[1] The biogeochemical cycling of carbon, water, energy, aerosols, and trace gases in the Amazon Basin was investigated in the project European Studies on Trace Gases and Atmospheric Chemistry as a Contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). We present an overview of the design of the project, the measurement sites and methods, and the meteorological conditions during the experiment. The main results from LBA-EUSTACH are: Eddy correlation studies in three regions of the Amazon Basin consistently show a large net carbon sink in the undisturbed rain forest. Nitrogen emitted by forest soils is subject to chemical cycling within the canopy space, which results in re-uptake of a large fraction of soilderived NO x by the vegetation. The forest vegetation is both a sink and a source of volatile organic compounds, with net deposition being particularly important for partially oxidized organics. Concentrations of aerosol and cloud condensation nuclei (CCN) are highly seasonal, with a pronounced maximum in the dry (burning) season. High CCN concentrations from biomass burning have a pronounced impact on cloud microphysics, rainfall production mechanisms, and probably on large-scale climate dynamics.

The observation of acclimation in leaf photosynthetic capacity to differences in growth irradiance has been widely used as support for a hypothesis that enables a simplification of some soil-vegetation-atmosphere transfer (SVAT) photosynthesis models. The acclimation hypothesis requires that relative leaf nitrogen concentration declines with relative irradiance from the top of a canopy to the bottom, in 1 : 1 proportion. In combination with a light transmission model it enables a simple estimate of the vertical profile in leaf nitrogen concentration (which is assumed to determine maximum carboxylation capacity), and in combination with estimates of the fraction of absorbed radiation it also leads to simple 'big-leaf' analytical solutions for canopy photosynthesis. We tested how forests deviate from this condition in five tree canopies, including four broadleaf stands, and one needle-leaf stand: a mixed-species tropical rain forest, oak ( Quercus petraea (Matt.) Liebl), birch ( Betula pendula Roth), beech ( Fagus sylvatica L.) and Sitka spruce ( Picea sitchensis (Bong.) Carr). Each canopy was studied when fully developed (mid-to-late summer for temperate stands). Irradiance ( Q , µ µ µ µ mol m Relative V a also declined linearly with relative Q , but with a significant intercept at zero irradiance ( P < 0·01). This intercept was strongly related to L a of the lowest leaves in each canopy ( P < 0·01, r 2 = 0·98, n = 5). For each canopy, daily ln Q was also linearly related with ln V a (P < 0·05), and the intercept was correlated with the value for photosynthetic capacity per unit nitrogen (PUN: Key-words :Acclimation; beech; birch; canopy photosynthesis model; leaf mass per unit area; leaf nitrogen; oak; photosynthetic capacity; Sitka spruce; tropical rain forest.Abbreviations : A max , photon saturated leaf photosynthetic rate at ambient CO 2 concentration ( µ mol m − 2 s − 1 ); A can , canopy photosynthetic rate ( µ mol m − 2 s − 1 ); H , mean canopy height (m); H r , height in canopy relative to H ; J a , maximum electron transfer rate (conventionally J max ) at 25 ° C, on an area basis ( µ mol m ; Q dr , the diffuse radiation component of Q r , obtained from hemispherical photographs; R da , daytime leaf respiration rate at 25 °C (µmol m −2 s −1 ); V a , V m , V ar , V ml , maximum carboxylation rate (conventionally V cmax ) at 25 °C on an area basis (µmol m −2 s −1 ), and a mass basis (nmol g −1 s −1 ), V a relative to V a of the highest measured leaves, V m of the lowest leaves in the canopy; α, is the apparent quantum efficiency, or initial slope of the J/ 344 P. Meir et al.

The vertical profile in leaf photosynthetic capacity was investigated in a terra firme rain forest in central Amazonia. Measurements of photosynthesis were made on leaves at five levels in the canopy, and a model was fitted to describe photosynthetic capacity for each level. In addition, vertical profiles of photosynthetic photon flux density, leaf nitrogen concentration and specific leaf area were measured. The derived parameters for maximum rate of electron transport (J(max)) and maximum rate of carboxylation by Rubisco (V(cmax)) increased significantly with canopy height (P < 0.05). The highest J(max) for a single canopy level was measured at the penultimate canopy level (20 m) and was 103.9 &mgr;mol m(-2) s(-1) +/- 24.2 (SE). The highest V(cmax) per canopy height was recorded at the top canopy level (24 m) and was 42.8 +/- 5.9 &mgr;mol m(-2) s(-1). Values of J(max) and V(cmax) at ground level were 35.8 +/- 3.3 and 20.5 +/- 1.3 &mgr;mol m(-2) s(-1), espectively. The increase in photosynthetic capacity with increasing canopy height was strongly correlated with leaf nitrogen concentration when examined on a leaf area basis, but was only weakly correlated on a mass basis. The correlation on an area basis can be largely explained by the concomitant decrease in specific leaf area with increasing height. Apparent daytime leaf respiration, on an area basis, also increased significantly with canopy height (P < 0.05). We conclude that canopy photosynthetic capacity can be represented as an average vertical profile, perturbations of which may be explained by variations in the environmental variables driving photosynthesis.