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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 (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.