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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 …