Mechanisms of Native Shrub Encroachment on a Virginia Barrier Island

 

Recent shrub encroachment is generally recognized as a response to anthropogenic disturbance and often a threat to ecosystems, although historically, shrubs represent a shift in successional states after a natural disturbance.  Negative effects associated with recent shrub expansion include decreased species diversity, extreme alteration in community structure, the creation of irreversible alternate stable states, nutrient cycling shifts, and increased susceptibility of shrubland compared to previous ecosystems.  On the Virginia barrier islands, Morella cerifera thickets clearly represent a different community structure compared to grasslands, and decreased plant diversity as well as increased soil nitrogen has been observed with the shift to shrubland. Joseph Thompson's master's research evaluated the effects of Morella cerifera on fine-scale abiotic and biotic factors upon encroachment into grassland. Species composition, temperature, soil nutrients, and leaf area index (LAI) were recorded across three encroaching M. cerifera thicket edges and three free-standing shrubs on Hog Island, Virginia to characterize the effect of shrub thickets on the plant community and microclimate. Electron transport rate (ETR) was taken on shrub leaves to determine if microclimate benefits M. cerifera physiology. Species richness was lowest inside shrub thickets. Soil water content and LAI were higher in shrub thickets compared to grassland. Soil organic matter, N, and C were higher inside shrub thickets. Summer and fall maximum temperatures were more moderate in shrub thickets and at free-standing shrubs. Fall and winter minimum temperatures were warmer inside shrub thickets. ETR was higher at the free-standing shrubs compared to the thicket edge. Morella cerifera significantly changes the microenvironment including temperature, edaphic factors, and plant species composition. These results show that expansion of M. cerifera in coastal systems has an immediate and significant impact on the surrounding environment.