Barrier islands are important for coastal protection but are extremely vulnerable to sea level rise and increasing storm frequency and intensity. Rapid and extensive vegetation change has been observed on the Virginia Coast Reserve islands as swale grasslands are invaded by shrubs, dominantly Morella cerifera. The shrubs eventually displace grasses completely and form dense, monospecific thickets. Islands undergoing shrub expansion have also experienced a decline in island area, likely resulting from shrubs interrupting normal sediment movement across the island. Therefore, it is important to understand mechanisms driving this shrub encroachment. One factor may be facilitation during early stages of encroachment of shrub seedlings by grasses through protection from excess heat and solar radiation. My study objectives were (1) examine facilitation by experimentally removing grass from plots with newly established M. cerifera seedlings and (2) quantify microclimatic effects on seedling survival, growth, and physiological performance. To examine the interactions between grass and shrub seedlings, grasses were clipped away from seedlings in half of my plots containing shrubs. Sub-grass canopy air temperatures, leaf temperatures, photosynthetically active radiation (PAR), soil moisture, and leaf area index (LAI) were monitored to quantify microclimatic effects of grasses.
Results/Conclusions
LAI was significantly higher in grassy areas where seedlings are found. Under grass canopy, seedling leaf temperature was lower (31.9 ± 0.3°C) relative to clipped plots (35.1 ± 0.4°C). Light was 55% lower in grassy plots and more quickly attenuated as grass density increased. Soil moisture was significantly lower in clipped plots. Grasses may encourage shrub seedling establishment by ameliorating harsh environmental conditions found on barrier islands, specifically high light and temperature. The interaction between the shrub and the grasses likely switches from facilitative to competitive as shrubs grow above the grass canopy. With changing climate, species’ range expansions may be moderated by facilitation. My research serves as the beginning of a long-term study to determine the role of biotic interactions and possible feedbacks in the grass-shrub transition. By following shrubs from the seedling life stage, we can better predict rates of the grassland – shrubland transition and the mechanisms that drive this. The fast transition from grassland to thicket, often occurring in only 15 years, and the impacts that this vegetation shift has on island sediment dynamics, make understanding this process important.