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We examined changes in branch hydraulic, leaf structure and gas exchange properties in coast redwood ( Sequoia sempervirens) and giant sequoia ( Sequoiadendron giganteum) trees of different sizes. Leaf-specific hydraulic conductivity ( kL) increased with height in S. sempervirens but not in S. giganteum, while xylem cavitation resistance increased with height in both species. Despite hydraulic adjustments, leaf mass per unit area (LMA) and leaf carbon isotope ratios ( δ13C) increased, and maximum mass-based stomatal conductance ( gmass) and photosynthesis ( Amass) decreased with height in both species. As a result, both Amass and gmass were negatively correlated with branch hydraulic properties in S. sempervirens and uncorrelated in S. giganteum. In addition, Amass and gmass were negatively correlated with LMA in both species, which we attributed to the effects of decreasing leaf internal CO2 conductance ( gi). Species-level differences in wood density, LMA and area-based gas exchange capacity constrained other structural and physiological properties, with S. sempervirens exhibiting increased branch water transport efficiency and S. giganteum exhibiting increased leaf-level water-use efficiency with increasing height. Our results reveal different adaptive strategies for the two redwoods that help them compensate for constraints associated with growing taller, and reflect contrasting environmental conditions each species faces in its native habitat. [ABSTRACT FROM AUTHOR]

Tracheid buckling may protect leaves in the dynamic environments of forest canopies, where rapid intensifications of evaporative demand, such as those brought on by changes in light availability, can result in sudden increases in transpiration rate. While treetop leaves function in reliably direct light, leaves below the upper crown must tolerate rapid, thermally driven increases in evaporative demand. Using synchrotron‐based X‐ray microtomography, we visualized impacts of experimentally induced water stress and subsequent fogging on living cells in redwood leaves, adding ecological and functional context through crown‐wide explorations of variation in leaf physiology and microclimate. Under drought, leaf transfusion tracheids buckle, releasing water that supplies sufficient temporal reserves for leaves to reduce stomatal conductance safely while stopping the further rise of tension. Tracheid buckling fraction decreases with height and is closely coordinated with transfusion tissue capacity and stomatal conductance to provide temporal reserves optimized for local variation in microclimate. Foliar water uptake fully restores collapsed and air‐filled transfusion tracheids in leaves on excised shoots, suggesting that trees may use aerial water sources for recovery. In the intensely variable deep‐crown environment, foliar water uptake can allow for repetitive cycles of tracheid buckling and unbuckling, protecting the tree from damaging levels of hydraulic tension and supporting leaf survival. If sudden changes in microclimate increase transpirational demand, tracheid buckling in the water‐filled transfusion tissue of Sequoia sempervirens leaves can release a quantity of water sufficient to briefly sustain increased transpiration, thereby providing time for stomatal adjustment. The temporal reserves supplied by tracheid buckling can be replenished through foliar water uptake, which allows collapsed tracheids to refill to their former volumes. [ABSTRACT FROM AUTHOR]