Abstract: [Objective] Stem CO₂ efflux rates of various DBH classes and its sensitivity to temperature were investigated of the important pioneer tree species Pinus massoniana in the degraded red soil region. The aims of this study were to providefundamental data for evaluating the carbon sink contribution of stem respiration from Pinus massoniana plantation to the ecosystem.[Method] We conducted year-round in situ chamber measurements of F_CO2 on twelve stems of Pinus massoniana using an automated, portable gas-exchange system (LC PRO-SD, ADC BioScientific Ltd., Hoddesdon, UK), which was equipped with an infrared gas analyzer and a transparent plexiglas cuvette. The sample trees were selected from four DBH classes (15.9-17.9, 18.9-20.5, 22.8-24.8, and 27.1-29.8 cm) with three replicates. At least 24 h prior to measurements, loose bark was removed from the surface of measurement and then collars were mounted horizontally on the north-facing stems at about 135 cm above-ground with 100% silicone sealant. During the process of measurements, the chamber was wrapped with aluminum foil to avoid excess heating by direct sun exposure. Stem temperature at 1 cm depth (T_s) were recorded synchronously. The measurements were conducted from 8:00 to 17:00 for the selected days each month between January and December in 2015 to ensure that the temperature range of data was collected as widely as possible. The temporal fluctuations of F_CO2 and difference between various DBH classes were analyzed. And the exponential regression models were also established between F_CO2 and T_s.[Result] There was clear seasonality in F_CO2 for four diameter classes following similar remarkable single-peak patterns, with the maximum values occurring in July and the minimum in January. Obviously, the monthly F_CO2 values were significantly higher(P<0.01)during the fast-growing season than in the slow-growing season. The values of F_CO2 differed significantly(P<0.01)among various DBH classes during the growing season but not in the non-growing season. Significant exponential relationships were found between F_CO2and T_s, with the correlations of determination higher in the bigger DBH classes than those in the smaller DBH classes. Furthermore, the values of temperature sensitivity (Q₁₀) for four diameter classes varied from 1.47 to 1.64, which were within the range of values previously reported for most conifers. Additionally, F_CO2were significantly correlated positively with DBH(R²=0.89), suggesting that DBH could be used as a proxy for predicting stem respiration for conifers.[Conclusion] Therefore, the temporal variations in F_CO2and difference between various DBH classes should be simultaneously considered when estimating forest carbon budgets so as to improve the accuracy of evaluation.

Key words: Pinus massoniana, stem temperature, stem CO₂ efflux ratio, diameter classes