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Several studies have suggested that CO 2  transport in the transpiration stream can considerably bias estimates of root and stem respiration in ring-porous and diffuse-porous tree species. Whether this also happens in species with tracheid xylem anatomy and lower sap flow rates, such as conifers, is currently unclear. We infused  13 C-labelled solution into the xylem near the base of two 90-year-old  Pinus sylvestris  L. trees. A custom-built gas exchange system and an online isotopic analyser were used to sample the CO 2  efflux and its isotopic composition continuously from four positions along the bole and one upper canopy shoot in each tree. Phloem and needle tissue  13 C enrichment was also evaluated at these positions. Most of the  13 C label was lost by diffusion within a few metres of the infusion point indicating rapid CO 2  loss during vertical xylem transport. No  13 C enrichment was detected in the upper bole needle tissues. Furthermore, mass balance calculations showed that c. 97% of the locally respired CO 2  diffused radially to the atmosphere. Our results support the notion that xylem CO 2  transport is of limited magnitude in conifers. This implies that the concerns that stem transport of CO 2  derived from root respiration biases chamber-based estimates of forest carbon cycling may be unwarranted for mature conifer stands.

Limited vertical CO 2 transport in stems of mature boreal Pinus sylvestris trees