Niche requirements and competitive effects of a new forest invader, Oplismenus hirtellus spp. undulatifolius (wavyleaf basket grass)

Background/Question/Methods

Exotic species pose an increasing threat to natural communities, with negative impacts on environmental processes and local and global economies. Understanding where a new species is likely to be most competitive and how it may impact native species is a crucial component of natural resource management. The goal of this study was to characterize the functional niche of a new invasive grass, Oplismenus hirtellus spp. undulatifolius (wavyleaf basket grass), its impact on native species, and its ability to compete with the exotic Microstegium vimineum (stilt grass). Wavyleaf was discovered in Maryland in 1996 and occurs on over 400 hectares in Maryland and Virginia. Preliminary observations indicate that wavyleaf forms a monoculture in forest understories but that stilt grass dominates at forest edges and gaps where light intensities are higher. To investigate habitat preferences and competitive abilities of wavyleaf, 280 1m² plots were established along 20 transects beginning at the forest edge and running 50 m into the forest interior. Percent cover by species, soil moisture, canopy cover, transmitted light, and cover and depth of leaf litter were measured in each plot. Correlation analyses were used to examine relationships between wavyleaf and stilt grass cover, species richness, and environmental variables. 

Results/Conclusions

Distance from forest edge and leaf litter depth were strong predictors of both wavyleaf and stilt grass cover. Wavyleaf and stilt grass cover were negatively correlated, with wavyleaf more abundant in forest interiors and areas with deep litter and stilt grass more abundant at forest edges and in areas with shallow litter. Cover of both species increased as light levels and/or canopy openness increased. Increases in both wavyleaf and stilt grass cover were related to decreases in species richness. Stilt grass, an annual species, may be less competitive in areas with deep litter where wavyleaf, a perennial species can push new growth through several centimeters of leaves. There was no support for the hypothesis that high light levels restrict wavyleaf abundance at forest edges, but edge plots may have been too close to the canopy to measure full light conditions. Decreased species richness associated with wavyleaf may be due to competition or may indicate that wavyleaf is only capable of invading areas with low richness. As a shade tolerant invader, wavyleaf poses a threat to forest ecosystems but more work is needed to identify how this species interacts with both native and exotic species in forest understory communities.