Determining vegetation response to ecosystem stress : an observational and modeling approach
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Forests play a fundamental role in linking energy, water, carbon, and biogeochemical cycles. Understanding the feedback mechanisms between these cycles is essential for comprehending the Earth's functioning as a dynamic system. Earth system models have evolved to incorporate complex interactions; however, incorporating the knowledge acquired about ecosystem functioning at the individual tree-level into large-scale climate models is challenging. This dissertation combined field data, data-based models, and physics-based models to enhance understanding of trees’ hydraulics. Our investigation unearthed a robust inverse relationship between wood water content measured in the field and raw sap flux data, especially evident during dry inter-storm intervals. Leveraging data-driven models such as artificial neural networks, we accurately predicted wood water content using publicly accessible sap flux data. Furthermore, we pioneered a novel concept correlating stem radius data with directly observed wood water content, advocating for the wider integration of capacitance sensors for direct monitoring of stem-resident water in fully grown trees. Our scrutiny of continuously monitored field data unveiled that following precipitation events, red oaks and white pines demonstrated expedited recovery of transpiration (within 2 and 3 days respectively), contrasting with red maples (6 days). Ultimately, by integrating species-specific information into a physics-based model to track water potentials across the soil-plant-atmosphere continuum, we showcased significant enhancement in simulating transpiration rates. Notably, this innovation led to improvements exceeding 20% for white pines and 15% for red maples in comparison to the widely employed Penman-Monteith model. Deciphering the intricate links between stressors, hydraulic tactics, and transpiration rates assumes paramount significance in advancing our comprehension of the Earth system and prognosticating the future trajectory of forest ecosystems. This dissertation stands as a testament to the pursuit of expanding our understanding of transpiration rates, trees’ water storage dynamics, and plant hydraulic function across both individual trees and ecosystem dimensions, with a distinct focus on the implications of drought and disturbances. The profound insights garnered through this research transcend the boundaries of species comprehension, propelling the broader scientific dialogue encompassing plant hydraulics and the remarkable resilience showcased by forest ecosystems.