The net impact of rising CO2 versus rising vapor pressure deficit on tree mortality

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Human-activities are leading to multiple, concurrent global changes including rising atmospheric carbon dioxide (CO2) levels, rising temperature, and an associated drying of the atmosphere. While higher CO2 levels could potentially have beneficial effects for trees, a drier atmosphere may outweigh those positive impacts and instead lead to increased forest mortality. Forest mortality has major consequences for clean air, clean water, and biodiversity. In order to address the uncertainty of the net effect of rising CO2 and a drier atmosphere on forests, this research will combine field experiments that directly expose trees to different CO2 and atmospheric dryness levels, from which plant physiological measurements will be taken. These measurements will be combined with a global tree transpiration dataset to parameterize and validate a model that will improve the ability to predict the effects of these impacts on forests. In addition, this project will recruit and train scientists from underrepresented groups in science to promote a diverse and globally competitive workforce and support outreach activities that engage K-12 students in hands-on science.The net impact of rising CO2 levels and rising atmospheric aridity, measured as vapor pressure deficit (VPD), on forests is one of the largest uncertainties in model projections of the terrestrial carbon sink and the water cycle. Current evidence suggests that the negative impacts of VPD will outweigh the positive impacts of increased CO2, but direct tests, physiological measurements, and a multi-scale understanding of mechanisms and consequences are lacking. To address this, this research will use a three-pronged approach by combining 1) the first field experiment that directly manipulates CO2 and VPD, 2) a global-scale sapflux dataset enabling examination of species- and ecosystem-specific variation in CO2 versus VPD impacts, and 3) a process-based model that will be parameterized and validated using datasets from both (1) and (2). Sensitivity analyses will be conducted on model outputs to investigate the role of CO2 and VPD on mortality, the associated mechanisms, and the possible role of acclimation.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.