Abstract. Understanding how tropical forest carbon balance will respond to global change requires knowledge of individual heterotrophic and autotrophic respiratory sources, together with factors that control respiratory variability. We measured leaf, live wood, and soil respiration, along with additional environmental factors over a 1-yr period in a Central Amazon terra firme forest. Scaling these fluxes to the ecosystem, and combining our data with results from other studies, we estimated an average total ecosystem respiration (R eco ) of 7.8 mol·m Ϫ2 ·s Ϫ1. Average estimates (per unit ground area) for leaf, wood, soil, total heterotrophic, and total autotrophic respiration were 2.6, 1.1, 3.2, 5.6, and 2.2 mol·m Ϫ2 ·s Ϫ1 , respectively. Comparing autotrophic respiration with net primary production (NPP) estimates indicated that only ϳ30% of carbon assimilated in photosynthesis was used to construct new tissues, with the remaining 70% being respired back to the atmosphere as autotrophic respiration. This low ecosystem carbon use efficiency (CUE) differs considerably from the relatively constant CUE of ϳ0.5 found for temperate forests. Our R eco estimate was comparable to the above-canopy flux (F ac ) from eddy covariance during defined sustained high turbulence conditions (when presumably F ac ϭ R eco ) of 8.4 (95% CI ϭ 7.5-9.4). Multiple regression analysis demonstrated that ϳ50% of the nighttime variability in F ac was accounted for by friction velocity (u*, a measure of turbulence) variables. After accounting for u* variability, mean F ac varied significantly with seasonal and daily changes in precipitation. A seasonal increase in precipitation resulted in a decrease in F ac , similar to our soil respiration response to moisture. The effect of daily changes in precipitation was complex: precipitation after a dry period resulted in a large increase in F ac , whereas additional precipitation after a rainy period had little effect. This response was similar to that of surface litter (coarse and fine), where respiration is greatly reduced when moisture is limiting, but increases markedly and quickly saturates with an increase in moisture.

Summary The temperature response of photosynthesis is one of the key factors determining predicted responses to warming in global vegetation models (GVMs). The response may vary geographically, owing to genetic adaptation to climate, and temporally, as a result of acclimation to changes in ambient temperature. Our goal was to develop a robust quantitative global model representing acclimation and adaptation of photosynthetic temperature responses. We quantified and modelled key mechanisms responsible for photosynthetic temperature acclimation and adaptation using a global dataset of photosynthetic CO2 response curves, including data from 141 C3 species from tropical rainforest to Arctic tundra. We separated temperature acclimation and adaptation processes by considering seasonal and common‐garden datasets, respectively. The observed global variation in the temperature optimum of photosynthesis was primarily explained by biochemical limitations to photosynthesis, rather than stomatal conductance or respiration. We found acclimation to growth temperature to be a stronger driver of this variation than adaptation to temperature at climate of origin. We developed a summary model to represent photosynthetic temperature responses and showed that it predicted the observed global variation in optimal temperatures with high accuracy. This novel algorithm should enable improved prediction of the function of global ecosystems in a warming climate.

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Satellite and tower-based metrics of forest-scale photosynthesis generally increase with dry season progression across central Amazônia, but the underlying mechanisms lack consensus. We conducted demographic surveys of leaf age composition, and measured the age dependence of leaf physiology in broadleaf canopy trees of abundant species at a central eastern Amazon site. Using a novel leaf-to-branch scaling approach, we used these data to independently test the much-debated hypothesis - arising from satellite and tower-based observations - that leaf phenology could explain the forest-scale pattern of dry season photosynthesis. Stomatal conductance and biochemical parameters of photosynthesis were higher for recently mature leaves than for old leaves. Most branches had multiple leaf age categories simultaneously present, and the number of recently mature leaves increased as the dry season progressed because old leaves were exchanged for new leaves. These findings provide the first direct field evidence that branch-scale photosynthetic capacity increases during the dry season, with a magnitude consistent with increases in ecosystem-scale photosynthetic capacity derived from flux towers. Interactions between leaf age-dependent physiology and shifting leaf age-demographic composition are sufficient to explain the dry season photosynthetic capacity pattern at this site, and should be considered in vegetation models of tropical evergreen forests.

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RESUMOAs três parcelas permanentes usadas neste estudo são testemunhas (não perturbadas) de um experimento de manejo florestal do Instituto Nacional de Pesquisas da Amazônia, no município de Manaus (AM). Essas parcelas têm sido monitoradas desde 1980, mas para efeito deste estudo, foram consideradas 12 medições repetidas no período 1986-2000. Durante este período, o fenômeno El Niño (seca anormal na região) ocorreu em duas ocasiões, em 1992-93 e 1997-98, sendo que o último foi seguido do La Niña (chuva anormal na região), em 1999. Devido a esses fenômenos, as taxas de recrutamento e mortalidade foram iguais, 0,7%, durante o período observado. No entanto, a acumulação (fixação na árvore) de carbono, foi de 16 toneladas métricas, dando um incremento periódico anual significativo (p = 0,039), em torno de 1,2 t/ha/ano. Palavras-chave: biomassa, parcelas permanentes, mudanças climáticas CARBON BALANCE AND DYNAMICS OF PRIMARY VEGETATION IN THE CENTRAL AMAZON ABSTRACTThe three permanent forest inventory plots used for this study were control plots (not disturbed) from a forest management project of the National Institute of Amazon Research (INPA) in the Brazilian State of Amazonas. These plots have been monitored since 1980, although for this study the period from 1986-2000 was considered. During this period, the El Niño phenomenon, which causes increased drought in the region, occurred on two occasions (1992-93 and 1997-98), followed by La Niña which causes increased precipitation in the region (1999)(2000). Despite of this change in climate, recruitment and mortality rates were equal throughout the period at 0.7% yr-1. During the same period, carbon accumulation in forest biomass was 16 Mg, resulting in a statistically significant (p = 0.039) increase of about 1.2 Mg biomass ha-1 yr-1.

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Com o objetivo de avaliar as respostas ecofisiológicas de mudas de macacaporanga (Aniba parviflora) em diferentes níveis de sombreamento foi conduzido um experimento no viveiro de mudas da Universidade Federal do Oeste do Pará, em Santarém-PA. Os níveis de sombreamento foram 30%, 50% e 70% de sombreamento e a pleno sol com 0% de sombreamento. O delineamento foi inteiramente casualizado, com quatro repetições, das quais cada uma foi composta por cinco plantas. Foram avaliados altura, diâmetro, número de folhas, área foliar, área foliar específica, massa seca da raiz, caule, folhas e total, taxa assimilatória líquida, condutância estomática, transpiração, eficiência do uso da água e teores de clorofila. Os maiores valores para altura, área foliar e área foliar específica foram obtidos sob 70% de sombreamento, no entanto, para diâmetro e número de folhas os maiores valores foram sob 50% de sombreamento. O sombreamento de 70% proporcionou os maiores valores na taxa assimilatória líquida, condutância estomática, transpiração, eficiência do uso da água, clorofilas a, b e total. O sombreamento de 50% ocasionou os maiores valores na massa seca de raiz, caule, folhas e total. O nível de sombreamento de 50% proporcionou o melhor crescimento e desenvolvimento das mudas.