EDAPHIC AND HUMAN EFFECTS ON LANDSCAPE-SCALE DISTRIBUTIONS OF TROPICAL RAIN-FOREST PALMS

We studied the landscape-level spatial variation in distribution and abundance of seven species of canopy and subcanopy palms in a neotropical rain forest. Within 568 ha of nonswamp old-growth lowland forest at the La Selva Biological Station, Costa Rica, we sampled at 516 intersections of a reserve-wide grid to systematically assess the species' distributions across the major edaphic gradients. We found mosaics of community structure related to marked within-forest variability in both soil and topography. Total stem density of this guild varied with both edaphic factors. Steep sites had twice as many large palms (>10 m tall) per hectare than those on gentler topography or at lower slope positions. The combined density of subcanopy and canopy palms also varied significantly among soil types in the smallest size class (1-5 m tall), but not for larger individuals. Local (point) species richness of the subcanopy and canopy palms varied among soil types. Of the seven species, two (Astrocaryum alatum and A. confertum) were rare. All others showed significant edaphic variation in their distribution and/or estimated density. Two closely related species had strong and opposite edaphic associations. Euterpe macrospadix was biased toward steep topography and less fertile sites. Prestoea decurrens was nearly ommipresent on soil types with gentle topography while absent from half the points on soils with steep slopes. Two other closely related species, Iriartea deltoidea and Socratea exorrhiza, while virtually omnipresent across soil types and topographic positions, showed marked reciprocal variation in density between related soils. Iriartea's spatial distribution further indicates local removal of this species from one sector of the old growth by human harvesting (with subsequent apparent ''release'' of Socratea in this site). The most abundant species, Welfia georgii, while present at all sample points, showed significant among-soil variation in density. This substructuring of the arborescent palm guild results from the interplay of edaphic variation and past human activity. These findings and evidence from other sites suggest that marked spatial heterogeneity in community structure, at small to large scales (0.5-10(3) ha), may be general among tropical wet forests. Study of the spatial scales and causes of this variability will produce a more robust understanding of these complex ecosystems.