Ecological Interpretation of Leaf Carbon Isotope Ratios: Influence of Respired Carbon Dioxide

In a Neotropical moist forest at Barro Colorado Island, Panama, 6'3C values of CO2 in air and (13C values of leaf tissue exhibit parallel patterns of variation between the forest floor and the canopy. During the daytime, 613C values of CO2 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 CO2 in air at 0.5 m above the forest floor is from respiration. Respired CO2 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 CO2 accounts for 45-70% of the difference in 613C values between understory and canopy leaves for three tree species growing in large-scale irrigation and control treatments. Understory leaves of these species show 613C values consistent with higher ratios of intercellular to ambient CO2 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 CO2 in closed-canopy forests.