The effect of carbon dioxide enrichment on apparent stem respiration from Pinus taeda L. is confounded by high levels of soil carbon dioxide

Respiration supports growth and maintenance processes and returns a substantial portion of the CO2 fixed by photosynthesis to the atmosphere each year. Investigating stem respiration using CO2 flux measurements is complicated by uncertainty surrounding the source of CO2 diffusing from tree stems. Over 2 years we measured the stem efflux from 24 trees exposed to ambient or elevated CO2. The rate of stem CO2 efflux increased with annual tree diameter increment and the estimated uptake of dissolved CO2 from the soil. To determine the source of CO2 diffusing from tree stems, we used the fumigation gas at the Duke Forest Atmosphere Carbon Transfer and Storage-1 elevated-CO2 experiment as a C-13 tracer and measured the presence of soil CO2 in stem efflux on a subset of these trees. The isotopic composition of soil CO2 explained a considerable portion of the variation in the composition of CO2 in stem efflux. We also found that direct measurements of the isotopic composition of phloem-respired CO2, unlike the CO2 found in stem efflux, was less variable and distinct from the isotopic composition of soil CO2. Tree growth rates and soil CO2 concentrations found at the site together explained 56% of the variance in stem CO2 efflux among trees. These results suggest that the uptake of CO2 dissolved in soil water and transported through the vascular system can potentially confound efforts to interpret stem efflux measurements in trees exposed to elevated CO2 and that previous studies may have overestimated the effects of elevated CO2 on autotrophic respiration in tree stems.