Temporal dynamics and vertical variations in stem CO₂ efflux of Styphnolobium japonicum

CO₂ efflux (E_CO2) from stems and branches is highly variable within trees. To investigate the mechanisms underlying the temporal dynamics and vertical variations in E_CO2, we measured the stem E_CO2 by infrared gas analysis (IRGA) and meteorological conditions at 10 different heights from 0.1 to 3.7 m aboveground on two consecutive days every month for 1 year in six Styphnolobium japonicum trees with a similar size. The results indicated that the seasonal change in E_CO2 roughly followed the seasonal variations in woody tissue temperature (T_W) and stem radial diameter increment (Di). Together, T_W and Di explained the monthly change in E_CO2, and the contributions of T_W and Di changed with the stem positions and growth stages. The diurnal patterns of E_CO2 differed greatly between the growing and dormant season, showing a bimodal distribution with an obvious midday depression in the former and a unimodal distribution in the latter. The strong vertical variation in the day-time E_CO2 of the growing season was mainly caused by the vertical gradients of T_W, Di and difference in sapwood volume per unit of the stem surface along the trunk. The temperature-sensitivity coefficient (Q₁₀) was not constant, as assumed in some models, but was instead vertically altered and highly dependent on the measurement temperature. For all stem positions, the highest Q₁₀ value appeared at approximately 5 °C, and both higher and lower temperatures decreased Q₁₀. Our study demonstrated that application of a constant Q₁₀ would cause an estimation error when scaling up chamber-based measurements to annual carbon budgets at the whole-stem level.