Collaborative Research: The coordinated structural and physiological responses of trees to water stress: an organismal approach

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Drought is a global and increasingly frequent problem that threatens the vitality of forests and the many benefits they provide to society (including carbon sequestration, timber provisioning, and water cycle regulation). Whether trees thrive or falter during drought depends on how they respond to these stressful conditions. Despite many studies of leaf-level changes to photosynthesis and water use during drought, it is still unclear how trees make integrated and coordinated changes at the scale of the entire plant. For example, trees that change the angles of their leaves can reduce the radiation load at different levels of the canopy, reducing heat stress and water loss. This study investigates the coordinated adjustments of tree canopy structure and physiology under water stress using drought experiments in both growth chambers and in a forest that has a rainfall removal experiment. The investigators bring together traditional measurements of leaf and canopy gas exchange with novel remote sensing techniques that measure canopy structure and physiology. This project will provide interdisciplinary teaching, training, and learning experiences by collaborating with a non-profit organization (Math4Science) that promotes STEM, creating videos of 3D forests, mentoring undergraduate students, and exposing K-12 students from under-represented groups to STEM research.The degree to which forests will continue to serve as carbon sinks and mitigate climate change hinges on whether trees can make physiological and structural adjustments to cope with changing environmental stressors. Most research to date on tree responses to drought has focused on how trees alter leaf-level physiology, phenology, and carbon mobilization and allocation. However, trees also make structural adjustments in response to drought (e.g., by changing leaf angles) to reduce radiation load and leaf temperature. The work extends current research on plant water use strategies, which is mostly focused on physiological responses at the leaf or ecosystem scales, to a new dimension - canopy structure (leaf angles). The investigators aim to answer the question "To what extent do trees respond to water stress through coordinated adjustments of canopy structure and leaf physiology, and by what mechanisms?" combining measurements of leaf and canopy gas exchange with novel remote sensing techniques that measure canopy structure and leaf physiology, including the Terrestrial Laser Scanning (TLS) and the Unmanned Autonomous Vehicle (UAV). By taking advantage of both classical and novel techniques, this project will provide new insights into the coordination of plant response to water stress over spatial (leaf to canopy) and temporal scales (short to long term). Including mechanical responses in physiological studies of drought at the scales of entire tree crowns will provide novel mechanistic insights into drought-science and will facilitate building comprehensive mechanistic approaches into ecosystem-scale models.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.