Tree species distributions in an upper Amazonian forest

Not a single tree species distribution in the Amazon basin has been reliabl y mapped, though speculation regarding such distributions has been extensiv e. We present data from a network of 21 forest plots in Manu National Park, Peru, totaling >36 ha and sited over an area of similar to 400 km(2), to e xplore 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 a sked 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 fou nd in several? (3) Is the species locally abundant anywhere or is it scarce everywhere? The answers served to classify a subset of species under Rabin owitz's classification scheme for rare species. Three main conclusions emer ged. First, the great majority of tree species at Manu are geographically w idespread. Every species identified to date occurs elsewhere in South Ameri ca, outside the department of Madre de Dies; more than two-thirds of them h ave been collected 1500 km away in Amazonian Ecuador. Second, 15-26% of spe cies appear to be restricted to a single forest type, when forest types are defined by historical river dynamics (i.e., terra firme forest, mature flo odplain forest, swamp forest, and primary successional floodplain forest). The proportion of restricted species declined with increasing sampling effo rt, making 15% a more reliable figure. Third, while 88% of species occurred at densities of <1 individual/ha over the entire network of plots, at leas t half occurred somewhere at densities of >1.5 individuals/ha. Extrapolatin g these results provides a first guess at how tree species are distributed across the western portion of the Amazon basin. We conclude with the sugges tion that most tree species in the region are habitat generalists occurring over large areas of the Amazonian lowlands at low densities but large abso lute population sizes.