Within-stem variation of respiration in Pseudotsuga menziesii (Douglas-fir) trees.

•  A technique for measuring in vitro respiration was investigated to understand why rates were higher than those reported in vivo and to elucidate trends within mature Pseudotsuga menziesii (Douglas-fir) trees. •  Extracted increment cores were divided into 3-4 radial depths and a gas chromatograph was used to compare respiration rates radially and vertically within stems. •  Respiration of inner bark was 2-3 times greater than sapwood, and 50-70% higher in outer than inner sapwood. Inner bark and outer sapwood released > 40% more CO2 at treetops than at bases. Trends were robust for CO2 production on a core dry-mass, volume, or total carbon basis. By contrast, CO2 production on a nitrogen basis showed almost no significant variation. •  This in vitro technique provided an effective index for relative differences in respiration within tree stems. Discrepancies between in vitro and in vivo measurements might be related to the gaseous environment in stems. The estimated within-stem gradients in respiration were possibly determined by enzyme quantity and availability and could be useful in scaling to whole-trees.