Removal of the exotic forb, Galium verum, from grassland communities: The role of native biodiversity in invasions and community recovery

Background/Question/Methods

Invasive species can have serious impacts on natural communities and ecosystems. Researchers suggest that native species diversity may play an important role in the ability of communities to resist invasion. Others suggest that these “diversity” patterns may actually be driven by particular dominant species, which control community productivity and invasibility regardless of species richness. Species diversity and the presence of dominant species may also play important roles in community recovery after invasive species removal.  Galium verum is an emerging invasive forb in Virginia’s Shenandoah Valley. We sought to determine the effect of native grass diversity on Galium success and the effect of Galium removal on community composition. Our experiment consisted of three blocks of five grass species combinations. In two of these combinations, we planted either the dominant grass Andropogon gerardii in monoculture or a combination of A. gerardii with Schizachyrium scoparium and A. virginicus. Two other combinations were planted with Bouteloua curtipendula either in monoculture or with S. scoparium and A. virginicus. A fifth combination included six different grass species. Within each combination, we tested common removal methods (hand-pulling, 2,4-D herbicide) to control Galium verum, which had heavily invaded all plots. We followed these plots for two growing seasons.

Results/Conclusions Both herbicide and hand-removal effectively suppressed Galium growth (p = 0.003) and enhanced biomass production of warm-season grasses (p = 0.01). Grass species diversity had a substantial effect on above-ground biomass production (p = 0.02) and overall species richness (p = 0.002). Plots containing the dominant tallgrass, A. gerardii, possessed significantly higher aboveground biomass and lower species richness than plots containing the less dominant Bouteloua curtipendula. These results suggest that species identity is more important for controlling community processes than species richness per se. Although there were significant main effects of removal method and grass diversity, we found no significant interactions. This indicates that Galium removal and community recovery were equally successful at all levels of grass diversity. Hand removal successfully controlled Galium, but is highly labor intensive and impractical for large areas. However, application of the herbicide 2,4-D may control Galium spread while maintaining forb species diversity in invaded tallgrass prairie.