A common hypothesis suggests that increasing path length with increasing height decreases leaf-specific hydraulic conductance as trees grow in height so that leaf water potential decreases, resulting in increasing water stress at the tree top. In addition, increasing gravitational potential leads to water stress at the tree top. However, there have been no direct measurements of leaf water potential in the world's tallest trees and the level of water stress of the tree-top leaves is unknown.

We sampled leaves from various heights in the crown of 70-100m tall Sequoia sempervirens trees and measured their water relations characteristics and morphology. The study site is Prairie Creek Redwoods State Park and Pfeiffer-Big sur Redwoods State Park, California, USA. With increasing height, leaf water potential at turgor loss remained constant and leaf osmotic potential at water saturation increased. We observed temporary leaf rehydration due to excess apoplastic water during the pressure-volume measurements. Decreasing leaf size and increasing leaf thickness limit photosynthesis with increasing height, but the succulent morphology of the tree-top leaves may have water storage function, which compensates water stress.