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

干旱导致的森林死亡是一种全球性现象，具有深远的生态和经济影响。在美国西部，由于干旱、高温和树皮甲虫暴发导致的树木死亡现在超过了森林的生长。尽管与干旱有关的昆虫暴发的频率和严重程度有所增加，但影响树木对昆虫食草动物易感性的因素，如化学防御的存在，几乎没有受到关注。特别是，目前尚不清楚干旱如何导致树木将资源转移到或远离化学防御，化学防御可以阻止昆虫攻击，以及随后存在的与其他重要植物功能的权衡。为了解决这一关键的知识缺口，本研究重点研究了在干旱胁迫增加的情况下，松树如何将其碳资源分配到防御和其他生理过程中，以避免因干旱或树皮甲虫攻击而死亡。这项研究在温室和田间试验中使用了同位素标记方法来跟踪干旱引起的碳分配到特定化合物的变化，这些变化影响了树皮甲虫的选择和成功。该项目通过招募本科生申请暑期研究助理奖学金，增加了美国原住民学生的参与，并通过开发两个批判性思维模块，以及支持在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较低的少数族裔的参与。全球树木死亡率有所上升，但人们对树木死亡机制的了解仍然令人惊讶地有限。到目前为止，大多数关于与干旱相关的树木死亡的工作都集中在了解碳饥饿和水力衰竭的双重作用上，但干旱往往伴随着引起或促成树木死亡的昆虫暴发。研究尚未确定树木何时停止投资于有效的化学防御，以抵御干旱胁迫的连续统一体的生物攻击，以及这些碳可获得性的变化如何同时影响植物的其他生理过程。这项研究试图从机理上理解干旱胁迫如何影响树木的水力功能、碳水化合物的有效性和化学防御之间的相互作用。通过温室实验和田间干旱处理，这项研究将结合酶分析和稳定同位素的使用，在已知对树皮甲虫行为有影响的单个化合物的水平上，识别导致松树最近固定碳分配变化的机制。这些方法还将允许识别在不同干旱程度下合成防御化合物所涉及的权衡，同时也促进了对树木生理和整个树木C预算的基本理解。通过全面了解干旱诱导的生理胁迫对决定树皮甲虫防御机制的影响，将开发一个新的、更完整的评估树木死亡机制的框架。这项研究是由IOS/BIO的综合生态生理学计划和既定的激励竞争研究计划(EPSCoR)共同资助的。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。

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.