资源环境科技发展态势分析平台

Previous research has indicated that a potentially large portion of root‐respired CO₂ 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 CO₂ that moves either internally through the xylem (F_T) or externally through the soil to the atmosphere (F_S) 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_T as the product of sap flow and dissolved [CO₂] in the xylem (i.e., [CO₂*]) and calculated F_S using the [CO₂] gradient method. During the study, stem and soil CO₂ concentrations, temperature, and sap flow were measured continuously. We determined that F_T accounted for 33% of daily total belowground CO₂ flux (i.e., F_S + F_T; F_B) during our observation period that spanned most of a growing season. Cumulative daily F_T was lower than F_S 74% of the time, equivalent to F_S 26% of the time, and never exceeded F_S. One‐third of the total CO₂ released by belowground respiration over most of the growing season in this forest stand followed the F_T pathway rather than diffusing into the soil. The magnitude of F_T indicates that measurements of F_S alone substantially underestimate total belowground respiration in some forest ecosystems by systematically underestimating belowground autotrophic respiration. The variability in F_T 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.