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The response of terrestrial vegetation to a globally changing environment is central to predictions of future levels of atmospheric carbon dioxide (1,2). The role of tropical forests is critical because they are carbon-dense and highly productive (3,4). Inventory plots across Amazonia show that old-growth forests have increased in carbon storage over recent decades (5-7), but the response of one-third of the world's tropical forests in Africa (8) is largely unknown owing to an absence of spatially extensive observation networks (9,10). Here we report data from a ten-country network of long-term monitoring plots in African tropical forests. We find that across 79 plots (163 ha) above-ground carbon storage in live trees increased by 0.63 Mg C [ha.sup.-1] [yr.sup.-1] between 1968 and 2007 (95% confidence interval (CI), 0.22-0.94; mean interval, 1987-96). Extrapolation to unmeasured forest components (live roots, small trees, necromass) and scaling to the continent implies a total increase in carbon storage in African tropical forest trees of 0.34 Pg C [yr.sup.-1] (CI, 0.15-0.43). These reported changes in carbon storage are similar to those reported for Amazonian forests per unit area (6,7), providing evidence that increasing carbon storage in old-growth forests is a pan-tropical phenomenon. Indeed, combining all standardized inventory data from this study and from tropical America and Asia (5,6,11) together yields a comparable figure of 0.49 Mg C [ha.sup.-1] [yr.sup.-1] (n = 156; 562 ha; CI, 0.29-0.66; mean interval, 1987-97). This indicates a carbon sink of 1.3 Pg C [yr.sup.-1] (CI, 0.8-1.6) across all tropical forests during recent decades. Taxon-specific analyses of African inventory and other data (12) suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations, may be the cause of the increase in carbon stocks (13), as some theory (14) and models (2,10,15) predict., Tropical forests cover 7-10% of the global land area, store 40-50% of carbon in terrestrial vegetation and annually process approximately six times as much carbon via photosynthesis and respiration as [...]

Tree architecture determines a tree's light capture, stability, and efficiency of crown growth. The hypothesis that light demand and adult stature of tree species within a community, independently of each other, determine species' architectural traits was tested by comparing 53 Liberian rain forest tree species. We evaluated whether species differed in their tree height, crown depth, and crown diameter, when compared at a standardized size of 15-cm diameter at breast height; and how their architecture changed early during ontogeny. Tree height was positively correlated with adult stature and light demand. By producing a relatively slender stem, large-stature species are able to rapidly reach their reproductive size, at a low cost for construction and support. Light-demanding species need a slender stern to be able to attain or maintain a position in the canopy. Both crown depth and crown diameter were negatively correlated with adult stature, but not with light demand. This is in contrast with the hypothesis that shade-tolerant species should have a shallow crown to reduce self-shading in a light-limited environment. Investing energy in height growth rather than lateral crown growth allows a rapid vertical stem extension, but crown diameter has to be sufficiently small to reduce the risk of mechanical failure. All architectural patterns were maintained during ontogeny. The key factors driving interspecific differences in tree architecture are the costs of height extension and mechanical stability. In general, light demand and adult stature represent independent axes of architectural differentiation, affecting tree architecture in different ways. Key words: crown traits; Liberia; plant strategy; shade tolerance; tree architecture; light demand vs. adult stature; tree height; tropical rain forest.

1 A height-light trajectory (HLT, a fitted curve relating canopy exposure to tree height) was determined for populations of individuals of each of 47 tree species in a Liberian lowland rainforest. The HLTs were compared and related to tree allometry and adult stature. Crown exposure was measured for 7460 trees and related to tree height using a multinomial regression analysis. Individual trees were followed for 2.89.8 years. 2 The trajectories of the 47 species were compared with the average vertical light profile in the forest canopy. Evidence was found for the existence of all nine trajectories hypothesized on the basis of three possible light environments (high, intermediate and low) for juveniles and adults. The classical paradigm of pioneer vs. shade tolerant, based on seed and seedling responses, does not therefore apply to post-seedling stages. 3 The majority of the species followed the vertical light profile in the forest canopy, starting in low light environments in the juvenile stage and ending up in high light environments in the adult stage. Only two species complied with the classic notion of whole-life shade tolerants and whole-life shade intolerants (one each). 4 The predictable vertical light gradient in the forest canopy has led to a close association between adult height, light trajectories and allometric traits. Large-stature species tend to have relatively slender stems and narrow crowns, and therefore realize a faster temporal and height-related increase in crown exposure. 5 Tree species have different heightlight trajectories when they grow from seedling to adulthood. This may have profound repercussions for our current views on plasticity and adaptation, light partitioning and species coexistence, and on silviculture and management.