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

Abstract

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 New Zealand 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. © 2008 Heron Publishing.