Influence of edaphic specialization on pteridophyte distribution in neotropical rain forests

The abundance of terrestrial pteridophytes was studied in eight sites in Western Amazonia, all representing lowland tropical rain forest in non-inundated terrain (tierra firme). In each site, a sample plot ranging from 0.25 ha to 1.00ha in area was inventoried and all terrestrial pteridophytes were recorded. In total, 27,000 individuals representing sixty-four species were encountered, with each plot having between fourteen and thirty-three species. The plots represented a gradient from nutrient-poor sandy soils to relatively fertile clay soils, and it was found that their floristic compositions clearly correlated with the differences in the soils. Two-thirds of the species were restricted either to poor, intermediate or rich soils, while less than a tenth were found growing on all soil types. Analyses of species lists from another twenty neotropical localities supported the result that some pteridophyte species associate, whereas other species are never found growing together. The floristic similarities among sites showed no relationship to the geographical distances between them, but rather reflected similarities in soils. It is concluded that pteridophytes are good indicators of such ecological conditions, and that when interpreting biogeographical patterns in the neotropical rain forests, it is important to take into account the distribution of edaphically different habitats.