SALINITY AND THE SMALL-SCALE DISTRIBUTION OF 3 BARRIER-ISLAND SHRUBS

The importance of salinity to small-scale distribution patterns was ex amined for three shrubs common on barrier islands of the southeastern United States. Field measurements focused on the salt marsh - upland i nterface zone on Hog Island, Virginia, where Myrica cerifera, Bacchari s halimifolia, and Iva frutescens form distinct distributional zones. Although considerable variation in salinity occurred throughout the gr owth season (June through October), total soil chlorides and groundwat er salinity were lowest for M. cerifera, intermediate for B. halimifol ia, and highest for I. frutescens. All three species showed similar di urnal and seasonal patterns in stomatal conductance and leaf xylem pre ssure potential, despite the differences in salinity. However, a labor atory experiment revealed interspecific differences in water relations when the three shrubs were exposed to identical salinity regimes. The field data and water relations experiment indicated M. cerifera is le ast tolerant to salinity, I. frutescens is most tolerant, and B. halim ifolia is intermediate. Seed germination experiments revealed a simila r response, except that B. halimifolia was more sensitive to salinity than M. cerifera. The interspecific differences in soil and groundwate r salinity, along with the physiological response differences, indicat ed that salinity may be one of the major environmental factors influen cing zonation among the three shrubs; however, the absence of I. frute scens and B. halimifolia in low salinity areas implied that other fact ors also influence zonation patterns on barrier islands.