COS 61-8: Storage and vertical transport of CO2 have negligible effects on δ13C of stem CO2 efflux in large conifer trees
Nerea Ubierna1, John D. Marshall1, Lucas A. Cernusak2, and Arjun Kumar3. (1) University of Idaho, (2) Charles Darwin University, (3) The Georgia Institute of Technology
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.