Temporal dynamics and vertical variations in stem CO efflux of Styphnolobium japonicum.

CO efflux (E) from stems and branches is highly variable within trees. To investigate the mechanisms underlying the temporal dynamics and vertical variations in E, we measured the stem E 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 roughly followed the seasonal variations in woody tissue temperature (T) and stem radial diameter increment (Di). Together, T and Di explained the monthly change in E, and the contributions of T and Di changed with the stem positions and growth stages. The diurnal patterns of E 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 of the growing season was mainly caused by the vertical gradients of T, Di and difference in sapwood volume per unit of the stem surface along the trunk. The temperature-sensitivity coefficient (Q) 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 Q value appeared at approximately 5 °C, and both higher and lower temperatures decreased Q. Our study demonstrated that application of a constant Q would cause an estimation error when scaling up chamber-based measurements to annual carbon budgets at the whole-stem level.