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

人类活动正在导致多种并发的全球变化，包括大气二氧化碳（CO2）水平上升，温度上升以及相关的大气干燥。虽然较高的二氧化碳水平可能对树木产生有利影响，但干燥的大气可能会超过这些积极影响，反而导致森林死亡率增加。森林死亡对清洁空气、清洁水和生物多样性有重大影响。为了解决二氧化碳浓度上升和大气干燥对森林的净影响的不确定性，本研究将结合联合收割机实地实验，直接将树木暴露于不同的二氧化碳和大气干燥水平，从植物生理测量将采取。这些测量结果将与全球树木蒸腾数据集相结合，以参数化和验证一个模型，该模型将提高预测这些影响对森林的影响的能力。此外，该项目还将从科学界代表性不足的群体中招募和培训科学家，以促进多样化和具有全球竞争力的劳动力队伍，并支持让K-12学生参与实践科学的外联活动。对森林的影响是陆地碳汇和水循环模型预测中最大的不确定性之一。目前的证据表明，VPD的负面影响将超过二氧化碳增加的积极影响，但缺乏直接测试，生理测量和多尺度的机制和后果的理解。为了解决这个问题，这项研究将使用三管齐下的方法，结合1）直接操纵CO2和VPD的第一个现场实验，2）全球规模的sapflux数据集，可以检查CO2与VPD影响的物种和生态系统特定变化，以及3）基于过程的模型，将使用（1）和（2）的数据集进行参数化和验证。将对模型输出进行敏感性分析，以调查CO2和VPD对死亡率的作用，相关机制以及驯化的可能作用。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.