Woody vegetation has expanded beyond historic ranges due to increased temperatures, altered precipitation patterns, and changed grazing pressures; woody expansion occurs globally in prairies, tundra, and coastal grasslands. Barrier islands protect dense populations and areas of economic importance along coasts and are affected by atmospheric and oceanic drivers of climate change. The mid-Atlantic represents a hotspot of sea-level rise, 3-4 times higher than the global mean. Despite significant reductions in island surface area due to sea-level rise, woody vegetation has increased by 40% over the last 30 years, encroaching into historic grassland. Morella cerifera, an evergreen shrub, associates with nitrogen-fixing Frankia, enhancing productivity in an otherwise nutrient-limited environment. Our objective was to quantify changes and feedbacks in local microclimate and biotic composition associated with shrub expansion at the Virginia Coastal Reserve LTER site. We hypothesized that moderation of microclimate by M. cerifera enhances transpiration and increases depth to the water table, while reducing species composition through increased canopy cover. Species composition and leaf-area index (LAI) were assessed annually during two growing seasons across three landscape types: grassland, transition, and shrub plots. Ground temperature, air temperature, and water table depth were measured hourly. Stomatal conductance was measured during one growing season.
Grassland exhibited significantly cooler winter and warmer summer temperatures (1 and 18oC, respectively). Higher variability in temperatures occurred in transition plots as shrub cover increased. Microclimate in shrub plots was moderated and summer temps were close to the photosynthetic temperature optimum (30oC) of M. cerifera. Depth to the freshwater lens was increased in the shrub and transitional plots compared to grassland plots. This was positively associated with higher stomatal conductance of M. cerifera. Differences in water table depth were mitigated by precipitation input seasonally. Lower water table, high stomatal conductance, and high LAI decreased the persistence of grasses and further facilitated establishment of shrubs and expansion of the thicket. Species composition was most diverse in the transition plots, possibly due to enhanced N input; however, conversion to thicket significantly reduced diversity as plots had 100% cover of M. cerifera, with LAI >6. Positive feedbacks between shrub growth and microclimate are mechanisms of shrub encroachment in coastal grasslands. Interactions between biotic and abiotic processes are important to quantify in systems known to be heavily influenced by physical drivers.