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

CO2 efflux (ECO2) from stems and branches is highly variable within trees. To investigate the mechanisms underlying the temporal dynamics and vertical variations in ECO2, we measured the stem ECO2 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 ECO2 roughly followed the seasonal variations in woody tissue temperature (TW) and stem radial diameter increment (Di). Together, TW and Di explained the monthly change in ECO2, and the contributions of TW and Di changed with the stem positions and growth stages. The diurnal patterns of ECO2 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 ECO2 of the growing season was mainly caused by the vertical gradients of TW, 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.