A simple experiment designed to challenge the idea that xylem characteristics are determined by cambial age

There is a prevailing belief that cambial age influences xylem properties such as conduit size. Typically, xylem conduits in the inner rings of a tree are narrower than those in the outermost rings. However, there is no established causal relationship between cambial age and conduit size. Instead, it is well known that the hydraulic path length (leaf-to-root) primarily influences conduit diameter variation along the stem axis, given the unavoidable tip-to-base conduit widening. The study aimed at disentangling the effects of path length and cambial age on conduit size. We exploited a phenomenon common in coppice-with-standards forest stands: after harvesting, isolated non-coppiced trees produce numerous low epicormic branches, significantly altering crown structure and reducing hydraulic path length, while cambial age continues to increase. This setup provided a unique opportunity to test if hydraulic path length, rather than cambial age, dictates conduit diameter at the stem base. Wood cores were collected from the stem base of seven broadleaved trees across four species, particularly focusing on those exhibiting significant epicormic branching, alongside two control trees from a non-coppiced area. Median vessel area was measured in several (3-4) annual rings before and after coppicing. Across all analyzed samples, median vessel area consistently decreased 2-3 years after coppicing (P<0.0001), with reductions ranging from -7% to -44% compared to pre-coppicing values, despite a greater width of the tree rings. Conversely, trees under constant canopy cover did not exhibit significant changes in vessel size. The study effectively challenges the misconception that cambial age dictates conduit size, highlighting instead the pivotal role of hydraulic path length.