The extent to which pre-Columbian societies altered Amazonian landscapes is hotly debated. We performed a basin-wide analysis of pre-Columbian impacts on Amazonian forests by overlaying known archaeological sites in Amazonia with the distributions and abundances of 85 woody species domesticated by pre-Columbian peoples. Domesticated species are five times more likely than nondomesticated species to be hyperdominant. Across the basin, the relative abundance and richness of domesticated species increase in forests on and around archaeological sites. In southwestern and eastern Amazonia, distance to archaeological sites strongly influences the relative abundance and richness of domesticated species. Our analyses indicate that modern tree communities in Amazonia are structured to an important extent by a long history of plant domestication by Amazonian peoples

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.

Using a simple isotope mixing model, we evaluated the relative proportion of water vapour generated by plant transpiration and by soil evaporation at two sites in the Amazon basin. Sampling was carried out at two different soil covers (forest and pasture), in a seasonal tropical rainforest at eastern Amazon where major deforestation is the result of land‐use change, and compared to a less seasonal central Amazon forest. In both forests, vapour from transpiration was responsible for most, if not all, of the water vapour generated in the forest, while it could not be detected above the grassy pastures. Thus the canopy transpiration may be a major source of water vapour to the forest and perhaps to the atmosphere during the dry season. The results are discussed in relation to predictive models based on net radiation that usually are not able to distinguish between transpiration and evaporation.

Analyses of forest loss and protected areas suggest that 36 to 57% of Amazonian tree flora may qualify as “globally threatened.”