This project was designed to understand how drought affects tree physiological function, and in turn, the ability of trees to produce chemical defenses against bark beetles. We conducted experiments in the greenhouse with mature, transplanted piñon pine, and with trees of the same species growing naturally in the field in central New Mexico. To manipulate water availability in the field, we installed three large plots (1600 m2) with rainout shelters that consisted of troughs that funneled rain and snow away from trees. We installed tree sets of rain-out shelters in January 2020 that covered approximately 45%, 75%, and 90% of the plots. We also leveraged a 45% shelter with the same design that had been installed at the site since 2010, a legacy of a previous experiment. This enabled us to assess recent responses to drought, but also long-term responses with our legacy plot.

Our experiment benefitted from several years of background drought in New Mexico, due to the influence of La Niña, as well as a bark beetle outbreak. This resulted in almost half of the trees in our newly installed rainout shelters dying by the end of 2022, providing us the opportunity to understand tree stress and its effect on defenses across a full range of conditions. Over four years of the field drought experiment, we made detailed, high temporal resolution measurements of tree photosynthesis, water loss, tension in the xylem (water transporting tissue), growth, exudation of defensive resin from the tree trunk, and concentrations of sugar, starch, and a variety of defense-related compounds in various organs of the tree. Defense-related measurements of chemical compounds and the flow of resin from the trunk were also made after trees were exposed to a fungal species vectored by bark beetles, to understand how trees defenses might be altered when experiencing varying levels of drought stress and being challenged by bark beetles. In our greenhouse experiment we made similar measurements but also restricted transport of sugars to the site of fungal attack and added detailed assessments of the water pressure in tree tissues.

We believe the dataset produced from these experiments is novel and globally unique, as no other research has carried out a set of such comprehensive and integrative measurements on how tree drought stress influences defense, and ultimately, death or survival within natural conifer populations. With large-scale tree mortality events increasing in frequency and extent world-wide because of hotter droughts and bark beetles, it is more important than ever to understand the mechanism driving forest susceptibility to destructive disturbances. The commonly held assumption has been that drought stress reduces tree carbon reserves and thus the production of carbon-based chemical defenses, allowing for successful bark beetle attack. However, multiple lines of evidence from the field and greenhouse contradict this claim and the mechanisms involved appear to be far more complex than models assume. We found that carbon limitation takes time in these drought tolerant trees and that bark beetles seek out and successfully attack drought stressed trees that are unable to mobilize stored terpenes when attacked. Thus, defensive capacity is related to pre-formed resin and mobilizing it to the site of attack, and this transport of resin is more tightly coupled to cellular water relations than to carbohydrate availability for resin synthesis.

We expect that publication of the results summarized here will change the overall understanding of drought x bark beetled induced tree mortality. This project has already led to the successful publication of eight peer-reviewed papers, with another currently in review, and three more nearly ready to submit. Project results have also been communicated through 45 presentations at scientific meetings of professional societies, in seminars given at multiple universities, and in two episodes of an environmental science podcast run by high school students. This research led to the training of three postdocs, six graduate students, ~30 undergraduates including one REU students, and two field technicians. These early-career researchers were trained in a variety of field measurements and laboratory procedures, including through three workshops where they received hands-on training of the primary methods used in each laboratory of the principal investigators (PIs). Many of these project participants have now taken their careers to the next stage by building on the experience they gained on this project, and some are now advising students in these same techniques in their own lab groups. The project PIs also included the hypotheses, approach, and results of this research into multiple courses as their respective institutions. We also conducted an authentic research experience for teachers, where three high school science educators participated in field research and developed lesson plans organized around translating this research for an AP Environmental Science class. These lessons will be disseminated to their classrooms but also beyond those of our teacher participants through the creation of open-access data-driven learning modules.

Last Modified: 01/25/2025
Modified by: William T Pockman