The apparent respiratory quotient (ARQ) of tree stems, defined as the ratio of net stem CO
2 efflux (E
S_CO2) to net stem O
2 influx (E
S_O2), offers insights into the balance between local respiratory CO
2 production and CO
[...] Read more.
The apparent respiratory quotient (ARQ) of tree stems, defined as the ratio of net stem CO
2 efflux (E
S_CO2) to net stem O
2 influx (E
S_O2), offers insights into the balance between local respiratory CO
2 production and CO
2 transported via the xylem. Traditional static chamber methods for measuring ARQ can introduce artifacts and obscure natural diurnal variations. Here, we employed an open flow-through stem chamber with ambient air coupled with cavity ring-down spectrometry, which uses the molecular properties of CO
2 and O
2 molecules to continuously measure E
S_CO2, E
S_O2, and ARQ, at the base of a California cherry tree (
Prunus ilicifolia) during the 2024 growing season. Measurements across three stem chambers over 3–11-day periods revealed strong correlations between E
S_CO2 and E
S_O2 and mean ARQ values ranging from 1.3 to 2.9, far exceeding previous reports. Two distinct diurnal ARQ patterns were observed: daytime suppression with nighttime recovery, and a morning peak followed by gradual decline. Partitioning E
S_CO2 into local respiration and xylem-transported CO
2 indicated that the latter can dominate when ARQ exceeds 2.0. Furthermore, transported CO
2 exhibited a higher temperature sensitivity than local respiration, with both processes showing declining temperature sensitivity above 20 °C. These findings underscore the need to differentiate stem CO
2 flux components to improve our understanding of whole-tree carbon cycling.
Full article
