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. The ratio of CO2 efflux to O2 influx (ARQ,apparent respiratory quotient) in tree stems is expected to be 1.0 forcarbohydrates, the main substrate supporting stem respiration. In previousstudies of stem fluxes, ARQ values below 1.0 were observed and hypothesizedto indicate retention of respired carbon within the stem. Here, wedemonstrate that stem ARQ   1.0 values are common across 85tropical, temperate, and Mediterranean forest trees from nine differentspecies. Mean ARQ values per species per site ranged from 0.39 to 0.78, withan overall mean of 0.59. Assuming that O2 uptake provides a measure ofin situ stem respiration (due to the low solubility of O2 ), the overall meanindicates that on average 41 % of CO2 respired in stems is notemitted from the local stem surface. The instantaneous ARQ did not vary withsap flow. ARQ values of incubated stem cores were similar to those measuredin stem chambers on intact trees. We therefore conclude that dissolution of CO2 in the xylem sap and transport away from the site of respirationcannot explain the low ARQ values. We suggest refixation of respired CO2 in biosynthesis reactions as possible mechanism for low ARQ values.

biogeosciences

Comparison of CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; fluxes demonstrate retention of respired CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; in tree stems from a range of tree species