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In this paper, we review some critical issues regarding carbon cycling in Amazonia, as revealed by several studies conducted in the Large Scale Biosphere Atmosphere Experiment in Amazonia (LBA). We evaluate both the contribution of this magnificent biome for the global net primary productivity/net ecosystem exchange (NPP/NEE) and the feedbacks of climate change on the dynamics of Amazonia. In order to place Amazonia in a global perspective and make the carbon flux obtained through the LBA project comparable with global carbon budgets, we extrapolated NPP/NEE values found by LBA studies to the entire area of the Brazilian Amazon covered by rainforest. The carbon emissions due to land use changes for the tropical regions of the world produced values from 0.96 to 2.4 Pg C year−1, while atmospheric CO2 inversion models have recently indicated that tropical lands in the Americas could be exchanging a net 0.62±1.15 Pg C year−1 with the atmosphere. The difference calculated from these two methods would imply a local sink of approximately 1.6–1.7 Pg C year−1, or a source of 0.85 ton C ha−1 year−1. Using our crude extrapolation of LBA values for the Amazon forests (5 million km2) we estimate a range for the C flux in the region of −3.0 to 0.75 Pg C year−1. The exercise here does not account for environmental variability across the region, but it is an important driver for present and future studies linking local process (i.e. nutrient availability, photosynthetic capacity, and so forth) to global and regional dynamic approaches. [ABSTRACT FROM AUTHOR]

The humid forests of Amazonia are experiencing longer and more intense dry seasons, which are predicted to intensify by the end of the 21st century. Although tree species often have long generation times, they may still have the capacity to rapidly respond to changing climatic conditions through adaptive phenotypic plasticity. We, therefore, predicted that Amazonian trees have shifted their leaf morphology in response to the recent drier climate. We tested this prediction by analysing historical herbarium specimens of six Amazonian tree species collected over a 60-year period and comparing changes in leaf morphology with historical precipitation data. Moreover, we explored spatial and temporal biases in herbarium specimens and accounted for their potentially confounding effect in our analysis. We found pronounced biases in herbarium specimens, with nearly 20% of specimens collected in close geographic proximity and around the 1975s. When accounting for such biases, our results indicate a trend of decreasing leaf size after the 1970s, which may have been spurred by an observed reduction in rainfall. Our findings support the hypothesis that (some) Amazonian trees have the capacity to adaptively change their leaf phenotypes in response to the recent drier climate. Nevertheless, the unavoidable spatial and temporal biases in herbarium specimens call for caution when generalizing our findings to all Amazonian trees. [ABSTRACT FROM AUTHOR]