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Premise of the study: Leaves are the sites of greatest water stress in trees and a key means of acclimation to the environment. We considered phenotypic plasticity of Pseudotsuga menziesii leaves in their ecological context, exploring responsiveness to natural gradients in water stress (indicated by sample height) and light availability (measured from hemispherical photos) to understand how leaf structure is controlled by abiotic factors in tall tree crowns. Methods: After measuring anatomy, morphology, and carbon isotope composition (δ13C) of leaves throughout crowns of P. menziesii >90 m tall, we compared structural plasticity of leaves among the three tallest conifer species using equivalent data from past work on Sequoia sempervirens and Picea sitchensis. Key results: Leaf mass per projected area (LMA) and δ13C increased and mesoporosity (airspace/area) decreased along the water‐stress gradient, while light did not play a detectable role in leaf development. Overall, leaves of P. menziesii were far less phenotypically responsive to within‐crown abiotic gradients than either P. sitchensis, whose leaves responded strongly to light availability, or S. sempervirens, whose leaves responded equally strongly to water stress. Conclusions: P. menziesii maintain remarkably consistent leaf structure despite pronounced vertical gradients in abiotic factors. Contrasting patterns of leaf structural plasticity underlie divergent ecological strategies of the three tallest conifer species, which coexist in Californian rainforests. [ABSTRACT FROM AUTHOR]

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]

• Premise of the study: Leaves at the tops of most trees are smaller, thicker, and in many other ways different from leaves on the lowermost branches. This height-related variation in leaf structure has been explained as acclimation to differing light environments and, alternatively, as a consequence of hydrostatic, gravitational constraints on turgor pressure that reduce leaf expansion. • Methods: To separate hydrostatic effects from those of light availability, we used anatomical analysis of height-paired samp1es from the inner and outer tree crowns of tall redwoods (Sequoia sempervirens). • Key results: Height above the ground correlates much more strongly leaf anatomy than does light availability. Leaf length, width, and mesophyll porosity all decrease linearly with height and help explain increases in leaf-mass-to-area ratio and decreases in both photosynthetic capacity and internal gas-phase conductance with increasing height. Two functional traits--leaf thickness and transfusion tissue--also increase with height and may improve water-stress tolerance. Transfusion tissue area increases enough that whole-leaf vascular volume does not change significantly with height in most trees. Transfusion tracheids become deformed with height, suggesting they may collapse under water stress and act as a hydraulic buffer that improves leaf water status and reduces the likelihood of xylem dysfunction. • Conclusions: That such variation in leaf structure may be caused more by gravity than by light calls into question use of the terms "sun" and "shade" to describe leaves at the tops and bottoms of tall tree crowns. [ABSTRACT FROM AUTHOR]

Redwood forests contain some of the largest and most structurally complex trees on Earth. The most abundant vascular epiphyte in these forests is the fern Polypodium scouleri (Polypodiaceae). We measured dimensions of all 765 P. scouleri mats on 32 trees (27 Sequoia sempervirens, 5 Picea sitchenais). Eighteen P. scouleri mats from 11 trees were randomly selected for removal and dissection in the laboratory. The total fern mat mass consisted of live fronds (3.3%), dead fronds (2.4%), live rhizomes (4.2%), dead rhizomes (8.9%), roots (34.4%), humus (28.0%), and debris (18.8%). We used multiple regression analysis to develop equations for estimation of fern masses, and we applied these equations to undisturbed fern mats on the 32 trees. Individual trees supported up to 742 kg dry mass of P. scouleri mats. These are the highest whole-tree epiphyte masses ever reported. We also quantified crown structure and counted the number of vascular plant species occurring as epiphytes on each tree. Very large, complex trees had more fern mat mass and higher vascular epiphyte species richness than smaller, simpler trees. Desiccation-sensitive organisms dependent on water stored in fern mats may be unable to survive in managed redwood forests lacking large, complex trees with abundant P. scouleri. [ABSTRACT FROM AUTHOR]

Premise: Trees in wet forests often have features that prevent water films from covering stomata and inhibiting gas exchange, while many trees in drier environments use foliar water uptake to reduce water stress. In forests with both wet and dry seasons, evergreen trees would benefit from producing leaves capable of balancing rainy‐season photosynthesis with summertime water absorption. Methods: Using samples collected from across the vertical gradient in tall redwood (Sequoia sempervirens) crowns, we estimated tree‐level foliar water uptake and employed physics‐based causative modeling to identify key functional traits that determine uptake potential by setting hydraulic resistance. Results: We showed that Sequoia has two functionally distinct shoot morphotypes. While most shoots specialize in photosynthesis, the axial shoot type is capable of much greater foliar water uptake, and its within‐crown distribution varies with latitude. A suite of leaf surface traits cause hydraulic resistance, leading to variation in uptake capacity among samples. Conclusions: Shoot dimorphism gives tall Sequoia trees the capacity to absorb up to 48 kg H2O h−1 during the first hour of leaf wetting, ameliorating water stress while presumably maintaining high photosynthetic capacity year round. Geographic variation in shoot dimorphism suggests that plasticity in shoot‐type distribution and leaf surface traits helps Sequoia maintain a dominate presence in both wet and dry forests. [ABSTRACT FROM AUTHOR]

Here we respond to the communication in American Journal of Botany (96: 542-544 in this issue) by Neting, who proposes several ways in which our paper "The Limits to Tree Height" (Nature 428: 851-854) may have erred in estimating the biophysical limits to height growth in Sequoia sempervirens. We first explain that because embolism repair requires long time periods and is generally incomplete, xylem vulnerability characteristics offer a sound basis for estimating performance limits in woody plants. We reaffirm our earlier use of vertical gradients of foliar carbon isotope composition with new data for S. sempervirens. We support these arguments with reference to studies in other tree species. We take exception with Netting's view that the turgor pressure-cell expansion relationship for Zea mays is applicable to S. sempervirens. Finally, we second Netting's call for more work on carbon allocation vis a vis height growth limits. [ABSTRACT FROM AUTHOR]