International audienceThe seasonal climate drivers of the carbon cycle in tropical forests remain poorly known, although these forests account for more carbon assimilation and storage than any other terrestrial ecosystem. Based on a unique combination of seasonal pan-tropical data sets from 89 experimental sites (68 include aboveground wood productivity measurements and 35 litter productivity measurements), their associated canopy photosynthetic capacity (enhanced vegetation index, EVI) and climate, we ask how carbon assimilation and aboveground allocation are related to climate seasonality in tropical forests and how they interact in the seasonal carbon cycle. We found that canopy photosynthetic capacity seasonality responds positively to precipitation when rainfall is < 2000 mm yr(-1) (water-limited forests) and to radiation otherwise (light-limited forests). On the other hand, independent of climate limitations, wood productivity and litterfall are driven by seasonal variation in precipitation and evapotranspiration, respectively. Consequently, light-limited forests present an asynchronism between canopy photosynthetic capacity and wood productivity. First-order control by precipitation likely indicates a decrease in tropical forest productivity in a drier climate in water-limited forest, and in current light-limited forest with future rainfall < 2000 mm yr(-1)

Elevated CO(2) concentrations ([CO(2)]) affect plant water relations and photosynthesis, and the increase in atmospheric [CO(2)] over the past 100-200 years has been related to changes in stomatal density and the carbon isotope ratio (delta(13)C) in tree rings and leaves from herbarium specimens. Because many tropical trees do not produce annual growth rings and their wood is therefore difficult to date, no trends in delta(13)C of tropical trees have been reported. Wood from Cedrela odorata L. (tropical cedar) and Swietenia macrophylla King (bigleaf mahogany), which do produce annual rings, was collected from a primary rain forest in Aripuanã, Brazil (10 degrees 09' S, 59 degrees 26' W). We measured wood cellulose delta(13)C in 10-year growth increments from 37 Cedrela trees (between 11 and 151 years old in 2001) and 16 Swietenia trees (48-126 years old). A comparison of delta(13)C in cellulose of trees from different decades and of trees of different cambial ages showed that the amount of delta(13)C was largely related to the decade the wood was produced in, and not, or only to a minor extent, to tree age. Cellulose delta(13)C decreased from -26.0 to -27.3 per thousand in Cedrela and from -25.7 to -27.1 per thousand in Swietenia, with the largest changes occurring during the past 50 years. Based on these data and the trends in atmospheric [CO(2)] and delta(13)CO(2), we calculated that the internal [CO(2)] increased from about 220 to 260 ppm and that intrinsic water-use efficiency increased by 34% in Cedrela and by 52% in Swietenia. This may have implications for the water cycle and may explain the trend toward increased tree growth and turnover observed in some tropical forests.

The width of the increment zones in

The pattern of growth increment zones, the cambial growth dynamics and the structural variation in wood formation of Swietenia macrophylla King, Carapa guianensis Aubl., and Cedrela odorata L. (Meliaceae) were investigated in order to understand the relationship of site conditions and sustainable growth in Central Amazonian plantations. Trees were available from 8-, 17-, 23-, and 57-year-old plantations, and from primary forests in Manaus (Amazônia), Santarem (Pará), and Aripuanã (Mato Grosso). The wood anatomical structure and the annual increments of 61 Swietenia, 94 Carapa, and 89 Cedrela trees were studied for different tree heights. The curves of annual increments were cross-dated and tested for synchronisation. The cambial growth dynamics of up to 52 trees per species were dated by means of dendrometer measurements, monthly labelling by pinmarkers, and extracted cambium samples investigated using a microscope. The intraannual course of the growth and structural variation was compared with the water supply of the soil and insect attacks (Hypsipyla grandella (Zeller) Lep.).In Swietenia and Carapa parenchyma and vessel bands as well as bands of resin canals were observed. Within the xylem of Cedrela, alternating bands of fibres and vessels surrounded by paratracheal parenchyma were found; bands of resin canals were only occasional. In the juvenile wood of Swietenia and Carapa no synchronization of the increment curves was possible, whereas the increment curves obtained in the juvenile wood of Cedrela showed parallel run in growth. The increment curves obtained in adult wood of Swietenia and Cedrela indicate an annual formation of increment zones, whereas the number of increment zones in the xylem of Carapa was approximately 50% higher than the tree age (years) indicating that the growth increments of Carapa also were not annual during the adult phase of growth.The study of the intraannual growth dynamics of the trees showed that the formation of parenchyma bands in Swietenia is induced by dry periods before a cambial dormancy. The formation of parenchyma bands of Carapa was induced by extremely dry and extremely wet periods before a cambial dormancy, whereas fibre bands in Cedrela were induced by dry periods before a cambial dormancy and the formation of vessel bands embedded in paratracheal parenchyma was induced by wet periods after a cambial dormancy. In addition, insect attack (Hypsipyla grandella) induced locally restricted formation of parenchyma bands and bands of resin canals in Swietenia, Carapa and Cedrela.