RAPID: Leveraging an experimental drought on mature trees to study mechanisms of drought mortality and legacies with radiocarbon

全球环境变化增加了树木死亡的频率和分布。随着树木的死亡，它们将储存的二氧化碳释放到大气中，对全球变化和加速变暖产生了积极的反馈。为了预测森林健康的未来状况，需要对树木死亡的方式和时间有预测性的了解，但是在干旱导致树木死亡的生理学方面仍然存在重大的不确定性。该项目的研究人员将测量树木组织中储存的碳（糖和淀粉）在严重干旱中死亡时的年龄。这些数据将揭示在严重干旱下不同树木器官之间的连接如何失效的新信息。要测试的假设是，在干旱之前经历过干燥条件的树木会改变碳的储存，并且死亡得更快，这揭示了一种预测森林树木未来死亡风险的方法。这项研究的结果将与科罗拉多高原上的当地社区分享，包括纳瓦霍族的学生，以帮助提供有关未来干旱下地区森林可能发生的变化的科学知识。十年来的树木生理学研究主要集中在干旱、水力衰竭和碳饥饿两种假设的树木死亡模式之间的二分法，对这两种模式之间的相互作用的探索有限。该项目将利用塞维莱塔LTER对小松进行的实验性干旱处理，以测量致命干旱下树木多个器官中储存和呼吸的非结构碳（NSC；糖和淀粉）的年龄。通过测定干旱条件下树木使用的NSC年龄，揭示水力胁迫对不同年龄碳库利用和迁移的影响。具体来说，该项目将解决储存（深边材和根与浅边材和分支）和最近的NSC在致命干旱下的作用。该实验还包括一个“遗留”处理，该处理将现有的75%降水排除处理升级为90%排除处理，从而可以评估先前的干旱胁迫如何影响树木对后续干旱的生理反应。利用放射性碳技术测定NSC的年龄，将对从树枝和树干中呼出的NSC、储存在树枝、不同边材深度和粗根中的NSC进行取样。随着干旱的发展，干旱树木会使用更老的NSC，而经历运输或韧皮部限制的树木会在不同的树木器官中显示出NSC使用年龄的差异，因为深层储存和代谢活跃组织（叶片）之间的联系被破坏。该奖项反映了美国国家科学基金会的法定使命，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。

Global environmental changes have increased the frequency and distribution of tree mortality. As trees die, they release stored carbon dioxide to the atmosphere, creating a positive feedback on global change and accelerating warming. To predict the future state of forest health, a predictive understanding of how and when trees die is needed, but there are still major uncertainties regarding the physiology of tree mortality from drought. Investigators in this project, will measure the age of stored carbon (sugars and starches) in tree tissues as they die from a severe drought. These data will reveal novel information about how the connections between different tree organs fail under severe drought. The hypothesis to be tested is that trees which experienced drier conditions ahead of the drought will have altered stores of carbon, and die more quickly, revealing a way to predict future mortality risk in forest trees. Results from this study will be shared with local communities on the Colorado Plateau, including students from the Navajo Nation, to help provide scientific knowledge about the changes that will likely take place in regional forests under future drought.A decade of tree physiological research has focused on the dichotomy among two posited modes of tree mortality from drought, hydraulic failure and carbon starvation, with limited exploration of the interaction between these two modes. This project will leverage an experimental drought treatment being imposed upon pinyon pine trees at the Sevilleta LTER, to measure the age of non-structural carbon (NSC; sugars and starch) stored in and respired from multiple tree organs under lethal drought. By measuring the age of NSC used in trees under drought, the impacts of hydraulic stress upon the use and mobility of different-aged carbon pools will be revealed. Specifically, the project will address the role of stored (deep sapwood and roots vs. shallow sapwood and branches) and recent NSC under lethal drought. This experiment also includes a 'legacy' treatment, which will upgrade an existing 75% precipitation exclusion treatment to 90% exclusion, allowing evaluation of how previous drought stress influences the physiological responses of trees to subsequent drought. NSC being respired from branches and the trunk, and NSC stored in branches, different sapwood depths, and coarse roots, will be sampled in order to measure the age of the NSC with radiocarbon techniques. The expectation is that drought trees use older NSC as the drought progresses, and that trees experiencing transport or phloem limitation will show divergence of NSC age in use among different tree organs as the connections between deep storage and metabolically active tissues (leaves) are disrupted.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.