Abstract
Abstract. A simple, nondestructive method for the estimation of canopy CO2 uptake is important for understanding the CO2 exchange between forest and atmosphere. Canopy CO2 uptake (FCO2) of a subtropical mature \\textit{A. mangium} plantation was estimated by combining sap flow measurements and stable carbon isotope discrimination (Δ) in Southern China from 2004 to 2007. The mechanistic relationship linking FCO2, Δ in leaf sap, and sap flow-based canopy stomatal conductance (Gs) was applied in our study. No significant seasonal variations were observed in Δ or in the ratio of the intercellular and ambient CO2 concentrations (Ci/Ca), although diurnal Ci/Ca varied between sunlit and shaded leaves. A sensitivity analysis showed that estimates of FCO2 were more sensitive to dynamics in Gs than in Ca and Δ. By using seasonally and canopy averaged Ci/Ca values, we obtained an acceptable estimate of FCO2 compared to other estimates. FCO2 exhibited similar diurnal variation to that of Gs. Large seasonal variation in FCO2 was attributed to the responsiveness of Gs to vapor pressure deficit, photosynthetically active radiation, and soil moisture deficit. Our estimate of FCO2 for a mature A. mangium plantation (2.13 ± 0.40 gC m−2 d−1) approached the lower range of values for subtropical mixed forests, probably due to lower mean canopy stomatal conductance, higher Ci/Ca, and greater tree height than other measured forests. Our estimate was also lower than values 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 gas exchange results. Differences in results between the two approaches likely reflected variability due to leaf position and age, which should be reduced for the combined sap flux and isotope technique, as it uses canopy average values of Gs and Ci/Ca.
Highlights
The continued increase in atmospheric carbon dioxide levels due to anthropogenic emissions has led to significant climate changes (Schneider, 1989; Pachauri and Reisinger, 2007)
We demonstrate in this paper that with appropriate sampling, CO2 uptake obtained from the SF/SI approach is representative at the tree canopy scale, and is comparable with results from other methods, including the gas exchange method, carbon allocation studies, and modeling approaches
Our study demonstrated that combining sap flow measurements and stable isotope discrimination provides a simple and accurate way of estimating canopy-scale photosynthesis
Summary
The continued increase in atmospheric carbon dioxide levels due to anthropogenic emissions has led to significant climate changes (Schneider, 1989; Pachauri and Reisinger, 2007). Rising atmospheric CO2 is due to the imbalance between the rates that sources emit CO2 into the atmosphere and the rates that sinks remove CO2 from the atmosphere (Baldocchi et al, 1996). In this context, forests act as vital CO2 sinks by storing carbon in woody biomass (Nowak et al, 2002). Estimation of canopy scale photosynthesis has not been common. Such uncertainties hinder our ability to estimate forest CO2 sequestration for global carbon budgets, as well as to increase CO2 sequestration through forest management
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