22 Years of Counting Plants: the St. George Island Census

by Dani Davis

It’s no secret; we love our beaches. Floridians across the state take pride in our gorgeous coastline, and visitors from across the world travel to enjoy the sunshine state’s beaches. On the Forgotten Coast, St. George Island is a jewel, adored by locals and visitors alike. For some, this love is couched in a deep understanding of the ecology of barrier islands. Investigating how they grow and change, how they respond to hurricanes, and how they may respond to changing climate conditions – including sea-level rise and stronger hurricanes.

Dr. Alice Winn, an ecologist at Florida State, and Ashley Hennessey, an undergraduate researcher, identify plants in the interdune

Dr. Tom Miller, a professor at Florida State University, is one of these people. Since 1999, Dr. Miller has conducted an annual census of the plant communities at St. George Island State Park to understand how the island changes over time. Through his work, Dr. Miller has seen how the island responds to devastating storms, which plants may be the best for restoration, and how tiny changes in elevation can lead to significant changes in the habitat.

Censusing plants takes a team!
Sea oats blowing in a gentle sea breeze
Sea oats (Uniola paniculata) blowing in the ocean breeze

The Three Habitats of a Beach Dune System

Just across Apalachicola Bay lies the long and narrow St. George Island. A popular destination for tourists, birders, and beachgoers, the island brings visitors from across the county. Many of these visitors find their way into St. George Island State Park to take in the soft white sands and blue-green waters. A beachgoer often interacts with one of the three habitat types on their way to the water, the foredune.

beach morning glory in bloom
A common foredune plant is Beach Morning-glory (Ipomoea imperati) also known as Railroad Vine, often spotted trailing down dune fronts

Foredunes are the first line of dunes running parallel to the ocean; they’re characterized by the ever-lovely sea oats (Uniola paniculata) that top the ridges and beach morning glory (Ipomoea imperati) that winds its way down dune tops and onto the beach.

wetland loving plants in the interdune - a close-up of Largeleaf Pennywort
Behind the foredune lies the interdune, an area full of water loving (or at least tolerating) plants, like the Largeleaf Pennywort (Hydrocotyle bonariensis).

Going behind the foredunes, you enter the wet interdunes. These low-lying areas often fill with fresh water during the wet times of the year and hold onto water in the soils. Wetland plants are easy to find here; species like cattails (Typha) and marsh grass (Spartina) abound, along with treefrogs, wetland birds, and a variety of different snake species.

The backdune. Muhly grass blows in the wind at sunset. Trees are visible on the horizon
The backdune, an area of high biodiversity. Muhly grass (Muhlenbergia capillaris) blooms in the foreground.

After wading through the wet interdune, you enter into the last habitat on the island before reaching the bay, the backdune. This is the oldest habitat on the island, plants have had time to establish, and the conditions aren’t as harsh as in the foredune and interdune. The backdune supports longer-lived species, like pine and oak trees, and shrubby plants like wax myrtle and groundsel. The backdune is also a great place to spot cactus, rattlesnakes, and birds of prey (like eagles and harriers) hunting for their prey.

The census crew walking with their equipment through the interdune.
The plant census crew walks back through the interdune after censusing plants

Digging into the Data- How do St. George Island Dunes Respond to Hurricanes?

Understanding how these habitats change over time has been a research goal of the Miller Lab for the past 22 years. In addition to investigating the ecology of the ecosystems to better understand how barrier islands grow and change, collecting these data is also critical for understanding the effects that anthropogenic and natural disasters have on our coastlines. “We have very little background information, particularly in the Gulf of Mexico, about natural ecosystems. Dr. Miller says while sitting next to one of his foredune plots, “so, then when you have something like an oil spill occur, or you have a major drought occur, or you have climate change – a much slower, going phenomenon – you don’t know what’s changing.”

a plot showing the change in average number of plant species in a plot per year. major hurricanes are noted by grey lines
From the St. George census data, we can see how plant species abundance changes through time. This plot from Dr. Tom Miller shows how the average species number for plot changes over the years, major hurricanes are noted on the year that they occurred

With the long-term data collected during the annual census, Dr. Miller has established a baseline of what the plant communities look like on the dunes and can better understand the effects of large-scale disturbances. This baseline data has been beneficial for understanding the impacts of hurricanes on species diversity on the dunes. The graph above shows us the average number of species per plot for 1999 – 2020, with major hurricanes marked on the year they occur. Looking closely, we can see a general trend – hurricanes cause a drop in biodiversity on the plots.

a small camphorweed plant grows in the foredune
A notoriously salt-intolerant plant – Camphorweed (Heterotheca subaxillaris) disappears after hurricanes

This is often due to a loss of sensitive species that are salt intolerant and don’t survive the overwash. We can also see that the dunes rebound from storms reasonably quickly in some cases, either with a gradual increase in biodiversity after hurricane Dennis or a more dramatic increase as with the years following Michael. From the census data and a strong understanding of baseline conditions, Dr. Miller has proposed which species may be best for restoring dunes after a significant disturbance.

a plot showing the change in specific species through time. Three species are highlighted in the plot, sea oats, camphorweed, and frog fruit
Through the long-term data set, we can take a deep dive into understanding how each specific species changes through time. Plot and images courtesy of Dr. Tom Miller

Not all species respond to disturbance the same. With detailed data on the species level, we can gather insight on how each species responds to storms and how they may be similar or different from one another. The graph above shows the average percent cover from 1999 – 2020 for three common species (Sea oats, Camphorweed, and Frog Fruit) with major hurricanes marked by grey bars.

How do Different Plant Species Respond?

Some species, like Camphorweed, are entirely wiped out by hurricanes – driven to near extinction on the plots when the hurricanes bring a large storm surge. Others, like Sea oats, don’t respond very strongly- as they’re more salt-tolerant- but will decline during strong storms. Then, there’s Frog fruit – an odd plant that seems to do whatever it wants. Many questions are still unanswered about the lives of the plants that live out on the dunes. The census data contains a wealth of information, with plenty of stories still to tell.

For a different perspective on barrier islands responding to hurricanes, read about FSU oceanographer Dr. Jeff Chanton’s research on neighboring Saint Vincent Island.

a ghost crab sits in freshly deposited wrack on the beach
Fresh seaweed (also called wrack) washed up in the early morning on St. George Island.

Future Directions

Now, the Miller lab has set their sights on understanding how the plants get the nutrients they need to grow and establish themselves. Our Gulf beaches are famous for their soft, white sand – but soft white sand is nearly devoid of nutrients, so where are plants getting their nutrients?

Piles of seaweed (mostly seagrass) washed up on the beach at St. George Island State Park

This is a crucial question to understand since the plants build the dunes that protect our coastlines – so healthy plants mean healthy dunes and more resilient coastlines. Understanding nutrient input is a work in progress, but the Miller lab suspects that seaweed may have something to do with it. This is where my own work comes in. Seaweed may act like a fertilizer for dune plants, much like decaying leaves in a forest, or compost added to your garden. I’ll be carrying out a series of experiments in 2022 to test this using a stable isotope tracing study to see how our primary foredune species (Sea oats, Coastal Searocket, and Beach Morning-glory) use the nutrients given off by seaweed as it decays. More on this another time.

a blue crab sits on the shore on the bay-facing side of the island
The census usually lasts at least a weekend, so there is plenty of time to interact with the other creatures roaming the island. A grumpy blue crab was out on the shore one morning.

And finally, a trip to St. George wouldn’t be complete without saying a quick hello to the shorebirds and other creatures that roam the island. Want to see this incredible ecosystem for yourself? St. George Island State Park is a gorgeous 2 hour drive from Tallahassee, and is well worth the trip!

a bird's nest in the interdune
The interdune is a popular place for birds with plenty of cover to forage and raise young in.
a gulf fritillary visits a woody goldenrod plant
A Gulf Fritillary stops to visit a woody goldenrod (Chrysoma pauciflosculosa)
a sanderling forages in the shallow water
No trip to the island would be complete without visiting with a Sanderling (Calidris alba), a very common shorebird on St. George Island
blooming muhly grass blows as the sun sets behind.
The census occurs each year in the fall- since that’s when the grasses flower. That makes them much easier to identify and they’re also just stunningly beautiful during this time.
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