PS 43-173 - Resin response of drought-induced Pinus edulis is an early indicator of mortality following attack by bark beetles

Abstract: Pinyon pine (Pinus edulis), a slow growing species and ethnobotanical food source, is facing increased mortality rates due to an upsurge of bark beetle attack, drought, and higher temperatures. Although this drought-resistant conifer is highly adapted to the aridity of its native environment, it’s been experiencing repeated tree die-off for 20 years. Resin-based defenses have long been recognized as the primary mechanism by which conifers respond to attacks by bark beetles and pathogens. Studies have linked resin defenses to survivorship during periods of increased beetle activity, but few have observed resin responses to drought in experimentally manipulated mature trees in the field. Rain-out shelters were installed at a site in early 2020 at the Sevilleta National Wildlife Refuge in New Mexico with five levels of drought induction: 45% removal, 45% removal (legacy), 75% removal, 90% removal, and control. The legacy plot, installed in 2010 for a previous experiment, had experienced 10 years of precipitation removal prior to our study. Six individual P. edulis were targeted per plot for intensive measurement over several years. Measurements included resin flow, water potential, and other physiological data. Resin flow was collected from a 1.3 cm diameter punch to the cambium-xylem boundary at 24, 48, and 168 hours, although we only discuss 168-hour results here.

Trees in the 45% removal plots produced the highest amount of resin flow among treatments in 2020 and 2021. Yet by June 2022, resin flow in the 45% removal plot was the lowest of any treatment. Subsequently, all trees in this treatment were confirmed dead by August of 2022. In 2021 between very dry June and wet August following monsoon rains, control trees doubled their resin flow, while trees in the 45% treatment reduced resin flow by ~30%. Although trees in the 45% legacy were subjected to severe drought for a decade at the start of the experiment, these trees had consistently low resin flow over 3 years and all survived the experiment. Across trees in all treatments, we did not find a relationship between resin flow and plant water potential, and indicator of tree drought stress. This could suggest acclimation to drought not reflected in resin defenses, or that long-stressed trees were of little nutritional value to bark beetles. High resin flow did not indicate successful defense response in our study, but instead was an early indicator that tree mortality from bark beetle attack would follow.