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Stem respiration rates are often quantified by measuring the CO 2  efflux from stems into chambers. It has been suggested that these measurements underestimate respiration because some of the respired CO 2  can be either retained or transported upwards in the transpiration stream. If the stem CO 2  efflux does not represent all respired CO 2 , then the interpretation of its isotopic signal may be compromised as well. The C-isotope composition of the respired CO 2  and the measured efflux could differ due to (i) the release of CO 2  produced elsewhere into the stem and transported upwards in xylem water (soil CO 2  or root respired CO 2 ); (ii) the retention or release of CO 2  storage pools within the tree stem and (iii) the removal of CO 2  by the transpiration stream. We investigated the effects of these processes in large conifer trees using two manipulative experiments: a labelling experiment and a crown removal experiment. The labelling experiment used an extreme enrichment of dissolved CO 2  in soil water to assess the C uptake by the roots. In this experiment, we found no contamination of the stem CO 2  pool despite clear evidence that the water itself had been taken up. The crown removal experiment tested for vertical CO 2  flux in xylem water by eliminating transpiration. Here, we found no change in the δ 13 C of stem CO 2  efflux (δ EA ;  P  &gt; 0.05). We concluded that for these large conifers, sap-flow influenced neither δ 13 C of stem efflux nor that of the stem CO 2  pool. By parameterizing Henry’s Law for conditions inside the stem, we estimated the transport flux to represent 1–3% of the stem CO 2  efflux to the atmosphere. Finally, assuming a 2‰ difference between δ 13 C of root and stem respiration, we estimated that potential contamination of δ EA  by root respired CO 2  would be &lt; 0.1‰. Thus, neither the release of soil or root CO 2 , nor storage in the stem, nor vertical transport of CO 2  in the xylem sap had any detectable influence on δ 13 C of the CO 2  measured in stem efflux.

Storage and transpiration have negligible effects on δ 13 C of stem CO 2 efflux in large conifer trees