Savannah River Ecology Lab, University of Georgia, Aiken, SC, USA.
Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA, USA.
Glob Chang Biol. 2021 Jun;27(12):2991-3000. doi: 10.1111/gcb.15624. Epub 2021 Apr 12.
Previous research has indicated that a potentially large portion of root-respired CO can move internally through tree xylem, but these reports are relatively scarce and have generally been limited to short observations. Our main objective was to provide a continuous estimate of the quantity and variability of root-respired CO that moves either internally through the xylem (F ) or externally through the soil to the atmosphere (F ) over most of a growing season. Nine trees were measured in a Populus deltoides stand for 129 days from early June to mid-October. We calculated F as the product of sap flow and dissolved [CO ] in the xylem (i.e., [CO *]) and calculated F using the [CO ] gradient method. During the study, stem and soil CO concentrations, temperature, and sap flow were measured continuously. We determined that F accounted for 33% of daily total belowground CO flux (i.e., F + F ; F ) during our observation period that spanned most of a growing season. Cumulative daily F was lower than F 74% of the time, equivalent to F 26% of the time, and never exceeded F . One-third of the total CO released by belowground respiration over most of the growing season in this forest stand followed the F pathway rather than diffusing into the soil. The magnitude of F indicates that measurements of F alone substantially underestimate total belowground respiration in some forest ecosystems by systematically underestimating belowground autotrophic respiration. The variability in F observed during the growing season demonstrated the importance of making long-term, high-frequency measurements of different flux pathways to better understand physiological and ecological processes and their implications to global change.
先前的研究表明，很大一部分根系呼吸产生的 CO 可以在树木木质部内部移动，但这些报告相对较少，且通常仅限于短期观测。我们的主要目的是提供一个连续的估计，即在整个生长季节的大部分时间内，通过木质部内部（F ）或通过土壤外部到大气（F ）移动的根呼吸 CO 的数量和可变性。从 6 月初到 10 月中旬，我们对一个杨树林中的 9 棵树进行了 129 天的测量。我们将 F 计算为木质部中 sap 流和溶解的[CO ]的产物（即[CO *]），并使用[CO ]梯度法计算 F 。在研究过程中，连续测量了树干和土壤中的 CO 浓度、温度和 sap 流。我们确定，在我们的观测期内，F 占每天总地下 CO 通量（即 F + F ；F ）的 33%，该观测期跨越了大部分生长季节。F 的累积日值低于 F ，占 74%，而 F 占 26%，且从未超过 F 。在这个森林群落中，大部分生长季节地下呼吸释放的 CO 有三分之一通过 F 途径而不是扩散到土壤中。F 的大小表明，仅通过 F 测量会系统地低估地下自养呼吸，从而大大低估某些森林生态系统中的总地下呼吸。在生长季节观察到的 F 变异性表明，进行长期、高频的不同通量途径测量对于更好地理解生理和生态过程及其对全球变化的影响非常重要。
