Edaphic variation and the mesoscale distribution of tree species in a neotropical rain forest

1 Do local edaphic factors over short environmental gradients affect the distribution and abundance of tree species in tropical rain forests? We addressed this question by examining the responses of tree species to soil type, topographic position and slope angle in an upland old-growth tropical rain forest landscape in Costa Rica, Central America. 2 The study area covered 216 ha of non-swamp old growth and included replicated landscape units such as ridgetop to swale catenas, small watersheds, and alluvial terraces. An existing soils map was refined using additional sampling along a regularly spaced grid. Three soil units were defined: residual soils derived from in-place weathering of basaltic parent material; old alluvial terrace soils formed by precontemporary flooding; and soils of stream valleys. A Geographic Information System was used to assign soil type to 2190 post-establishment individuals of nine tree species in a longterm demographic study. Topographic position and slope angle were measured in the field. 3 Data from 433 regularly spaced sample points were used to generate an expected distribution of edaphic variables, which was compared with the number of individuals of each species in each edaphic category. 4 Non-random distributions among soil types were found for seven of the nine species, with topographic positions for six species, and with slope angle classes for four species. 5 The issue of what constitutes an independent sample of establishment was analysed by considering the old-growth character of the landscape and evidence for frequent dispersal among edaphic units. A re-analysis of species' distributions using only individuals > 4cm diameter showed that results from the original analyses were robust. 6 Soil type (residual vs. old alluvial) was not significantly related to diameter growth. Three species showed significant differences in size class distributions between soil types with increasing diameter. 7 Tree species in tropical rain forests are frequently non-randomly distributed along relatively short edaphic gradients on upland soils. Future studies should increase the number of species and spatial scale analysed, incorporate better analyses of edaphic variables, and include experiments to identify the ecological processes that generate these non-random distributions.