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PREMISE OF THE STUDY: Leaves respond to environmental signals and acclimate to local conditions until their ecological limits are reached. Understanding the relationships between anatomical variation in leaves and the availability of water and light improves our ab ility to predict ecosystem-level impacts of foliar response to climate change, as it expands our knowledge of tree physiology. METHODS: We examined foliar anatomy and morphology of the largest plant species, Sequoiadendror giganteum, from leafy shoot samples collected th roughout crowns of trees up to 95 m tall and assessed the functionality of with in-crown variation with a novel drought/recovery experiment. KEY RESULTS: We found phenotypic variation in response to water availability in 13 anatomical traits of Sequoiadendron leaves. Shoot expansion was constrained by the hydrostatic gradient of maximum water potential, while functional traits supporting succulence and toughness were associated with sites of peak hydraulic limitation. Water-stress tolerance in experimental shoots increased dramatically with height. CONCLUSION: We propose a heat-sink function for transfusion tissue and uncover a suite of traits suggesting rapid hydraulic throughput and flexibility in water-stress tolerance investments as strategies that help this montane species reach such enormous size. Responses to water stress alter the amount of carbon stored in foliage and the rate of the eventual release of carbon. [ABSTRACT FROM AUTHOR]