We investigated the influences of height, light availability, leaf structure, and season on bigleaf maple (Acer macrophyllum Pursh) leaf physiology in northern California to improve our understanding of productivity and carbon sequestration in western forests. Using hydrated cuttings and lab-based measurements, we found that leaf mass-to-area ratio and maximum photosynthetic capacity increased with height and distance from the bole. In situ, we measured leaf physiology throughout tree crowns from June through September and found that predawn water potential was remarkably constant, indicating sustained access to water. We also found that midday water potential generally decreased with height and distance from the bole, noticeably decreased at the end of the growing season, and did not fall below –2 MPa, suggesting ample access to water and/or stomatal regulation to maintain hydrated water status. At midday, light availability and stomatal conductance of water vapor generally increased with height and distance from the bole. Stomatal conductance peaked in the treetop in June, in the mid-crown in July and August, and in the low-crown in September, demonstrating temporal and spatial optimization of crown resources in response to changing light quality, climatic conditions, and water status across the season to maximize carbon uptake at the tree level. Together, our water potential and light measurements suggest that light availability is a stronger determinant of leaf morphology and physiology than hydraulic limitation in this species. These findings provide information on seasonal physiology in a widespread deciduous hardwood species, thereby strengthening our understanding of forest productivity in a predominantly coniferous region.
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Vol. 94 • No. 2