Sapwood temperature gradients between lower stems and the crown do not influence estimates of stand-level stem CO₂ efflux

Temperature plays a critical role in the regulation of respiration rates and is often used to scale measurements of respiration to the stand-level and calculate annual respiratory fluxes. Previous studies have indicated that failure to consider temperature gradients between sun-exposed stems and branches in the crown and shaded lower stems may result in errors when deriving stand-level estimates of stem CO₂ efflux. We measured vertical gradients in sapwood temperature in a mature lowland podocarp rain forest in NewZealand to: (1) estimate the effects of within-stem temperature variation on the vertical distribution of stem CO₂ efflux; and (2) use these findings to estimate stand-level stem CO₂ efflux for this forest. Large within-stem gradients in sapwood temperature (1.6 ± 0.1 to 6.0 ± 0.5 °C) were observed. However, these gradients did not significantly influence the stand-level estimate of stem CO₂ efflux in this forest (536 ± 42 mol CO₂ ha–1 day–1) or the vertical distribution of stem CO₂ efflux, because of the opposing effects of daytime warming and nighttime cooling on CO₂ efflux in the canopy, and the small fraction of the woody biomass in the crowns of forest trees. Our findings suggest that detailed measurements of within-stand temperature gradients are unlikely to greatly improve the accuracy of tree- or stand-level estimates of stem CO₂ efflux