Xylem transport of root-derived CO caused a substantial underestimation of belowground respiration during a growing season.

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.