Xylem transport of root‐derived CO₂ caused a substantial underestimation of belowground respiration during a growing season

Abstract

Abstract Previous research has indicated that a potentially large portion of root‐respired CO 2 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 2 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 [CO 2 ] in the xylem (i.e., [CO 2 *]) and calculated F S using the [CO 2 ] gradient method. During the study, stem and soil CO 2 concentrations, temperature, and sap flow were measured continuously. We determined that F T accounted for 33% of daily total belowground CO 2 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 CO 2 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.

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