Life history variation in trees is a ubiquitous feature of tropical forests that may facilitate the niche partitioning of light. However, many tests have failed to detect light partitioning by saplings in gaps, which may reflect the stochastic nature of understory light penetration and recruitment. We argue that tree size is a critical component of niche partitioning that is more tightly linked to light availability. To account for size, we use a scaling framework to assess patterns of growth, abundance, mortality, and richness across life histories from >114,000 trees in a primary, neotropical forest. Relative abundance, productivity, and richness shift ~1−2 orders of magnitude with tree size: from shade tolerant, slow trees dominating the understory to parity with rapidly growing fast and long-lived pioneer species in the canopy. Life history tradeoffs promote vertical niche partitioning in tropical forests.