Pteridophyte diversity and species composition in four Amazonian rain forests

Local variation in individual density, species composition, species richnes s and species diversity of terrestrial pteridophytes were studied at four s ites in the tropical lowland rain forest of western Amazonia. 15568 pterido phyte individuals representing 40 species were recorded in four plots. The variability among subplots within the same plot was considerable in all the characteristics measured (number of individuals, number of species, specie s diversity); the square 1-ha plot was more homogeneous in these respects t han any of the three 5 m by 1300 m transects. Species richness was affected by the density of individuals both within and among plots. Density of indi viduals was not affected by topographical position within any of the plots, whereas in some of the plots both species richness and species diversity w ere. Clustering and ordination analyses showed that floristically similar s ubplots could be found in different plots: although there was a tendency fo r subplots from the same plot to be floristically similar and therefore to group together, many recognized groups included subplots from two or more p lots. Both within and among plots, the floristic differences corresponded t o topographic position and were probably related to soil drainage. This was also evident in that the abundance patterns of many species followed the t opography.