Phenotypic traits and their associated trade-offs have been shown to have globally consistent effects on individual plant physiological functions 1-3 , but how these effects scale up to influence competition, a key driver of community assembly in terrestrial vegetation, has remained unclear 4 . Here we use growth data from more than 3 million trees in over 140,000 plots across the world to show how three key functional traits-wood density, specific leaf area and maximum height-consistently influence competitive interactions. Fast maximum growth of a species was correlated negatively with its wood density in all biomes, and positively with its specific leaf area in most biomes. Low wood density was also correlated with a low ability to tolerate competition and a low competitive effect on neighbours, while high specific leaf area was correlated with a low competitive effect. Thus, traits generate trade-offs between performance with competition versus performance without competition, a fundamental ingredient in the classical hypothesis that the coexistence of plant species is enabled via differentiation in their successional strategies 5 . Competition within species was stronger than between species, but an increase in trait dissimilarity between species had little influence in weakening competition. No benefit of dissimilarity was detected for specific leaf area or wood density, and only a weak benefit for maximum height. Our traitbased approach to modelling competition makes generalization possible across the forest ecosystems of the world and their highly diverse species composition.Phenotypic traits are considered fundamental drivers of community assembly and thus species diversity 1,6 . The effects of traits on individual plant physiologies and functions are increasingly understood, and have been shown to be underpinned by well-known and globally consistent trade-offs 1-3 . For instance, traits such as wood density and specific leaf area capture trade-offs between the construction cost and longevity or strength of wood and leaf tissues 2,3 . By contrast, we still have a limited understanding of how such trait-based trade-offs translate into competitive interactions between species, particularly for long-lived organisms such as trees. Competition is a key filter through which ecological and evolutionary success is determined 4 . A long-standing hypothesis is that the intensity of competition decreases as two species diverge in trait values 7 (trait dissimilarity). The few studies [8][9][10][11][12][13] that have explored links between traits and competition have shown that linkages were more complex than this, as particular trait values may also confer competitive advantage independently from trait dissimilarity 9,13,14 . This distinction is fundamental for species coexistence and the local mixture of traits. If neighbourhood competition is driven mainly by trait dissimilarity, this will favour a wide spread of trait values at a local scale. By contrast, if neighbourhood interactions are mainly driven by the c...

For the period 1980-89, we estimate a carbon sink in the coterminous United States between 0.30 and 0.58 petagrams of carbon per year (petagrams of carbon = 10(15) grams of carbon). The net carbon flux from the atmosphere to the land was higher, 0.37 to 0.71 petagrams of carbon per year, because a net flux of 0.07 to 0.13 petagrams of carbon per year was exported by rivers and commerce and returned to the atmosphere elsewhere. These land-based estimates are larger than those from previous studies (0.08 to 0.35 petagrams of carbon per year) because of the inclusion of additional processes and revised estimates of some component fluxes. Although component estimates are uncertain, about one-half of the total is outside the forest sector. We also estimated the sink using atmospheric models and the atmospheric concentration of carbon dioxide (the tracer-transport inversion method). The range of results from the atmosphere-based inversions contains the land-based estimates. Atmosphere- and land-based estimates are thus consistent, within the large ranges of uncertainty for both methods. Atmosphere-based results for 1980-89 are similar to those for 1985-89 and 1990-94, indicating a relatively stable U.S. sink throughout the period.

Summary 1We estimated the dry, living, above-ground biomass (AGB) standing stock and its turnover in a 50-hectare forest plot located in moist tropical forest on Barro Colorado Island, Panama. The estimates were obtained using inventory data collected every 5 years from 1985 to 2000, including measurements of all trees ≥ 1 cm diameter. 2 Four different allometric regressions relating trunk diameter and height with AGB were compared. Based on the most consistent method, we estimated that the Barro Colorado forest holds 281 ± 20 Mg ha − 1 (1 Mg = 10 3 kg) of AGB, lianas included. A third of the AGB is stored in trees larger than 70 cm in diameter. 3 Stand-level AGB increment (growth plus recruitment) was highest in the period 1985-90 (7.05 ± 0.32 Mg ha − 1 year − 1 , mean ± 95% confidence limits based on samples of multiple hectares) and smallest in the period 1990-95 (5.25 ± 0.26 Mg ha − 1 year − 1 ), while AGB losses were similar during the three intervals (mean 5.43 ± 0.72 Mg ha − 1 year − 1 ). This resulted in significant differences in AGB change (defined as increment minus loss) among census intervals; including branchfalls, the AGB of Barro Colorado Island increased in 1985-90 (+0.82 ± 0.84 Mg ha − 1 year − 1 ), decreased in 1990-95 ( − 0.69 ± 0.82 Mg ha − 1 year − 1 ), and increased again in 1995 -2000 (+0.45 ± 0.70 Mg ha − 1 year − 1 ). The 15-year average was +0.20 Mg ha − 1 year − 1 , but with a confidence interval that spanned zero ( − 0.68 to 0.63 Mg ha − 1 year − 1 ). 4 Branchfalls and partial breakage of stems had a significant influence on the AGB changes. They contributed an average of 0.46 Mg ha − 1 year − 1 to the AGB loss. About 5% of AGB increment was due to trees less than 10 cm in diameter. 5 To test whether the AGB of tropical forests is increasing due to climate change, we propose that in each forest type, at least 10 hectares of forest be inventoried, and that measurements of the small classes (< 10 cm diameter) as well as large size classes be included. Biomass loss due to crown damage should also be estimated.