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 ( Ψ 50 ) 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. • Ψ 50 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, Ψ 50 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.