Han Fengsen, Wang Xiaolin, Zhou Hongxuan, Li Yuanzheng, Hu Dan
State Key Laboratory of Urban and Regional Ecology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, Republic of China.
University of Chinese Academy of Sciences, Beijing, 100049, Republic of China.
J Plant Res. 2017 Sep;130(5):845-858. doi: 10.1007/s10265-017-0951-3. Epub 2017 May 23.
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.
树木茎干和枝条的CO排放通量（E）在树体内变化很大。为了探究E的时间动态和垂直变化背后的机制，我们在6棵大小相似的国槐树上，每月连续两天，使用红外气体分析法（IRGA）测量0.1至3.7米高度范围内10个不同高度处的茎干E以及气象条件，为期1年。结果表明，E的季节变化大致遵循木质组织温度（T）和茎干径向直径增量（Di）的季节变化。T和Di共同解释了E的月度变化，且T和Di的贡献随茎干位置和生长阶段而变化。生长季和休眠季E的日变化模式差异很大，前者呈双峰分布且中午明显降低，后者呈单峰分布。生长季白天E的强烈垂直变化主要是由T、Di的垂直梯度以及沿树干单位茎干表面积边材体积的差异引起的。温度敏感性系数（Q）并不像一些模型假设的那样恒定，而是随垂直位置变化，并且高度依赖于测量温度。对于所有茎干位置，最高Q值出现在约5°C时，温度过高或过低都会使Q值降低。我们的研究表明，在将基于气室的测量结果扩展到整茎水平的年度碳预算时，应用恒定的Q值会导致估计误差。
