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

•  To assess hydraulic architecture and limitations to water transport across whole trees, we compared xylem anatomy, vulnerability to cavitation (Ψ₅₀ ) and specific hydraulic conductivity (K_s ) 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 (D_h ) conduit diameters and K_s 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 K_s 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 D_h and K_s , 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.