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A simple, nondestructive method for the estimation of canopy CO2 uptakeis important for understanding the CO2 exchange between forest andatmosphere. Canopy CO2 uptake (FCO2) of a subtropicalmature \textit{A. mangium} plantation was estimated by combining sap flowmeasurements and stable carbon isotope discrimination (Δ) in SouthernChina from 2004 to 2007. The mechanistic relationship linkingFCO2, Δ in leaf sap, and sap flow-based canopy stomatalconductance (Gs) was applied in our study. No significantseasonal variations were observed in Δ or in the ratio of theintercellular and ambient CO2 concentrations(Ci/Ca), although diurnalCi/Ca varied between sunlit and shaded leaves. Asensitivity analysis showed that estimates of FCO2 were moresensitive to dynamics in Gs than in Ca andΔ. By using seasonally and canopy averagedCi/Ca values, we obtained an acceptable estimate ofFCO2 compared to other estimates. FCO2 exhibitedsimilar diurnal variation to that of Gs. Large seasonalvariation in FCO2 was attributed to the responsiveness ofGs to vapor pressure deficit, photosynthetically activeradiation, and soil moisture deficit. Our estimate of FCO2 for amature A. mangium plantation(2.13 ± 0.40 gC m−2 d−1) approached the lower range ofvalues for subtropical mixed forests, probably due to lower mean canopystomatal conductance, higher Ci/Ca, and greatertree height than other measured forests. Our estimate was also lower thanvalues determined by satellite-based modeling or carbon allocation studies,suggesting the necessity of stand level flux data for verification.Qualitatively, the sap flux/stable isotope results compared well with gasexchange results. Differences in results between the two approaches likelyreflected variability due to leaf position and age, which should be reducedfor the combined sap flux and isotope technique, as it uses canopy averagevalues of Gs and Ci/Ca

biogeosciences

CO&amp;lt;sup&amp;gt;2&amp;lt;/sup&amp;gt; uptake of a mature &amp;lt;i&amp;gt;Acacia mangium&amp;lt;/i&amp;gt; plantation estimated from sap flow measurements and stable carbon isotope discrimination