Collaborative Research: How to live on a (carbon and water) budget: Tree investment in chemical defenses across a gradient of physiological drought stress

干旱引起的森林死亡是一种全球现象，具有深远的生态和经济影响。在美国西部，干旱、高温和树皮甲虫爆发造成的树木死亡现在超过了森林的生长。尽管干旱相关的昆虫爆发的频率和严重程度增加，影响树木对昆虫食草动物的易感性的因素，如化学防御的存在，很少受到关注。特别是，目前还不清楚干旱如何导致树木将资源转向或远离阻止昆虫攻击的化学防御以及随后与其他重要植物功能存在的权衡。为了解决这一关键的知识差距，本研究的重点是如何在不断增加的干旱胁迫下，将其碳资源分配给防御和其他生理过程，以避免因干旱或树皮甲虫攻击而死亡。本研究在温室和田间实验中使用同位素标记方法来跟踪干旱引起的碳分配变化，以影响树皮甲虫的选择和成功。该项目通过招募本科生获得暑期研究助理，增加了美洲原住民学生的参与，并通过开发两个批判性思维模块和支持蒙大拿州农村中学课堂采用这些模块，增加了科学领域其他代表性不足的少数民族的参与。树木死亡率在地球仪上不断增加，但对树木死亡机制的了解仍然非常有限。迄今为止，大多数与干旱有关的树木死亡的工作都集中在了解碳饥饿和水力故障的耦合作用，但干旱往往伴随着昆虫爆发，导致或有助于树木死亡。研究尚未确定树木何时停止投资于有效的化学防御措施，以对抗沿着干旱胁迫的生物攻击，以及这些碳供应的变化如何同时影响其他植物生理过程。本研究旨在探讨干旱胁迫如何影响树木水力功能、碳水化合物利用率和化学防御之间的相互作用。使用温室实验和田间干旱操作，这项研究将耦合酶测定与使用稳定同位素，以确定机制，负责在最近固定的碳在单个化合物的水平上的小松树分配的变化与已知的影响树皮甲虫的行为。这些方法还将允许识别在不同干旱严重程度下合成防御化合物所涉及的权衡，同时也推进了对树木生理学和整个树木C预算的基本理解。通过提供一个全面的了解干旱引起的生理压力的影响机制，确定防御小蠹，一个新的，更完整的框架，评估树木死亡率的机制。 该研究由IOS/BIO的综合生态生理学计划和刺激竞争研究的既定计划（EPSCoR）共同资助。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。

Drought-induced forest die-off is a global phenomenon with far-reaching ecological and economic impacts. In the western US, tree death from drought, high temperatures, and bark beetle outbreaks now exceeds forest growth. Despite increases in the frequency and severity of drought-related insect outbreaks, factors influencing a tree's susceptibility to insect herbivores, such as the presence of chemical defenses, have received little attention. In particular, it is unclear how drought causes trees to shift resources toward or away from the chemical defenses that deter insect attacks and the subsequent trade-offs that exist with other important plant functions. To address this critical knowledge gap, this study focuses on how pinon pine trees allocate their carbon resources toward defense and other physiological processes under increasing drought stress to avoid death by drought or bark beetle attack. This study uses an isotope labeling approach in both greenhouse and field experiments to track drought-induced changes in carbon allocation to specific chemical compounds that affect bark beetle choice and success. The project increases participation of Native American students through undergraduate recruitment for summer research assistantships as well as other underrepresented minorities in science through the development of two critical thinking modules and support for their adoption in rural middle school classrooms across Montana. The rate of tree mortality has increased across the globe yet the understanding of the mechanisms underlying tree death remains surprisingly limited. Most work to date on drought-related tree death has focused on understanding the coupled roles of carbon starvation and hydraulic failure, but drought is oftentimes accompanied by insect outbreaks that cause or contribute to tree mortality. Research has yet to determine when trees cease investment in effective chemical defenses against biotic attack along the continuum of drought stress, and how these shifts in carbon availability simultaneously impact other plant physiological processes. This study seeks a mechanistic understanding of how drought stress affects the interactions among tree hydraulic function, carbohydrate availability, and chemical defense. Using both greenhouse experiments and field drought manipulations this research will couple enzyme assays with the use of stable isotopes to identify mechanisms responsible for shifts in pinon pine allocation of recently fixed carbon at the level of individual compounds with known impacts on bark beetle behavior. These methods will also allow identification of trade-offs involved in the synthesis of defense compounds at different drought severities while also advancing fundamental understanding of tree physiology and whole tree C budgets. By providing a comprehensive understanding of the effects of drought-induced physiological stress on mechanisms determining defense against bark beetles, a new, more complete framework for assessing mechanisms of tree mortality will be developed. This research was co-funded by the Integrated Ecological Physiology Program in IOS/BIO and by the Established Program to Stimulate Competitive Research (EPSCoR).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.