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 (Psi(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. Psi(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, Psi(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.