Moving water well : comparing hydraulic efficiency in twigs and trunks of coniferous, ring-porous, and diffuse-porous saplings from temperate and tropical forests

Coniferous, diffuse-porous and ring-porous trees vary in their xylem anatomy, but the functional consequences of these differences are not well understood from the scale of the conduit to the individual. Hydraulic and anatomical measurements were made on branches and trunks from 16 species from temperate and tropical areas, representing all three wood types. Scaling of stem conductivity (Kh) with stem diameter was used to model the hydraulic conductance of the stem network. Ring-porous trees showed the steepest increase in Kh with stem size. Temperate diffuse-porous trees were at the opposite extreme, and conifers and tropical diffuse-porous species were intermediate. Scaling of Kh was influenced by differences in the allometry of conduit diameter (taper) and packing (number per wood area) with stem size. The Kh trends were mirrored by the modeled stem-network conductances. Ring-porous species had the greatest network conductance and this value increased isometrically with trunk basal area, indicating that conductance per unit sapwood was independent of tree size. Conductances were lowest and most size-dependent in conifers. The results indicate that differences in conduit taper and packing between functional types propagate to the network level and have an important influence on metabolic scaling concepts.