Floristic composition across a climatic gradient in a neotropical lowland forest

This study deals with the floristic composition of lowland tropical forest in the watershed of the Panama Canal, The floristic composition of large tr ees in 54 forest plots was analysed with respect to environmental factors, including precipitation, geologic parent material, stand age, topography, a nd soils. The plots contain 824 species of trees with a diameter at breast height greater than or equal to 10 cm and represent a regional flora with e xceptional beta -diversity. Plot data indicate that the Panamanian forest i s strongly spatially structured at the landscape scale with floristic simil arity decreasing rapidly as a function of inter-plot geographic distance, e specially for distances < 5 km. The ordinations and patterns of endemism ac ross the study area indicate broad floristic associations well correlated w ith Holdridge life zones. The results indicate the positive aspects of life zone classification at regional scales, while simultaneously highlighting its inadequacy for finer scales of analysis and resource management. Multiv ariate gradient analysis techniques (Non-metric Multidimensional Distance S caling and Detrended Correspondence Analysis) show clear patterns of floris tic variability correlated with regional precipitation trends, surficial ge ology, and local soil attributes. Geologic and edaphic conditions, such as acidic soils or excessively drained limestone substrates. appear to overrid e the effects of precipitation and modify forest composition. We conclude t hat the Panamanian forest shows clear patterns of spatial organization alon g environmental gradients. predominantly precipitation. The rapid decline i n floristic similarity with distance between stands also suggests a role fo r dispersal limitation and stochastic events.