The process of sapwood/heartwood transition in trees is not fully understood. We tested whether the ontogenetically-stable apex-to-base conduit widening generates path length effects limiting the conductance of inner sapwood rings. The axial scaling ( b) of conduit hydraulic diameter ( Dh) was estimated at annual resolution in a spruce and beech tree. We compiled a global dataset of sapwood ring number ( NSWr), their average width ( SWrw), tree height ( H) and stem elongation rate ( ΔH) in conifer and angiosperm trees. A numerical model simulated the effects of H and ΔH on the conductance of each xylem ring ( K_RING). b resulted ontogenetically stable. Simulations well predicted the observed patterns of increasing NSWr with H and decreasing NSWr with ΔH, assuming that heartwood forms when the marginal conductance gain of maintaining the functionality of an inner ring becomes negligible. Sapwood/heartwood transition minimizes the C costs associated to allocation to secondary growth and maintenance of living sapwood required to attain a given sapwood conductance. The number of sapwood rings depends on the effects of H and ΔH on the conductance of inner sapwood rings. The width of sapwood rings contributes to compensate for the lower conductance of inner sapwood rings at high ΔH.