The vast extent of the Amazon Basin has historically restricted the study of its tree communities to the local and regional scales. Here, we provide empirical data on the commonness, rarity, and richness of lowland tree species across the entire Amazon Basin and Guiana Shield (Amazonia), collected in 1170 tree plots in all major forest types. Extrapolations suggest that Amazonia harbors roughly 16,000 tree species, of which just 227 (1.4%) account for half of all trees. Most of these are habitat specialists and only dominant in one or two regions of the basin. We discuss some implications of the finding that a small group of species-less diverse than the North American tree flora-accounts for half of the world's most diverse tree community

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Not a single tree species distribution in the Amazon basin has been reliably mapped, though speculation regarding such distributions has been extensive. We present data from a network of 21 forest plots in Manu National Park, Peru, totaling Ͼ36 ha and sited over an area of ϳ400 km 2 , to explore how tree species are distributed across upper Amazonia at a variety of spatial scales. For each of 825 tree species occurring in the plots we asked three questions: (1) Does the species have a large or small geographic range? (2) Is the species restricted to a single forest type, or is it found in several? (3) Is the species locally abundant anywhere or is it scarce everywhere? The answers served to classify a subset of species under Rabinowitz's classification scheme for rare species. Three main conclusions emerged. First, the great majority of tree species at Manu are geographically widespread. Every species identified to date occurs elsewhere in South America, outside the department of Madre de Dios; more than two-thirds of them have been collected 1500 km away in Amazonian Ecuador. Second, 15-26% of species appear to be restricted to a single forest type, when forest types are defined by historical river dynamics (i.e., terra firme forest, mature floodplain forest, swamp forest, and primary successional floodplain forest). The proportion of restricted species declined with increasing sampling effort, making 15% a more reliable figure. Third, while 88% of species occurred at densities of Ͻ1 individual/ha over the entire network of plots, at least half occurred somewhere at densities of Ͼ1.5 individuals/ha. Extrapolating these results provides a first guess at how tree species are distributed across the western portion of the Amazon basin. We conclude with the suggestion that most tree species in the region are habitat generalists occurring over large areas of the Amazonian lowlands at low densities but large absolute population sizes.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Ecological Society of America is collaborating with JSTOR to digitize, preserve and extend access to Ecology. Abstract.We inventoried two Amazonian tree communities separated by -1400 km of continuous lowland tropical forest, in an effort to understand why one was more diverse than the other. Yasuni National Park, near the equator in eastern Ecuador, has one of the most diverse tree communities in the world. Manu National Park, at 120 S in Peru's Madre de Dios region, is only moderately diverse by upper Amazonian standards. Following the field inventories, a database of morphological, ecological, and other traits was compiled from the taxonomic literature for 1039 species from the plots. Our goals were (1) to describe how terra firme tree communities at the two sites differed in composition, diversity, and structure; (2) to characterize the "extra" species responsible for the higher diversity at Yasuni; and (3) to assess, in the light of those observations, some explanations for why forests near the equator are so diverse.Yasuni has -1.4 times as many tree species as Manu at all three spatial scales we examined: local (1 ha), landscape (<10000 kM2), and regional (<100000 kM2). Yasuni samples contain more families and genera, more individual trees per unit area, and a larger proportion of small trees. Tree species at Yasuni have smaller stature, larger leaves, larger seeds, and smaller geographic and altitudinal ranges than those at Manu, and disproportionate increases in species diversity are observed within the Myrtaceae, Lauraceae, Melastomataceae, and several other families. Community structures were strikingly similar, with the same species (Iriartea deltoidea, a palm) dominating both sites at identical densities. Common species at Yasuni occur at the same densities as equally ranked species at Manu, but there are substantially more very rare species at Yasuni. The poorer tree flora is not a nested subset of the richer tree flora, though a majority of species in each inventory do occur at the other site.Several models that offer explanations for geographic variation in tropical tree species diversity are assessed in light of these data. Most do a poor job of accounting for the patterns revealed by the inventories. We speculate that the most important factor in producing the higher diversity in Yasuni is its rainier, aseasonal climate, and we discuss two specific rainfall-related mechanisms that appear to be supported by the data: (1) year-round water availability allowing more species to persist in the understory at Yasuni and (2) a newly described "mixing effect" related to the higher stem density there.