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

In a Neotropical moist forest at Barro Colorado Island, Panama, ° 1 3 C 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 3 C 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 3 C 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.