Defining a biochemical and mechanistic link among Jeffrey pine stand density, drought, and risk of mortality from Jeffrey pine beetle, Dendroctonus jeffreyi

Jeffrey pine forests in California and Nevada experience chronic bark beetle outbreaks and tree mortality caused by the Jeffrey pine beetle, Dendroctonus jeffreyi Hopkins (JPB), during extended droughts. Reducing stand density may improve stand health by increasing individual tree access to water, carbon, and nutrient resources. The availability of these resources may determine tissue palatability to bark beetles as well as resin production, but the biochemical mechanisms behind these correlations are poorly understood. Oleoresin provides both a physical barrier and chemical impedance to bark beetle attack. Ironically, the volatile organic compounds (VOCs) emitted from defensive resins also attract bark beetles. In this project we examine this complete drought-response mechanism in Jeffrey pine over a three-year, field-oriented study across its geographic distribution. The objectives of this project are to: demonstrate how stand density influences tree drought stress and bole subcortical tissue quality, demonstrate how drought stress influences level of zeaxanthin and jasmonates, and both quantity and quality of resins (monoterpenes, alkanes), demonstrate how drought stress influences foliar and bole VOC emissions, and demonstrate how VOC emissions, resins, and phloem palatability influence colonization success of JPB.