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.

Ecologists have long sought to understand the controls of species’ geographic distributions. Two important hypotheses have been that range limits are determined 1) predominantly by climate or 2) by competition in addition to climate, with competitive interactions dominating where climate is benign. If the first hypothesis is correct, the effect of changing climate on species’ distributions should be adequately predicted by climate-only models. Here, we used demographic range models (DRMs) to evaluate the influence of climate and competition on the geographic distribution of Pinus edulis, a desert pine in the southwestern U.S. We parameterized DRMs with data on 23,426 trees in 1,941 forest inventory plots. Vital rate responses were consistent with the predictions of the second hypothesis: negative effects of warm-dry conditions at low-elevation sites and competition at cool-humid, high-elevation sites. Further, including both climate and competition in the DRM yielded a better fit of DRM-predicted population growth rate to the observed distribution than a climate-only DRM. However, at cool-humid sites, negative effects of competition were too weak to offset positive climate effects, resulting in a mismatch between predicted population growth rate and observed distribution. This result leads us to formulate a new hypothesis: climate has complex influences on species’ distributions through its effects on range-limiting processes at a variety of scales, for example, biotic and disturbance feedbacks at community, ecosystem, landscape, and macrosystem scales. The implications of this complex systems hypothesis are that range dynamics may be poorly predictable from static, climate-only models, and may be characterized by sudden changes in abundance, even distribution collapse, with climate change.