Origin, fate and significance of CO2 in tree stems

Although some CO₂ released by respiring cells in tree stems diffuses directly to the atmosphere, on a daily basis 15–55% can remain within the tree. High concentrations of CO₂ build up in stems because of barriers to diffusion in the inner bark and xylem. In contrast with atmospheric CO₂] of c.  0.04%, the CO₂] in tree stems is often between 3 and 10%, and sometimes exceeds 20%. The CO₂] in stems varies diurnally and seasonally. Some respired CO₂ remaining in the stem dissolves in xylem sap and is transported toward the leaves. A portion can be fixed by photosynthetic cells in woody tissues, and a portion diffuses out of the stem into the atmosphere remote from the site of origin. It is now evident that measurements of CO₂ efflux to the atmosphere, which have been commonly used to estimate the rate of woody tissue respiration, do not adequately account for the internal fluxes of CO₂. New approaches to quantify both internal and external fluxes of CO₂ have been developed to estimate the rate of woody tissue respiration. A more complete assessment of internal fluxes of CO₂ in stems will improve our understanding of the carbon balance of trees.