SEA-LEVEL RISE AND THE REDUCTION IN PINE FORESTS IN THE FLORIDA KEYS

Forests dominated by Pinus elliottii var densa have undergone a reduct ion in area in the Florida Keys (USA). A previous investigation interp reted the presence of halophytic species in a former pine forest in Ke y Large as evidence of sea-level rise. We therefore examined aerial ph otos and field evidence to learn how the 15-cm rise in local sea level over the last 70 yr had affected the distribution of pines on a secon d island, where intact pine forests still remained in 1991. The distri bution of in situ dead pine stems showed that the area occupied by pin es on Sugarloaf Key was 88 ha at some time prior to the earliest avail able aerial photographs, in 1935. The area of pine forest was reduced to 46 ha by 1935, and continued to decrease through 1991, when it cove red 30 ha. The pattern of pine mortality was related to topographic po sition, with the areas where pines died earliest occupying the lowest elevations. Our analysis of current vegetation patterns showed that th e areas of earliest pine mortality are now populated by a higher propo rtion of halophytic plant assemblages than areas of more recent pine m ortality. We also compared the physiological responses of pines in two portions of the island: one where pine forest reduction had been most pronounced, and a second where the extent of the forest had changed l ittle over the past 50 yr. Both groundwater and soil water salinity we re higher in the area of rapid pine forest reduction, and the pines sa mpled there exhibited higher physiological stress, as indicated by pre -dawn water potential and stemwood carbon isotope ratios. These result s suggest that the salinization of ground- and soil water that occurs as sea level rises is a major factor in the reduction of pine forests of Sugarloaf Key. If sea level continues to increase, the Florida Keys will experience a decline in both landscape and species diversity, as species-rich upland communities are replaced by simpler mangrove comm unities. This pattern may also occur in other low-lying island ecosyst ems with limited freshwater resources.