Two path length effects emerging from ontogenetically stable axial xylem
design affect the conductance of inner sapwood rings
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 (
KRING). 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.