The taxonomic composition of the vascular epiphyte flora at the Río Changuinola, Panama, was examined and complemented with an analysis of biogeographic affinities and physiological parameters related to plant water and nutrient relations. In an area of ca. 1000 ha, we found a total of 476 species of vascular epiphytes. This marks a new diversity record among lowland rainforest sites. Species composition was closely related to nearby lowland forest sites but not to montane sites. The floristic similarity with lowland sites decreased with distance and relative position towards the Andes. On basis of isotope discrimination, the proportion of species with Crassulacean Acid Metabolism (CAM) was found to be low compared to other studies, and many of these species showed a rather weak expression of this photosynthetic pathway. This observation and distributional shifts in 15% of the species in the study area towards lower elevations suggest that local water availability is high which in turn is arguably responsible for the high species richness.