Resistance to radial CO₂ diffusion contributes to between-tree variation in CO₂ efflux of Populus deltoides stems

Rates of CO₂ efflux of stems and branches are highly variable among and within trees and across stands. Scaling factors have only partially succeeded in accounting for the observed variations. In this study, the resistance to radial CO₂ diffusion was quantified for tree stems of an eastern cottonwood (Populus deltoides Bartr. ex Marsh.) clone by direct manipulation of the CO₂ concentration ([CO₂]) of xylem sap under controlled conditions. Tree-specific linear relationships between rates of stem CO₂ efflux (J_O) and xylem [CO₂] were found. The resistance to radial CO₂ diffusion differed 6-fold among the trees and influenced the balance between the amount of CO₂ retained in the xylem v. that which diffused to the atmosphere. Therefore, we hypothesised that variability in the resistance to radial CO₂ diffusion might be an overlooked cause for the inconsistencies and large variations in woody tissue CO₂ efflux. It was found that transition from light to dark conditions caused a rapid increase in J_O and xylem [CO₂], both in manipulated trees and in an intact tree with no sap manipulation. This resulted in an increased resistance to radial CO₂ diffusion during the dark, at least for trees with smaller daytime resistances. Stem diameter changes measured in the intact tree supported the idea that higher actual respiration rates occurred at night owing to higher metabolism in relation to an improved water status and higher turgor pressure.