The ratio of CO 2 efflux to O 2 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 < 1.0 values are common across 85 tropical, temperate, and Mediterranean forest trees from 9 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 O 2 uptake provides a measure of in situ stem respiration (due to the low solubility of O 2 ), the overall mean indicates that on average 41 % of CO 2 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 CO 2 in the xylem sap and transport away from the site of respiration cannot explain the low ARQ values. We suggest to examine refixation of respired CO 2 in biosynthesis reactions as possible mechanism for low ARQ values.