Phytosociology of Vascular Plants on an International Biosphere Reserve: Virgin Islands National Park, St. John, US Virgin Islands

—We investigated the relationships of vegetation communities to environmental variables and compared the relative contribution of native and introduced species in extant forest communities on St. John, US Virgin Islands, using an island-wide forest vegetation inventory and monitoring network of permanent plots. We detected 2,415 individuals of 203 species, 5 percent of which were introduced. Cluster analysis, Indicator Species Analysis, and Nonmetric Multidimensional Scaling (NMS) ordination detected four broad species communities divided primarily by moisture and disturbance gradients. Group 1 was characterized by rocky, low-to-mid elevation dry scrub forest on soils with fairly low soil nutrient content; Group 2 was distinguished by low soil nutrient content, high-elevation moist basin forests on steep slopes; Group 3 was indicative of disturbed communities on a wide range of elevations with gentle or no slope across a range of soil types; and Group 4 represented mid-elevation moist forests across a range of steep slopes on nutrientrich soils. Though introduced species are present and widespread on the island, they do not appear to be dominating most plant communities. Exceptions may be those communities with long-standing histories of human disturbance. Achieving an adequate sample of forest types of limited extent or linear spatial patterns such as mangroves and gallery moist forests is difficult with a systematic design. Future sampling should consider some form of stratification targeting these under-sampled forest types.