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

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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.