Axial conduit widening, tree height, and height growth rate set the hydraulic transition of sapwood into heartwood

The sapwood/heartwood transition is hydraulically determined by the combined effects of conduit widening, tree height, and height growth rate on the contribution of inner rings to the total xylem conductance.The size-related xylem adjustments required to maintain a constant leaf-specific sapwood conductance (K-LEAF) with increasing height (H) are still under discussion. Alternative hypotheses are that: (i) the conduit hydraulic diameter (Dh) at any position in the stem and/or (ii) the number of sapwood rings at stem base (NSWr) increase with H. In addition, (iii) reduced stem elongation (& UDelta;H) increases the tip-to-base conductance through inner xylem rings, thus possibly the NSWr contributing to K-LEAF. A detailed stem analysis showed that Dh increased with the distance from the ring apex (DCA) in all rings of a Picea abies and a Fagus sylvatica tree. Net of DCA effect, Dh did not increase with H. Using sapwood traits from a global dataset, NSWr increased with H, decreased with & UDelta;H, and the mean sapwood ring width (SWrw) increased with & UDelta;H. A numerical model based on anatomical patterns predicted the effects of H and & UDelta;H on the conductance of inner xylem rings. Our results suggest that the sapwood/heartwood transition depends on both H and & UDelta;H, and is set when the carbon allocation to maintenance respiration of living cells in inner sapwood rings produces a lower gain in total conductance than investing the same carbon in new vascular conduits.