The canopy effect, carbon isotope ratios and foodwebs in amazonia

In forests, the canopy effect produces 13C-depleted plants and a gradient of leaf δ13C values from ground to canopy; the most negative values are near the ground. Explanations for this phenomenon include recycling of 13C-depleted CO2 in the forest, fractionation due to photosynthesis in low light, and other physiological causes. We report δ13C measurements for CO2 at different heights in two forests of the upper Amazon basin. The results show that much of the 13C depletion in leaves derives from photosynthetic recycling of CO2 produced by forest soil respiration. Recycling does not, however, account adequately for the observed height gradient in δ13C values which must be due to an additional factor(s). 13C-depleted forest CO2 is also photosynthesized by plants in forest clearings. Furthermore, the canopy effect is passed along the foodchain to forest and aquatic fauna, as well as humans. This is of particular importance when using carbon isotopes to reconstruct prehistoric dietary regimes, e.g. the use of maize in Amazonia. © 1991.