Xylem transport of root-derived CO2: Is this pathway of respired CO2 important throughout the growing season

Background/Question/Methods

Forest ecosystems account for the majority of terrestrial net primary productivity and forest ecosystem respiration (R_E) releases one of the largest annual CO₂ fluxes of the global carbon cycle. Forest R_E is dominated by belowground contributions of autotrophic and heterotrophic respiration which account for up to 88% of total annual R_E. A mechanistic understanding of forest respiratory flux pathways is imperative to understanding forest carbon cycles and forest ecosystem responses to climate change. We recently demonstrated that, on a daily basis, the amount of CO₂ that fluxes upward from tree root systems into stems via the xylem stream rivals the amount of CO₂ diffusing from the soil surface. However, our original observations were limited to only four individual trees over a single week where environmental conditions remained similar. Here, we further investigate this alternative flux pathway for root-derived CO₂ throughout a growing season using nine eastern cottonwood (Populus deltoides L.) trees. We calculated the internal transport of root-derived CO₂ as the product of sap flow and dissolved CO₂ concentration ([CO₂]) in the xylem at the base of the stem and measured soil CO₂ efflux using the [CO₂] gradient approach.

Results/Conclusions

We found the internal transport of root-derived CO₂ accounted for a large portion of total belowground respiration throughout the growing season. However, the relative importance of internally transported CO₂ for total belowground respiration was influenced by both seasonal and daily environmental factors that influenced sap flow rates. We observed large variations in internal gaseous [CO₂] among and within individual trees through time (1-20%). However, temporal patterns of internal [CO₂] were inconsistent across individual trees. For example, some individuals contained higher internal [CO₂] in June than in October whereas others exhibited an opposing pattern. Our results provide further indication that belowground autotrophic respiration consumes a larger amount—and stem respiration consumes a smaller amount—of carbohydrates than previously realized. Our findings have important implications for how we understand physiological functioning of trees and carbon cycling in forests. Acknowledgement of the alternative pathway for root-derived CO₂ flux highlights the inadequacy of using soil CO₂ efflux alone to measure and model belowground respiration. We suggest the internal transport of root-derived CO₂ should be measured concurrently with CO₂ efflux from soil and other tree organs to more fully understand the mechanisms regulating carbon cycling at the tree and ecosystem levels.