Does water transport scale universally with tree size?

1. We employed standardized measurement techniques and protocols to describe the size dependence of whole-tree water use and cross-sectional area of conducting xylem (sapwood) among several species of angiosperms and conifers. 2. The results were not inconsistent with previously proposed 3/4-power scaling of water transport with estimated above-ground biomass. However, for a given size, angiosperms transported considerably greater quantities of water than conifers. 3. In the angiosperms studied, the scaling of water transport with sapwood area, stem diameter and above-ground biomass was best described by sigmoid functions rather than a power function, consistent with the previously reported size dependence of other processes such as growth. 4. At least three distinct species groupings for relationships between sapwood area and stem basal area were observed. Scaling of sapwood area with stem radius was well described by a power function of the form Y = Y₀X^b. However, exponents obtained for two of the three species groups differed significantly from a recently proposed theoretical value of 2·33.