Storage and transpiration have negligible effects on delta 13C of stem CO2 efflux in large conifer trees

Stem respiration rates are often quantified by measuring the CO2 efflux from stems into chambers. It has been suggested that these measurements underestimate respiration because some of the respired CO2 can be either retained or transported upwards in the transpiration stream. If the stem CO2 efflux does not represent all respired CO2, then the interpretation of its isotopic signal may be compromised as well. The C-isotope composition of the respired CO2 and the measured efflux could differ due to (i) the release of CO2 produced elsewhere into the stem and transported upwards in xylem water (soil CO2 or root respired CO2); (ii) the retention or release of CO2 storage pools within the tree stem and (iii) the removal of CO2 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 CO2 in soil water to assess the C uptake by the roots. In this experiment, we found no contamination of the stem CO2 pool despite clear evidence that the water itself had been taken up. The crown removal experiment tested for vertical CO2 flux in xylem water by eliminating transpiration. Here, we found no change in the delta 13C of stem CO2 efflux (delta (EA); P > 0.05). We concluded that for these large conifers, sap-flow influenced neither delta 13C of stem efflux nor that of the stem CO2 pool. By parameterizing Henry’s Law for conditions inside the stem, we estimated the transport flux to represent 1–3% of the stem CO2 efflux to the atmosphere. Finally, assuming a 2 ppt difference between delta 13C of root and stem respiration, we estimated that potential contamination of delta (EA) by root respired CO2 would be < 0.1 ppt. Thus, neither the release of soil or root CO2, nor storage in the stem, nor vertical transport of CO2 in the xylem sap had any detectable influence on delta 13C of the CO2 measured in stem efflux.