Height-related change in xylem structure of large Cinnamomum camphora trees

Abstract

As trees increase in size, water transport from roots to leaves becomes constrained due to increasing path length, hydraulic resistance, and hydrostatic limitation. In trees, hydraulic conductivity decreases with increasing size due, not only to hydrostatic constraints and increasing path length, but also increasing number of annual nodes and junctions through which water must travel to reach terminal leaves. Axial tapering of xylem conduits contributes to increasing efficiency of whole-tree water conduction and minimizing hydraulic constraints on productivity and growth. Here, we compared xylem structure along the vertical gradient in large, old Cinnamomum camphora trees (with height ca. 25 m and age ca. 100 years) to infer the height effect on xylem structure and its variation with height. Vessel diameter (D) was smaller and vessel density per sapwood area was higher for upper than lower trunk. Within annual rings, vessel diameter decreased from early- to late-wood and the rate of this change was nearly constant with height. Potential hydraulic conductivity, calculated from D, was markedly lower for upper than lower trunk and lack of vessels with D > 250 μm at upper trunk contributed to this difference. Vertical change in xylem structure observed here, may contribute to maintaining hydraulic efficiency and minimizing hydraulic limitation in large, old trees.