Ecological interpretation of leaf carbon isotope ratios: influence of respired carbon dioxide

In a Neotropical moist forest at Barro Colorado Island, Panama, @?^1^3C values of CO"2 in air and @?^1^3 values of leaf tissue exhibit parallel patterns of variation between the forest floor and the canopy. During the daytime, @?^1^3C values of CO"2 from air sampled at 1 m and 0.5 m were significantly less than that at 25 m. Based on mass balance equations, up to 18% of the CO"2 in air at 0.5 m above the forest floor is from respiration. Respired CO"2 is responsible for 31 and 37% of the variation in isotope composition in leaves of two species of herbaceous bamboo grown in a well-ventilated sun treatment and in the forest understory. Respired CO"2 accounts for 45-70% of the difference in @?^1^3C values between understory and canopy leaves for three species growing in large-scale irrigation and control treatments. Understory leaves of these species show @?^1^3 values consistent with higher ratios of intercellular to ambient CO"2 in irrigated relative to control treatments. Estimates of water-use efficiency from leaf carbon isotope content should be corrected for the contribution of the carbon isotope composition of respired CO"2 in closed-canopy forests.