The response of terrestrial vegetation to a globally changing environment is central to predictions of future levels of atmospheric carbon dioxide. The role of tropical forests is critical because they are carbon-dense and highly productive. Inventory plots across Amazonia show that old-growth forests have increased in carbon storage over recent decades, but the response of one-third of the world's tropical forests in Africa is largely unknown owing to an absence of spatially extensive observation networks. 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(-1) yr(-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(-1) (CI, 0.15-0.43). These reported changes in carbon storage are similar to those reported for Amazonian forests per unit area, 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 together yields a comparable figure of 0.49 Mg C ha(-1) yr(-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(-1) (CI, 0.8-1.6) across all tropical forests during recent decades. Taxon-specific analyses of African inventory and other data suggest that widespread changes in resource availability, such as increasing atmospheric carbon dioxide concentrations, may be the cause of the increase in carbon stocks, as some theory and models predict.

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The future of tropical forests under global environmental change is uncertain, with biodiversity and carbon stocks at risk if precipitation regimes alter. Here, we assess changes in plant functional composition and biomass in 19 plots from a variety of forest types during two decades of long-term drought in Ghana. We find a consistent increase in dry forest, deciduous, canopy species with intermediate light demand and a concomitant decrease in wet forest, evergreen, sub-canopy and shade-tolerant species. These changes in composition are accompanied by an increase in above-ground biomass. Our results indicate that by altering composition in favour of drought-tolerant species, the biomass stocks of these forests may be more resilient to longer term drought than short-term studies of severe individual droughts suggest.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 132.211. INTRODUCTIONThe most recent attempts to classify the forest vegetation of Ghana are those of Taylor (1960) and Mooney (1961), who based their classifications on inventories of trees in forest reserves. Their work has two major shortcomings. Firstly, forest which had not been enumerated could not be included. Secondly, the failure of the authors to describe the methods by which their classifications were derived makes it impossible to identify a forest stand other than by its map position, thus greatly reducing their practical value (cf. Greig-Smith 1969a). We have attempted to produce a floristic classification with a defined empirical basis, applicable to all forests in Ghana, and readily permitting identification.The structural complexity and species-richness of tropical forest make difficult or impossible its study by the traditional phytosociological procedures, but numerical methods have proved successful (Greig-Smith 1969b), especially in demonstrating correlations between vegetation and environmental factors in fairly small areas (Austin, Ashton & Greig-Smith 1972; Greig-Smith, Austin & Whitmore 1967). There are relatively few published accounts of the use of numerical methods in a survey of a large area of tropical forest, mainly because of the problem of dealing with a data matrix of the size involved. Webb et al. (1970) analysed tropical and subtropical forests in Australia spanning 20? of latitude, but took only sixty-eight plots and used only eighty-two species in the final analysis. Recent improvements in methodology and computer capacity encouraged us to undertake sampling more intensive than this, using a much larger number of species. Conditions in Ghana are particularly suitable for a survey of tropical forest. Of a total forest zone covering 82 260 km2, the area under forest amounts to 20 530 km2 including 16 790 km2 within forest reserves distributed more or less throughout the forest zone (Anon. 1973). Most reserves can be reached by motor road. The forest flora of West Africa is relatively well known, and is less diverse than that of tropical Asia or America. All Ghanaian forest falls within the categories of Tropical Evergreen Seasonal forest and Tropical Semi-deciduous forest (UNESCO 1973). It has a more or less uneven tree canopy at 10 to 40 m above the ground (emergent trees may reach 60 m); woody climbers are always present. Vascular epiphytes are present except in the driest forests, but not abundant except in the most humid; some of the canopy trees are deciduous in the dry season, but the understorey trees and shrubs are evergreen. Gymnosperms and stem succulents are absent, and palms are generally uncommon; a herb...

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Seasonal changes in the densities of dormant seeds in the soil around eight pioneer trees in the 50-ha Forest Dynamics Plot, on Barro Colorado Island, Panamá were studied, and how seed dispersal and seed dormancy influenced patterns of seed abundance and distribution were examined. Twenty-four, 3-cm-deep soil samples were collected on 30 m transects radiating out from each of the trees in each of four time-intervals through the year, and four 21-cm-deep samples were collected beneath the focal tree crowns. In the surface 0–3 cm of soil, germinable seed densities of all species combined declined from a peak of 1090 seeds m−2 in the mid-wet season in August, to 330 seeds m−2 by the end of the wet season in November. In contrast, at soil depths >3 cm, there was little variation in soil seed bank density through the year. Some variation in soil seed bank density for individual species could be accounted for by distance to reproductive conspecifics. Among species, abundance in the soil was negatively correlated with seed size. Seed persistence varied greatly among species at this site; after 1 y of burial in mesh bags, seed germinability of four species was near zero, while four other species showed no consistent decline in seed germinability after >2 y of burial. For at least one species, Trema micrantha, prolonged seed dormancy was also possible under natural conditions. Twenty-five percent of Trema seeds extracted from the soil at a site occupied by an isolated Trema tree that died between 1982 and 1985 were still germinable in 1994.

The sensitivity of tropical forest carbon to climate is a key uncertainty in predicting global climate change. Although short-term drying and warming are known to affect forests, it is unknown if such effects translate into long-term responses. Here, we analyze 590 permanent plots measured across the tropics to derive the equilibrium climate controls on forest carbon. Maximum temperature is the most important predictor of aboveground biomass (−9.1 megagrams of carbon per hectare per degree Celsius), primarily by reducing woody productivity, and has a greater impact per °C in the hottest forests (>32.2°C). Our results nevertheless reveal greater thermal resilience than observations of short-term variation imply. To realize the long-term climate adaptation potential of tropical forests requires both protecting them and stabilizing Earth’s climate.