Xylem biomechanics, water storage, and density within roots and shoots of an angiosperm tree species.

Xylem is a complex tissue that forms the bulk of tree bodies and has several functions, including to conduct water, store water and nutrients, and biomechanically support the plant body. We examined how xylem functional traits varied at different positions within Populus balsamifera spp. trichocarpa (Black Cottonwood). Whole trees were excavated, and xylem samples were collected at 1m increments along the main root-to-shoot axis of six trees, from root tip to shoot tip. We examined biomechanical and water storage traits of the xylem, including using a non-invasive imaging technique to examine water content within long, intact branches (high resolution computed tomography; microCT). Xylem density, strength, and stiffness were higher in shoots than roots. Along the main root-to-shoot axis, xylem strength and stiffness were greatest at shoot tips and tissue became linearly weaker and less stiff down the plant and through the root. Roots had greater water storage with reduced biomechanical support, and shoots had biomechanically stronger and stiffer xylem with reduced water storage. These findings support trade-offs among xylem functions between roots and shoots. Understanding how xylem functions differ throughout tree bodies is important in understanding whole-tree function and how terrestrial plants endure numerous environmental challenges over decades of growth.