The Floodplain

When Ph.D. candidates Linsey Haram and Rachel Smith began their research in the Odum School of Ecology, they did so with the intention of using science to improve conservation and benefit their community. Since both Smith and Haram’s work focuses on marine community ecology and invasion biology, the applications of their research in Athens, Georgia, were not immediately evident.

“I’ve been looking for something I could do locally, since all of my research is based on the coast,” says Smith.

Using their background as invasion biologists, Smith and Haram are working to improve restoration in the floodplain.

“There’s a lot of restoration that happens that is not taken advantage of by scientists to see if what is occurring in the restoration is actually ecologically beneficial or resilient,” says Haram.

The team is testing different management strategies to determine how to remove invasive species most efficiently.

What is an invasive species?

Nonnative species are species that have been introduced, usually by humans, into an area where they do not naturally occur. A nonnative species is considered to be invasive when it spreads and causes “harm” to the ecosystem it inhabits. When a species is introduced into an area outside of its native range, it often has an intrinsic advantage over the native species in the area because of its lack of natural predators or competitors. The resulting unchecked growth and prolific spread of an invasive species can limit the amount of resources available for native species, disturb the ecology of an ecosystem, lead to the extinction of other species, and impose a great economic expense.

Because invasive species lack the checks and balances that normally keep their numbers under control in their native range, human intervention is often required. In many cases, removing an invasive species from an ecosystem or habitat has been shown to improve biodiversity, but it is often very challenging and expensive to completely eradicate an invasive species. In the Georgia piedmont, invasive species like kudzu and the boll weevil are known for the damage they cause to property and crops. Invasive species like these, ones that pose an immediate economic expense to humans, tend to be well studied and addressed.

Chinese privet

Introduced as an ornamental hedge, Chinese privet (Ligustrum sinense) has since established itself in the floodplains of the southeastern United States. The shrub has been able to spread broadly because it is well suited for Georgia’s climate and has demonstrated a tolerance for shade. The spread of Chinese privet has impacted many forests’ understories: areas with a high density of privet have been shown to have lower plant diversity than areas free of privet. Once it has grown, Chinese privet can completely cover the forest floor and outcompete native plants for sunlight, water, and other resources (Merriam et al., 2002).

Because of Chinese privet’s impacts on biodiversity, efforts have been made to remove this invasive species from the riparian forests along the Oconee River. Several different sites, including Sandy Creek Nature Center, the State Botanical Garden of Georgia, Scull Shoals Experimental Forest, and Watson Springs Forest, were cleared of privet with the use of clear cutting and herbicides. As expected, areas with privet removal have shown signs of recovery and increased biodiversity. In addition to an increase in native plant species, sites cleared of privet also experienced an increase in bee abundance and diversity (Hanula et al., 2011) as well as an increase in native earthworm species and a subsequent decrease in exotic earthworms (Lobe et al., 2014). Somewhat unexpectedly, some of the sites have shown an increased abundance of the invasive grass Microstegium vimineum, an annual commonly known as Japanese stiltgrass, after being cleared of privet (Hanula et al., 2009).

Japanese stiltgrass

Japanese stiltgrass, while not as dense or destructive as Chinese privet, poses a challenge for conservation and restoration (Hanula et al., 2009). In the State Botanical Garden, sites that had previously been dominated by privet were cleared to encourage the establishment of native plants, only for these sites to become overrun with Japanese stiltgrass. These blankets of stiltgrass prevent the establishment of native plants and reduce species abundance. In addition to outcompeting native plants, Japanese stiltgrass has also had consequences for native fauna. The prevalence of the grass on the floodplain has indirectly impacted the abundance of toads. Wolf spider predation on toads has increased in areas with a high density of Japanese stiltgrass (DeVore et al., 2014).

The spread of Japanese stiltgrass after the removal of privet can be attributed to several different factors. Before its removal, Chinese privet in the upper Oconee River floodplain had reached very high densities, at one time covering 59% of the floodplain (Ward, 2002). The clearing of privet would have allowed more sunlight to reach the floodplain understory, to which Japanese stiltgrass responds positively (Hanula et al., 2009). Additionally, disturbances such as flooding aid in the dispersal of stiltgrass seeds. When these seeds reach large areas of bare soil, such as those found at privet removal sites, it is much easier for the grasses to establish and spread (Judge et al., 2008). These factors are important to understand, because knowing how the invasive grass establishes and spreads will help inform how it should be managed and removed.

Restoring the floodplain

The goal of privet removal in the State Botanical Garden was to increase biodiversity and return the floodplain to its pre-invaded state, but the recent explosion of Japanese stiltgrass is making that objective difficult. The stiltgrass must be removed before a native plant community can be established, but this is no easy feat. Japanese stiltgrass has seeds banks that can last up to three years, so even if all individuals could be removed from the site, this would not eliminate the invasive species completely. Additionally, eradicating individuals through mowing or hand removal could lead to the establishment of a different exotic species, as seen with privet removal (Judge et al., 2008).

Though privet removal on this part of the floodplain has been successful, continued monitoring and eradication is needed to maintain pre-privet conditions. Additionally, clear cutting privet alone does not restore the floodplain to pre-privet conditions due to the establishment of different exotic species, like Japanese stiltgrass. Therefore, complementary methods, such as planting native understory plants, may be necessary.

This is where Linsey Haram and Rachel Smith’s research project comes in. The team is testing the effectiveness of river oats as a potential competitor for stiltgrass. River oats are a native perennial that occur naturally in the floodplain. Once established, river oats can grow much taller than Japanese stiltgrass, which gives them a competitive edge against the invasive stiltgrass.

“The idea is that if you can get river oats to take hold they will likely be able to come back every year, and because they would have that lifecycle advantage over Microstegium, which is an annual, then hopefully you would not get an influx of stiltgrass,” says Haram. “The other thing is that river oats are supposed to be a dominant plant in the floodplain ecosystem but they’ve been punched back a lot by other invasions, like the privet invasion, nonnative bamboo… There are lot of other things that have outcompeted river oats in their native environment.” If the river oats prove to be a successful competitor against Japanese stiltgrass, planting river oats in the floodplain could discourage the spread of stiltgrass and support the spread of other native species.

The team set up ten plots on a site that had a very high density of stiltgrass. Every plot was further divided into ten subplots, which were each administered a different stiltgrass treatment. These treatments included herbicide, river oat seeds, a combination of herbicide and river oat seeds, and a combination of herbicide and adult river oats. Half of the subplots in each plot were treated on a frequent basis while the other half were only treated annually. Two of the subplots in each plot were left untreated as a control group. Every plot received each of these treatments, but the placement of each treatment was randomized to ensure that results were not influenced by external factors. The team then measured the average impact that each treatment had on both stiltgrass densities and native plant densities across all ten plots.

“The hope is that, in a couple of years once we’ve been able to look at the effect of the various treatments on the Microstegium densities as well as the native plant densities, that we can identify which practice makes the most sense for the native plant center at the Botanical Garden to move forward,” says Haram.

Looking forward

The challenge with restoring and preserving this valuable floodplain habitat is that frequent disturbance sometimes favors the spread of invasive species. Even if the floodplain is cleared of invasive stiltgrass, it is very possible that a heavy flood could reintroduce it.

“It’s not enough to simply remove the adult plants that are there, but you want to keep them out in the future by planting something else that will grow vigorously and outcompete any new seedlings coming in,” says Dr. Richard Hall, an assistant professor with the Odum School of Ecology. Dr. Hall, who has worked closely with the State Botanical Garden, advised that restoration sites must be revisited to prevent potential infestation in the future. Developing a management strategy that removes invasive species from the floodplain and replaces them with competitive native species is essential for restoring the habitat.

Though the resurgence of Japanese stiltgrass was an unwanted side effect, the riparian forests of the Georgia piedmont have mostly benefitted from the removal of Chinese privet. The appearance of several other invasive species at privet removal sites is troubling, but the resurgence of other native species has given the State Botanical Garden hope about future restoration efforts. Corydalis flavula, also known as yellow fumewort, is a rare native plant that was found in the gardens after the removal of privet. Only found in four counties in Georgia, the yellow fumewort is a testament to native plant diversity and invasive species management.

The project can be found near Moon Parking next to the Native Plant Center in the State Botanical Gardens.

Works Cited

DeVore, J. L., & Maerz, J. C. (2014). Grass invasion increases top-down pressure on an amphibian via structurally mediated effects on an intraguild predator. Ecology, 95(7), 1724-1730.

Hanula, J. L., Horn, S., & Taylor, J. W. (2009). Chinese privet (Ligustrum sinense) removal and its effect on native plant communities of riparian forests.

Hanula, J. L., & Horn, S. (2011). Removing an invasive shrub (Chinese privet) increases native bee diversity and abundance in riparian forests of the southeastern United States. Insect Conservation and Diversity, 4(4), 275-283.

Lobe, J. W., Callaham, M. A., Hendrix, P. F., & Hanula, J. L. (2014). Removal of an invasive shrub (Chinese privet: Ligustrum sinense Lour) reduces exotic earthworm abundance and promotes recovery of native North American earthworms. Applied soil ecology, 83, 133-139.

Judge, C. A., Neal, J. C., & Shear, T. H. (2008). Japanese stiltgrass (Microstegium vimineum) management for restoration of native plant communities. Invasive Plant Science and Management, 1(2), 111-119.

Merriam, R. W., & Feil, E. (2002). The potential impact of an introduced shrub on native plant diversity and forest regeneration. Biological Invasions, 4(4), 369-373.

Ward, R. W. (2002). Extent and dispersal rates of Chinese privet (Ligustrum sinense) invasion on the upper Oconee River floodplain, North Georgia. Southeastern Geographer, 42(1), 29-48.