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

人类活动正在导致多重的、同时发生的全球变化，包括大气二氧化碳(CO2)水平上升、气温上升以及相关的大气干燥。虽然更高的二氧化碳水平可能会对树木产生有益的影响，但更干燥的大气可能会超过这些积极影响，反而会导致森林死亡增加。森林死亡对清洁的空气、清洁的水和生物多样性具有重大影响。为了解决二氧化碳上升和大气更加干燥对森林的净影响的不确定性，这项研究将结合实地实验，直接将树木暴露在不同的二氧化碳和大气干燥水平下，并从这些实验中进行植物生理测量。这些测量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，该项目将从科学领域代表性不足的群体中招聘和培训科学家，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。二氧化碳水平上升和大气干旱加剧(以水汽压赤字衡量)对森林的净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、生理测量以及对机制和后果的多尺度理解。为了解决这一问题，这项研究将使用三管齐下的方法，1)第一个直接操纵二氧化碳和VPD的现场实验，2)全球范围的SAP通量数据集，能够检查物种和生态系统特定的CO2对VPD影响的变化，以及3)一个基于过程的模型，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，以调查二氧化碳和VPD对死亡率的作用、相关机制以及适应的可能作用。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.