Effects of temperature and xylem CO2 concentration on stem respiration of Populus deltoides in autumn

Stem CO₂ efflux, sap flow, xylem sap CO₂ concentration ([CO₂*]), and stem temperature were measured in three-year-old Populus deltoides trees during sunny and rainy days in autumn. Stem respiration was calculated by mass balance as the sum of both external and internal fluxes of CO₂.  Generally, efflux was well-correlated with stem temperature, but on sunny mornings and afternoons, efflux was lower than expected for a given temperature, and on rainy days, efflux was higher than expected for a given temperature.  [CO₂*] showed clear daytime depressions on sunny days at high rates of sap flow and increased dramatically during rain, when sap flow was zero. When the temperature effect on efflux was accounted for, a clear positive relationship between residual efflux and [CO₂*] was found. We concluded that efflux was primarily related to stem temperature, but a part of the variation in efflux was caused by changes in [CO₂*]. Changes in [CO₂*] were not related to stem temperature but to changes in sap flow, which likely affected both the  rate of transport of dissolved CO₂ and the water status of the living stem tissues.  We expected a strong correlation between stem respiration calculated by mass balance and stem temperature, but the relationship was weak in two of the three trees.  The rate of stem respiration may be regulated by factors in addition to temperature, including water deficits or substrate availability. Alternatively, the measurements, or the calculation of the component fluxes used to estimate the respiration rate, may be insufficient.