SIMULATION OF TRANSPIRATION SENSITIVITY TO ENVIRONMENTAL-CHANGES FOR SHRUB (MYRICA-CERIFERA) THICKETS ON A VIRGINIA BARRIER-ISLAND

Shrub (Myrica cerifera) thickets are a major vegetation type on barrie r islands. To evaluate the effects of moisture stress on shrub thicket distribution, a water relation model, MCHOG, was developed for Hog Is land, a barrier island along the eastern shore of Virginia, USA, The m odel consists of three submodels: a transpiration submodel based on th e Penman-Monteith equation, a rooting-zone water-balance submodel, and water-table dynamics submodel using Darcy's law, Precipitation, relat ive humidity, wind speed, solar radiation, and air temperature data co llected on Hog Island in 1991 were used with an hourly time step to si mulate transpiration patterns for shrub thickets along a cross-island transect. Transpiration under sufficient soil moisture was defined as potential transpiration. Comparisons were made between the potential t ranspiration pattern and the transpiration patterns under various envi ronmental change scenarios. Simulation results suggest that the shrub thicket transpiration pattern is sensitive to climate change, landscap e change, and sea-level rise. Soil water limits transpiration of all s hrub thickets on barrier islands. Elevationally related capillary rise from groundwater also plays an important role in determining shrub di stribution patterns. Competition may exist for groundwater resources a mong individuals of shrub thickets and between shrubs and associated g rasses,