Our current understanding of respiratory processes in forests is based largely on indirect or ex situ measurements of CO2 fluxes. As such, measurements may fail to accurately reflect the magnitude and dynamics of the respiratory process we strive to understand and model. The fact that a portion of respired CO2 remains inside of trees where it can be transported via the transpiration stream (e.g., from roots to stem, or stem to leaf) warrants reconsideration of how respiration is measured and quantified. This alternative, internal pathway for CO2 transport also calls into question the adequacy of efflux-based estimates of stem respiration and belowground respiration used in models.
Results/Conclusions
I present belowground respiration budgets derived from measurements of internal and external flux pathways that demonstrate the magnitude of the internal transport of root-respired CO2 aboveground through the xylem. These refined budgets underscore our limited understanding of belowground respiratory processes and indicates tree root respiration and, therefore, the quantity of carbon allocated belowground for growth and maintenance of tree roots, have been substantially underestimated. Finally, I present a conceptual model that illustrates our modern understanding of the origin, fate, and significance of respired CO2. The conceptual model illustrates all flux pathways of which we are currently aware and thereby advances our ability to calculate above- and belowground respiration in forests using a holistic approach of measuring internal CO2 flux and external CO2 efflux pathways.