Aim: We sought to determine the relationship of forest composition and alpha diversity (the species diversity of a local assemblage) to altitude, soil, and spatial factors over a 440–2,950 m a.s.l gradient. Location: Altitudinal gradient on the Caribbean slope of the Talamanca Cordillera, Costa Rica. Taxon: Angiosperm and gymnosperm trees, palms, and tree ferns. Methods: We measured and identified all stems =10 cm dbh in 32 0.25-ha undisturbed rain forest plots over the gradient. We determined compositional patterns using Non-Metric Multidimensional Scaling (NMS) ordination, and used linear regressions to explore the relationship between four alpha diversity metrics and altitude. With variation partitioning (VARPART), we determined the compositional variation explained by altitude, soil, and spatial variables quantified using Principle Components of Neighbor matrices. Results: We identified 425 species. NMS axis 1 separated a lowland zone (440–1,120 m asl) from a transitional one dominated by holarctic Oreomunnea mexicana (1,400–1,600 m asl) and Quercus-dominated forests at altitudes >2,100 m asl. The lowland zone was separated into two clusters of plots on NMS axis 2, the first in the 430–620 m asl range and the second at 1,000–1,120 masl. Regressions showed that all alpha diversity metrics were strongly negatively related to altitude (R2 > 0.78). Overall, adjusted R2 from VARPART was 0.43, with 0.30, 0.21, and 0.17 for altitude, soil, and space respectively. The respective adjusted R2 of individual matrices, on controlling for the other two, was 0.06, 0.05 and 0.09 (p < 0.001). Main conclusions: There are two well-defined forest compositional zones on this gradient—lowlands 430–1,120 m asl and montane forests >2,150 m asl—with a transitional zone at 1,400–1,600 m asl, where lowland tropical and montane holarctic species are found together. Montane forests are very distinct in their composition and low alpha diversity. Vegetation and soil respond to altitude, and t