Hydraulic path length as a determinant of xylem conduit size at the stem base, regardless of cambial age.

Xylem properties, such as wood density and conduit diameter, are linked to crown shape and size. Suppressed trees with smaller crowns tend to have denser wood and narrower conduits at the stem base, whereas dominant trees exhibit wider conduits and lower wood density. Given the tip-to-base widening of xylem conduits-an adaptation to counteract increasing hydraulic resistance with growth-we hypothesize that hydraulic path length (i.e., the distance from leaves to the stem base) is the primary driver of conduit size, independent of cambial age. To test this, we leveraged a phenomenon in managed forests: partial harvesting reduces stand density, triggering epicormic shoot formation along the stems of uncut (standing) trees. This downward shift in leaf distribution shortens the average hydraulic path length, allowing us to assess its influence on conduit formation in the standing trees. If conduit size is governed by hydraulic path length, newly formed tree rings should contain narrower conduits following epicormic shoot sprouting, despite the older cambial age. We analysed wood samples from nine broadleaved trees across four species (Acer opalus obtusatum (Waldst. & Kit. Ex Willd.) Gams, Ostrya carpinifolia Scop., Carpinus betulus L., Sorbus aria (L.) Crantz.), comparing the median conduit area in three to four annual rings before and after the harvesting of neighbouring trees. In trees with epicormic shoots, conduit size decreased by a factor ranging from 0.93 to 0.56 (P < 0.01). Conversely, the two trees without epicormic shoots exhibited no significant changes in conduit size. Our findings indicate that conduit size at the stem base is determined by hydraulic path length, rather than by cambial age. This suggests that newly formed leaves regulate the vascular conduits supplying them, leading to a hydraulic network structured by multiple, axially sectored pathways.