The allocation and cycling of carbon (C) within forests is an important component of the biospheric C cycle, but is particularly understudied within tropical forests. We synthesise reported and unpublished results from three lowland rainforest sites in Amazonia (in the regions of Manaus, Tapajos and Caxiuana), all major sites of the Large-Scale Biosphere-Atmosphere Programme (LBA). We attempt a comprehensive synthesis of the C stocks, nutrient status and, particularly, the allocation and internal C dynamics of all three sites. The calculated net primary productivities (NPP) are 10.1 +/- 1.4 Mg C ha(-1) yr(-1) (Manaus), 14.4 +/- 1.3 Mg C ha(-1) yr(-1) (Tapajos) and 10.0 +/- 1.2 Mg C ha(-1) yr(-1) (Caxiuana). All errors bars report standard errors. Soil and leaf nutrient analyses indicate that Tapajos has significantly more plant-available phosphorus and calcium. Autotrophic respiration at all three sites (14.9-21.4 Mg C ha yr(-1)) is more challenging to measure, with the largest component and greatest source of uncertainty being leaf dark respiration. Comparison of measured soil respiration with that predicted from C cycling measurements provides an independent constraint. It shows general good agreement at all three sites, with perhaps some evidence for measured soil respiration being less than expected. Twenty to thirty percent of fixed C is allocated belowground. Comparison of gross primary productivity (GPP), derived from ecosystem flux measurements with that derived from component studies (NPP plus autotrophic respiration) provides an additional crosscheck. The two approaches are in good agreement, giving increased confidence in both approaches to estimating GPP. The ecosystem carbon-use efficiency (CUEs), the ratio of NPP to GPP, is similar at Manaus (0.34 +/- 0.10) and Caxiuana (0.32 +/- 0.07), but may be higher at Tapajos (0.49 +/- 0.16), although the difference is not significant. Old growth or infertile tropical forests may have low CUE compared with recently disturbed and/or fertile forests

We developed an individual-based stochasticempirical model to simulate the carbon dynamics of live and dead trees in a Central Amazon forest near Manaus, Brazil. The model is based on analyses of extensive field studies carried out on permanent forest inventory plots, and syntheses of published studies. New analyses included: (1) growth suppression of small trees, (2) maximum size (trunk base diameter) for 220 tree species, (3) the relationship between growth rate and wood density, and (4) the growth response of surviving trees to catastrophic mortality (from logging). The model simulates a forest inventory plot, and tracks recruitment, growth, and mortality of live trees, decomposition of dead trees (coarse litter), and how these processes vary with changing environmental conditions. Model predictions were tested against aggregated field data, and also compared with independent measurements including maximum tree age and coarse litter standing stocks. Spatial analyses demonstrated that a plot size of ~10 ha was required to accurately measure wood (live and dead) carbon balance. With the model accurately predicting relevant pools and fluxes, a number of model experiments were performed to predict forest carbon balance response to perturbations including: (1) increased productivity due to CO2 fertilization, (2) a single semi-catastrophic (10%) mortality event, (3) increased recruitment and mortality (turnover) rates, and (4) turnover, increased tree growth rates, and decreased mean wood density of new recruits. Results demonstrated that carbon accumulation over the past few decades observed on tropical forest inventory plots (~0.5 Mg C ha"^ year"') is not likely caused by CO2 fertilization. A maximum 25% increase in woody tissue productivity with a doubling of atmospheric CO2 only resulted in an accumulation rate of 0.05 Mg C ha"' year"' for the period 1980-2020 for a Central Amazon forest, or an order of magnitude less than observed on the inventory plots. In contrast, model parameterization based on extensive data from a logging experiment demonstrated a rapid increase in tree growth following disturbance, which could be misinterpreted as carbon sequestration if changes in coarse litter stocks were not considered. Combined results demonstrated that predictions of changes in forest carbon balance during the twenty-first century are highly dependent on assumptions of tree response to various perturbations, and underscores the importance of a close coupling of model and field investigations.

RESUMOOs fatores que envolvem os processos da dinâmica da floresta influenciam a sua biodiversidade e, portanto, a qualidade da floresta. A definição de estratégias que envolve a proteção e o uso adequado da floresta manejada e a recuperação de áreas já degradadas tornam-se possível com o estudo da estrutura e dinâmica da floresta primária por meio de informações como a mortalidade, o recrutamento e a permanência das árvores no sistema florestal. Este trabalho teve como objetivo avaliar a dinâmica de uma floresta não perturbada e fazer projeções da dinâmica florestal usando a matriz de transição probabilística (Cadeia de Markov). (2008)). O teste Qui-quadrado mostrou que não há diferença significativa (1% de probabilidade) entre as informações coletadas e projetadas. Esses resultados permitem concluir que a Cadeia de Markov é um eficiente instrumento para projetar a dinâmica da floresta natural, contribuindo para o planejamento em curto prazo das atividades que utilizam os recursos florestais. PALAVRAS-CHAVEbiomassa, Recrutamento, Mortalidade, Incremento. Dynamics of the Terra-firme primary forest in Manaus-AM region using the Markov probabilistic transition matrix ABSTRACTTo combine protection and utilization of forest resources in the tropics, the understanding of forest dynamics is essential. It is also important in the definition of strategies for rehabilitation of degraded areas. In Forestry, forest dynamics could be translated as the understanding of recruitment, mortality and biomass increment rates over time. For this study, these rates were estimated based on measurements carried out in 2000 and 2004 over two transects measuring 20 by 2500 m (5 hectares) each, in Manaus region. This paper deals with forest dynamics of a pristine forest based on the probabilistic transition matrix (the first-order Markov Chain) approach. The main objective is to report 4-year (2000 to 2004) changes in the forest structure. Diameter distribution and tree mortality will be projected ahead to 2008 (t+2), based upon a 4-year period of observations completed in 2004 (t+1) and its immediate past in 2000 (t). In terms of fresh aboveground biomass, this site accumulated 8.34 t.ha -1 .ano -1 . The χ 2 test has shown no statistical difference (p = 0.01) between observed diameter frequency and the expected projected by Markov Chain. This result indicates that the Markov Chain approach is a reliable tool to project the forest dynamics on a short-term basis. In 2008, the total number of individuals will have a decrease of 2.7%, and the mortality rate will 15% higher than in 2004.

RESUMONa Amazônia, o fogo é ainda o principal trato cultural utilizado no preparo de solo para agricultura e pecuária, tanto pelos pequenos como pelos grandes fazendeiros. Combinando à baixa fertilidade do solo e ao baixo preço da terra, assim que as fontes naturais de nutrientes são exauridas, as áreas são abandonadas e novas florestas primárias são derrubadas e queimadas. Por conta disso, grandes extensões de área da Amazônia são cobertas por florestas secundárias originadas de áreas abandonadas pela agricultura ou pastagem. Este estudo foi conduzido em uma área experimental usada em uma pesquisa sobre eficiência de combustão e emissão de gás carbônico da floresta amazônica, localizada aproximadamente 50 km ao norte de Manaus. A vegetação da área experimental foi derrubada e queimada em 1991, simulando as condições em que o pequeno agricultor prepara o solo para plantios de subsistência. Dez anos após a queimada, a floresta secundária ainda é bastante diferente da floresta original. As espécies vegetais dominantes são, principalmente, das famílias botânicas Annonaceae, Arecaceae, Burseraceae, Cecropiaceae, Euphorbiaceae, Fabaceae, Lecythidaceae, Melastomataceae, Mimosaceae, Sapindaceae e Sterculiaceae. O estoque de biomassa recuperado, dez anos após a formação da capoeira estudada, é de aproximadamente 16%, ou seja, a capoeira apresenta um estoque médio de 56,2 t.ha -1 ± 12 (IC 95%), enquanto que o estoque da floresta primária é de 339,7 t.ha -1 ± 66,7 (IC 95 %). PALAVRAS-CHAVE

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