Variation in xylem structure and function in stems and roots of trees to 20 m depth


Author for correspondence: Andrew J. McElrone Tel: +1 919 660 7289 Fax: +1 919 660 7293 Email:


  • • To assess hydraulic architecture and limitations to water transport across whole trees, we compared xylem anatomy, vulnerability to cavitation (Ψ50) and specific hydraulic conductivity (Ks) of stems, shallow roots and deep roots (from caves to 20 m depth) for four species: Juniperus ashei, Bumelia lanuginosa, Quercus fusiformis and Quercus sinuata.
  • • Mean, maximum and hydraulically weighted (Dh) conduit diameters and Ks were largest in deep roots, intermediate in shallow roots, and smallest in stems (P < 0.05 for each). Mean vessel diameters of deep roots were 2.1–4.2-fold greater than in stems, and Ks was seven to 38 times larger in the deep roots.
  • • Ψ50 also increased from stems to roots with depth, as much as 24-fold from stems to deep roots in B. lanuginosa. For all species together, Ψ50 was positively correlated with both Dh and Ks, suggesting a potential trade-off exists between conducting efficiency and safety.
  • • The anatomical and hydraulic differences documented here suggest that the structure of deep roots minimizes flow resistance and maximizes deep water uptake.