As the morning fog lifts above the redwood canopy at Bull Creek (Humboldt Redwoods State Park, California, USA) a morphological mystery is revealed. Here at over 100 m above the ground, the small awl-like leaves (foreground) are half as wide as and four times shorter than the flat, expanded leaves growing more than 50 m below. As the plant species with the tallest individuals, redwood provides an unparalleled opportunity to examine the external factors controlling within-plant variation in foliar structure. To separate the effects of light availability on leaf morphology and anatomy from the effects of gravity-induced water stress we used arborist style climbing techniques to collect height-paired foliar samples from both the dark inner and bright outer crowns. We found that the hydrostatic gradient in water potential drives the observed reduction in leaf expansion with height as well as inducing investments in functional traits that promote water-stress tolerance in the upper crown, all with no detectable influence from light availability.