Comparison of CO2 and O2 fluxes demonstrate retention of respired CO2 in tree stems from a range of tree species

The ratio of <span classCombining double low line"inline-formula">CO2</span> efflux to <span classCombining double low line"inline-formula">O2</span> influx (ARQ, apparent respiratory quotient) in tree stems is expected to be 1.0 for carbohydrates, the main substrate supporting stem respiration. In previous studies of stem fluxes, ARQ values below 1.0 were observed and hypothesized to indicate retention of respired carbon within the stem. Here, we demonstrate that stem ARQ&thinsp;<span classCombining double low line"inline-formula">&lt;</span>&thinsp;1.0 values are common across 85 tropical, temperate, and Mediterranean forest trees from nine different species. Mean ARQ values per species per site ranged from 0.39 to 0.78, with an overall mean of 0.59. Assuming that <span classCombining double low line"inline-formula">O2</span> uptake provides a measure of in situ stem respiration (due to the low solubility of <span classCombining double low line"inline-formula">O2</span>), the overall mean indicates that on average 41&thinsp;% of <span classCombining double low line"inline-formula">CO2</span> respired in stems is not emitted from the local stem surface. The instantaneous ARQ did not vary with sap flow. ARQ values of incubated stem cores were similar to those measured in stem chambers on intact trees. We therefore conclude that dissolution of <span classCombining double low line"inline-formula">CO2</span> in the xylem sap and transport away from the site of respiration cannot explain the low ARQ values. We suggest refixation of respired <span classCombining double low line"inline-formula">CO2</span> in biosynthesis reactions as possible mechanism for low ARQ values.

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