaris and on local adaptation in Hydrocotyle bonariensis (Knight and Miller 2004). However, from the very beginning we were struck by how much the dune vegetation was a shifting mosaic of different species. From year to year, the identities and locations of the plants changed, sometimes dramatically. Although hurricanes were an obvious factor determining plant community patterns, it also appeared that there was significant variation in hurricane sensitivity among plant species, as well as perhaps other factors at play.

To better understand these patterns, we established at set of six 60 m x 60 m plots on the eastern tip of the island in 1998. Two plots were established each in the high but unstable front dunes, the wet and lower-lying interdunal areas, and the more stable rolling back dunes (this has since been expanded to nine plots, with three in each habitat). Within each plot, we set up permanent markers every 10 m in a 7 x 7 grid. We have sampled a 1 m squared plot at each of the 294 grid points since 1998 (Figure 1). These plots have been expanded to 3 replicates in each habitat in 2010, with 441 plots overall.

Some dynamics are apparent already (see Miller et al. 2010). For example, Heterotheca subaxillaris and Oenothera humifusa are intolerant of saltwater inundation. Such species spread widely across the dunes between storms (such periods may last several years), only to be dramatically restricted to the highest dune areas or even totally eliminated following high storm surge. Other species such as Uniola paniculata and Schizachyrium maritimum are relatively robust and may be particularly important for dune recovery following hurricanes and other major storms (Gornish and Miller 2010, 2013).

Patterns of vegetation change from 1999 to 2015 in animations:

Click on the links below to watch how populations of each species change through time

Centella asiatica Hydrocotyle bonariensis Phyla nodiflora
Eragrostis lugens Ipomea imperati Schizachyrium maritum
Fimbristylis spp. Muhlenbergia capillaris Smilax auriculata
Fuirena scirpoidea
Dichanthelium aciculare Spartina patens
Heterotheca subaxillaris Paspalum distichum Uniola paniculata

Coastal Vegetation Patterns and the Deepwater Horizon Oil Spill

In 2010, the Deepwater Horizon (DWH) oil spill occurred, which has significantly affected our research program. The details of the initial DWH incident are well known: a deepwater offshore oil-drilling rig exploded on 20 April 2010 at the Macondo Prospect off the coast of Louisiana. An estimated 4.9 million barrels of crude oil were released by the resulting sea-floor oil gusher until it was capped on July 15, 2010. Although the fate of much of the oil remains unknown, by 9 July 2011 some 790 kilometers of coastline in the northern NGM were contaminated by DWH oil, including much of the Louisiana, Mississippi, and Alabama coastlines, and the western panhandle in Florida (Fig. 2).

We realized that we had one of the very few long-term data sets for coastal vegetation in the Gulf of Mexico. Further, our data suggest that species are highly variable in these habitats, so that separating out any possible “signals” of oil on coastal vegetation may be difficult, given the ‘noise” created by normal stochastic forces. In such cases, long-term data are invaluable to distinguish if oil does or does not have any affects on the vegetation of barrier islands.

So we initiated a new census at six more barrier islands sites, scattered somewhat regularly around the northern Gulf of Mexico, with the intent to place the sites over a gradient of oil exposure (Fig. 2). With support from the National Science Foundationand the Northern Gulf Institute, we have now initiated six additional long-term sites on barrier islands spread across the northern Gulf of Mexico. These sites are approximately 160 km apart along a gradient of oil exposure and include Trinity Island (LA), Horn Island (MS), Santa Rosa Island (FL), East Crooked Island (FL), Anclote Key (FL),and Cayo Costa (FL), all censused using the methods developed for the on-going census on St. George Island, FL.

We have now censused most of these sites for 2010 and 2014; the data are publicly available through these pages (see below).

All questions about this research or about the data should be addressed to T. Miller, Dept. of Biological Sciences, Florida State University, Tallahassee, FL 32306, 850-644-9823 or


Species List

Census Data



Revised April 22, 2015