Background/Question/Methods
Stem respiration is often quantified by measuring the CO₂ efflux from stems into chambers. These measurements might underestimate respiration if some of the respired CO₂ were stored or transported axially along stems. If stem CO₂ efflux did not represent all respired CO₂, then the interpretation of its isotopic signal might also be compromised. We investigated the effects of these factors on δ¹³C of stem respiration in large conifer trees using two manipulative experiments: a labeling experiment and crown-removal experiment.
Results/Conclusions
The labeling experiment found no evidence of CO₂ uptake from the enriched soil pool (≈ 900‰) despite clear evidence of water uptake from the labeled root zone.  The crown removal experiment revealed no evidence of change in the δ¹³C of stem CO₂ efflux or the stem CO₂ pool. Based on these measurements, we estimated that the vertical transport flux represents 1% to 3% of the stem CO₂ efflux into the atmosphere. If there were a 2‰ difference between δ¹³C of root and stem respiration, then potential contamination of stem CO₂ efflux by root respired CO₂ would be < 0.1‰. Thus, we conclude that neither stored CO₂ in stems, released soil or root CO₂, nor the vertical transport of CO₂ in xylem sap had any influence on δ¹³C of CO₂ measured in stem efflux.