PNNL

Abstract

The Apparent respiratory quotient of tree stems is traditionally measured using static chambers that may introduce artifacts and obscure natural diurnal patterns. Using an open flow-through stem chamber with ambient air and cavity ringdown spectrometers, we continuously measured ES_CO2, ES_O2, and ARQ at the base of a California cherry tree (Prunus ilicifolia) during the 2024 growing season in Berkeley, CA, USA. Across three stem chambers, strong correlations between ES_CO2 and ES_O2 (R² > 0.99) were observed, with mean ARQ values ranging from 1.3 to 3.2, greatly exceeding previous reports (0.2 to 0.9). Two distinct diurnal ARQ patterns were identified: (1) daytime suppression with nighttime recovery and (2) a morning ARQ peak followed by gradual decline. Assuming a cellular RQ of 1.0 from aerobic respiration of non-structural carbohydrates, partitioning ES_CO2 into local respiratory production and xylem-transported CO2 revealed that transported CO2 can dominate, explaining ARQ > 2.0. Stem CO2 efflux deriving from transport showed a higher sensitivity to air temperature than local stem respiration, with both contributions showing a suppression at high temperatures, potentially reflecting hydraulic and/or substrate limitations on growth and respiration during warm afternoons. These findings underscore the need to quantify local respiration and CO2 transport processes to improve understanding of their roles in growth, development, and abiotic stress responses such as high temperature and drought. This study introduces a robust method based on ES_O2 stem O2 influx for quantifying different stem source CO2 flux contributions, offering insights into whole-tree stem respiration and its integration with water transport and carbon fluxes in process-based models.