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) lower stem elongation (ΔH) 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 apex (DCA) in all rings of a P. abies and a F. sylvatica tree. Net of DCA effect, Dh did not increase with H. Using sapwood traits from a global dataset, NSWr increased with H and decreased with ΔH, and the mean sapwood ring width (SWrw) increased with ΔH. A numerical model based on anatomical patterns predicted the effects of H and ΔH on the conductance of inner xylem rings.

Results suggested the sapwood/heartwood transition depends on both H and ΔH, and is set when the C allocation to maintenance respiration of living cells in inner sapwood rings produces a lower gain in total conductance than investing the same C in new vascular conduits.

The dataset of this article ("Axial conduit widening, tree height and height growth rate set the hydraulic transition from sapwood into heartwood") is composed of two datasets and Supplementay Tables and Figures.

Datasets:

1) Data of hydraulically weighted xylem conduit diameters of all the tree rings of stem discs extracted at different position along the stem of a Picea abies Karst. and a Larix decidua L. tree;

2) Compiled data of the number of sapwood rings, mean width of sapwood rings, tree height, and annual height growth from own sampling of trees across the Alps.

Tables:

• Table S1;
• Table S2;
• Table S3.

Figures:

• Figure S1;
• Figure S2;
  
• Figure S3;
  
• Figure S4.