A demonstration of the theoretical prediction that sap velocity is related to wood density in the conifer Dacrydium cupressinum.

•  The theoretical prediction of a close relationship between sap velocity (v) and wood density (ρb ) in conifers was tested in mature Dacrydium cupressinum trees. •  Thermal dissipation probes were used to measure v during summer in 12 trees of varying size and the data were analysed in relation to ρb . •  Variation in (1 - ρb )2 was found to explain 94% of variation in average sap velocity for trees growing in exposed canopy positions, in support of theoretical predictions. No relationship between v and ρb was found for trees growing in sheltered canopy positions. •  Differences in the relationship between v and ρb for exposed and sheltered trees are related to mixing of the air within the canopy and the response of stomatal conductance and transpiration rate to air saturation deficit. The results support the use of wood density to scale transpiration from individual trees to the stand in conifer forests.