Living in the shadow of shrub expansion

Background/Question/Methods

Woody expansion into grassland communities is a global phenomenon of current importance. In mid-Atlantic coastal grasslands, woody expansion is dominated by southern wax myrtle, Morella cerifera. This shrub expansion affects long-term barrier island processes through stabilizing sediment, creating a “resistant” island that does not migrate with sea-level rise. Due to winter warming and other climate related factors, Morella outcompetes grassland species. These shrubs modify local microclimate that may affect the adjacent plant community. The objective of this project was to quantify the effect of shrub microclimate on grassland physiology and traits, which can inform resilience of these ecosystems to increased disturbance. Data were collected on Hog Island, VA at the Virginia Coast Reserve, a Long-Term Ecological Research site. Plots (n=10) were established in Spring 2020 along the edge of Morella and in the adjacent grassland community. Microclimate measurements included temperature, soil moisture, and light level (PAR). To assess the plant communities and corresponding traits, measurements included height, percent cover, stem density, biomass, specific leaf area (SLA), and stem specific density (SSD). The dominant species included in the trait analyses were Solidago sempervirens, Spartina patens, and Panicum amarum. These species were chosen due to their presence within both communities.

Results/Conclusions

Shrub expansion microclimate modification alters grassland productivity and community structure along the shrub edge. Microclimate within the shrub edge is wetter (p< 0.05), more shaded (p< 0.001), and with moderated temperature compared to open grassland. This resulted in increased height (p< 0.001), biomass (p< 0.001), and percent cover (p=0.01) of total vegetation in grassland plots adjacent to Morella shrubs. These results were primarily due to dominance of S. patens on the edge. SLA was significantly higher along the shrub edge (p< 0.001), which is typical in shaded environments. These differences in community traits indicate a competitive environment at the shrub edge, due to moderated temperature, lower light availability, and increased water availability. This competitive environment led to decreased species richness in plots adjacent to shrubs (p=0.01). Expansion of woody cover results in cascading ecosystem effects which may alter future coastal resilience to climate change. Future analyses will determine the role of nitrogen from Morella (a known nitrogen fixer) to the adjacent community.