Comparison of CO 2 and O 2 fluxes demonstrate retention of respired CO 2 in tree stems from a range of tree species
2018
Abstract. 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 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 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 refixation of respired
CO 2 in biosynthesis reactions as possible mechanism for low ARQ values.
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