Grass density and dune elevation influence the spatial distribution of shrub encroachment in coastal grassland

Natasha Woods, Department of Biological Sciences, Moravian College, Bethlehem, PA, Natasha Woods, Caroline Baucom, Michael Sinclair and Julie Zinnert, Department of Biology, Virginia Commonwealth University, Richmond, VA

Background/Question/Methods
The phenomenon of woody plant encroachment into grasslands has rapidly increased worldwide over the last two centuries. Woody plant encroachment causing desertification in grasslands has been well documented. However, the effect of woody plant encroachment in coastal grasslands is not as well studied. Woody plant encroachment into barrier islands can have severe implications on the natural migration of islands when disturbance reinforcing species (e.g. grasses) are replaced by disturbance resisting species (e.g. shrubs). At the Virginia Coast Reserve, a Long-Term Ecological Research site, the expansion of evergreen Morella cerifera been most acute with a 40% increase in nearly 30 years. Morella cerifera adults have a positive impact on their own establishment, while precluding the establishment of shade intolerant species. Relatively little is known about the early establishment of M. cerifera. The extent to which abotic (i.e. light availability, soil characteristics, dune elevation) and biotic (i.e. proximity to adult shrubs, grass density, height and biomass) factors contribute to the success of M. cerifera at early stages of development still needs to be examined. The objective of this study was to examine the extent to which the establishment of M. cerifera is impacted by the abiotic and biotic environment. Morella cerifera seedlings were identified within distinct communities on the southern end of Hog Island, VA (i.e. edge of shrub thicket, grassy swale, dry grassland, and bare). Along this range 10 cm x 10 cm paired grass only and shrub seedling/grass plots were delineated. Abiotic and biotic factors were quantified by measurements of grass biomass and density, seedling and soil characteristics, and dune elevation.

Results/Conclusions
Seedlings were found predominately in areas of relatively low grass density and a narrow dune elevation of approximately 1.5 m. This suggests that shrub seedlings grow where they are less likely to have to compete for light and nutrients. Positive correlations were detected between soil organic matter and seedling height, as well as between grass biomass and seedling height. These findings collectively demonstrate that seedlings are favored in interdune grassland swales where conditions appear to be optimal for encroachment of M. cerifera seedlings. Grass density and dune elevation strongly impact the spatial distribution of shrub encroachment on Hog Island. The increase in the spatial distribution of M. cerifera could have detrimental consequences for Virginia barrier islands with sea-level rise.