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Organized Oral Session

Session: : Integrating Light Spectra to Understand the Causes and Consequences of Variation in Plant Function Across Biological Scales

OOS 51-1 - Tracking drought-induced shifts in tree susceptibility to bark beetles using leaf-level hyperspectral remote sensing

Thursday, August 10, 2023
8:00 AM – 8:15 AM PDT
Location: 254
Pinyon pine (Pinus edulis) has suffered extensive die-back over the last two decades due to the increased frequency of drought-induced bark beetle outbreaks. While drought-induced variation in plant secondary metabolism has been attributed to greater tree susceptibility to bark beetle attack, empirical evidence from mature trees is rare and mechanistic underpinnings are virtually unknown. This is likely due to a lack sufficient information about key metabolomic shifts across space and time during droughts and subsequent bark beetle attacks, which remains a challenge using traditional sampling methods.

The goal of this work is to unravel the trait syndromes that portend pinyon pine vulnerability to bark beetle attack, with an eye towards improving our mechanistic understanding of insect-caused tree mortality. We leverage a manipulative water-exclusion experiment located at the Sevilleta National Wildlife Refuge in New Mexico where mature pinyon pine experienced varying levels of short-term (2-years) or long-term (12 years) drought. Secondary metabolites (terpenes and phenolics) were measured from needles and phloem during the peak growing season along with non-structural carbohydrates, water potential, and other physiological variables. To more comprehensively monitor potential phytochemical ‘tipping points’ or trait syndromes of vulnerable individuals, we measured hyperspectral reflectance from fresh foliar samples using a field spectrometer to estimate additional leaf traits based on existing models for conifers.

Long-term drought stress resulted in lower constitutive concentrations of phloem monoterpenes and needle phenolics as well as lower levels of induced monoterpenes to simulated bark beetle challenge. Remarkably, this reduced investment in secondary metabolites overall translated into greater resilience to the in situ drought x bark beetle disturbance. Further, leaf-level phenolic content was inversely related to pre-dawn water potential and both traits were significant predictors of observed mortality one year later. Estimates from the spectral trait model overestimated phenolics in comparison to lab measurements and showed a positive correlation with water content, underscoring the need to build species-specific spectral models to further test the extent to which water content relates to water potential and other key functional traits.

Together these results highlight the importance of considering acclimation and drought legacy effects on secondary metabolism when assessing physiological thresholds of vulnerability to biotic agents. They also emphasize the importance of incorporating phytochemistry more broadly into our current predictive mortality frameworks and the utility of remote sensing for testing mechanistic hypotheses across broad geographic scales.