Ecosystem respiration (E_R) consumes up to 85% of annual gross primary productivity in forest ecosystems –a CO₂ flux nearly 16 times greater than annual fossil fuel emissions. Forest ecosystems are therefore an integral component of global carbon budgets. A mechanistic understanding of the origin and regulation of plant-respired contributions to E_R is imperative to accurately determine carbon budgets of forest ecosystems and predict responses to climate change. Recent reports indicate that CO₂ diffusing outward from tree stems is not necessarily produced locally but can be transported via the xylem stream from other sources. Further evidence suggests that a large portion of the CO₂ in the xylem may originate in the root system. The relative importance of this root-derived CO₂ flux pathway (F_T) remains unclear as no empirical investigations have quantified the magnitude and directly compared it with the flux of CO₂ from the soil surface. To determine the magnitude and relative importance of F_T, we instrumented eastern cottonwood (Populus deltoides L.) trees to measure xylem dissolved [CO₂] and sap flux and simultaneously measured soil CO₂ efflux near each tree.

Results/Conclusions

On a daily basis, the amount of CO₂ that fluxes upward from the roots into the stem via xylem stream rivals that fluxing from the soil surface. Thus, belowground autotrophic respiration consumes substantially more carbohydrates in mitochondrial respiration than previously recognized. It also indicates the carbohydrate cost of stem respiration has been significantly overestimated because a portion of F_T diffuses to the atmosphere. Root-derived CO₂ fluxing through the xylem stream may be recycled through leaf or stem photosynthesis. Our estimates indicate that approximately twice as much CO₂ released from root systems enters the xylem stream than diffuses into the soil. These results further suggest that belowground autotrophic respiration may exceed aboveground autotrophic (leaf and woody tissue) respiration. These considerably higher rates of root respiration also indicate the inadequacy of using soil CO₂ efflux alone to place upper limits on total belowground carbon allocation and thus challenge assumptions made in forest carbon cycling models. These findings have important implications for how we currently understand physiological functioning of trees, carbon cycling in forests and global carbon budgets and indicate the F_T should be measured concurrently with CO₂ efflux from soil to understand metabolism of root systems.