COS 165-1 - Primary and secondary physiological effects on trees prior to drought- and bark beetle-induced tree mortality in Pinus edulis.

Abstract: Tree die-off from heat, drought, and biotic attack is increasing globally, and expected to accelerate as climate change continues. Conifer forests in the northern hemisphere face attack by bark beetles (subfamily Scolytinae) and the symbiotic fungi they vector. Much research has investigated abiotic and biotic mortality drivers separately, but an integrated understanding is needed. With a field experiment we caused drought using rain-out shelters in a piñon-juniper woodland. Treatments were 45%, 70%, and 90% precipitation-removal installed in early 2020, a legacy 45% removal installed a decade prior, and ambient control. measured physiological responses on, targeting 6 trees in each treatment. Over three years we measured 6 Pinus edulis in each treatment for stem water potential, non-structural carbohydrates (NSCs; sugars and starch) in multiple organs and tissues, and concentrations of 40 separate monoterpenes and sesquiterpenes. Two trees died in 2021 (year 2) in the 90% removal treatment and an additional 10 trees died in 2022 (year 3): 5, 3, and 2 in the 45, 70, and 90% removal treatments. All had signs of bark beetle attack.

We compared trees that died to those which survived across all treatments. Pre-dawn water potential in trees that died diverged lower from survivors long before death, by 11 and 14 months for death in 2021 and 2022. Notably, some trees survived predawn water potentials as low as -4.1 MPa, although most in this range or lower eventually died. NSC differences with morality/survival were dynamic between May and June 2022, and among component and tissue measured. Starch was lower in trees that died by 16-50% relative to survivors. Sugar in May was 30-33% higher in trees that died in phloem and stem samples, but this reversed by June with 24-30% lower sugars in needles, root, and stems. June total NSC was lower, by 15-37%, in trees that died relative to survivors. We assessed constitutive phloem terpene concentrations in June 2021 and 2022, and induced terpenes in June 2022 after fungal inoculation with Ophiostoma. Total 2021 terpene concentrations were 31% lower in trees that died in 2022, but higher in 2022—by 26% for constitutive, 50% for induced responses. Specific differences in individual terpenes reflected overall results. Higher water potentials and reduced NSCs preceding mortality are consistent with previous research on pines. However secondary metabolites exhibited a counterintuitive result that raises questions why trees with greater defensive compounds were attacked and killed by bark beetles.