A comparison of tree species diversity in two upper Amazonian forests

Abstract. We inventoried two Amazonian tree communities separated by ϳ1400 km of continuous lowland tropical forest, in an effort to understand why one was more diverse than the other. Yasuní National Park, near the equator in eastern Ecuador, has one of the most diverse tree communities in the world. Manu National Park, at 12Њ 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 Yasuní; and (3) to assess, in the light of those observations, some explanations for why forests near the equator are so diverse. Yasuní has ϳ1.4 times as many tree species as Manu at all three spatial scales we examined: local (1 ha), landscape (Ͻ10 000 km 2 ), and regional (Ͻ100 000 km 2 ). Yasuní samples contain more families and genera, more individual trees per unit area, and a larger proportion of small trees. Tree species at Yasuní 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 Yasuní occur at the same densities as equally ranked species at Manu, but there are substantially more very rare species at Yasuní. 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 Yasuní 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 Yasuní and (2) a newly described ''mixing effect'' related to the higher stem density there