FRONT PAGE: Global Warming, Not Just Drought, Drives Bark Beetles To Kill More Ponderosa Pines

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Newswise — LOS ALAMOS, N.M., November 23, 2021–In California’s Sierra Nevada, western pine beetle infestations amped up by global warming were found to kill 30% more ponderosa pine trees than the beetles do under drought alone. A new supercomputer modeling study shows the danger of catastrophic tree deaths in the future and provides insights into how to mitigate the risk of both wildfires, insect outbreaks, and other natural disasters.

“Forests represent a crucial buffer against warming climate and are often touted as an inexpensive mitigation strategy against climate change,” said Zachary Robbins, a researcher at Los Alamos National Laboratory, graduate student at North Carolina State University, and lead author of the paper on beetles and ponderosa pine tree die-offs. Our research has shown that warming reduces the time between generations of beetles, which can lead to a rapid increase in beetle populations. This can lead to catastrophic forest mortality during droughts in the Sierra Nevada and the Western United States. Robbins and his collaborators have developed a new modeling framework that assesses the threat western pine beetles (or bark beetles) pose to forest ecosystems as a result of climate change. If the effects of compromised tree defenses (15% to 20%) and increased bark beetle populations (20%) are additive, the team determined that 35% to 40% more ponderosa pines would die from beetle attacks for each degree Celsius of warming. Robbins stated that this is the first study to link tree mortality to warming’s direct effects on bark beetles. He used a model that captured both beetle development and reproduction rates as well as host stress to determine the effect of warming. “We found that even slight increases in the number of annual generations of bark beetles due to warming can significantly increase tree mortality during drought.”

Using Los Alamos supercomputers, the team modeled bark beetle dynamics and tree die-off during the extreme drought of 2012-2015 and earlier periods. They then analyzed the results by using field observations of maximum temperature and precipitation, tree density and tree mortality. They found that the production of new offspring was more important than the survival of the winter without cold temperatures. However, it was not surprising that the increase in generations was very small.

“In Sierra, we only saw about one-third of the annual generations, but that really increased mortality,” Robbins stated. It shows that even a small change in the success rate of these populations can have a significant impact on tree mortality. This is a stark contrast to previous assumptions, which suggested that the beetle would need to increase its population by one generation to significantly impact mortality.

” These findings should be generalized to all species of pine forests in the West, even though the beetle species may be different,” stated Chonggang Xu (coauthor). Xu, a senior scientist at Los Alamos simulates forest-vegetation dynamics for his research.

“Beetle-instigated die-off may cause forests to act as carbon sources to the atmosphere for decades,” Xu said. “Dead trees don’t absorb carbon dioxide but instead release it into the atmosphere.” This could potentially raise global forecasts of atmospheric carbon, which has not yet been explicitly considered in current-generation earth-system models.”

The research has also implications for forest management under climate change. Robbins stated that a mechanistic understanding the interactions between climate, forests and disturbances could improve planning for forest management and better predict the impacts of climate change on biological systems.

Older ponderosas are more susceptible to beetle attack because of their large size, Xu stated. However, younger trees can survive.

” A diverse forest that includes both big and small trees, and also has a variety of species, is more resilient,” Xu stated. He said that forest management can reduce wildfire risk by removing smaller trees while preserving larger trees. This creates a forest with big trees. The beetle arrives and can cause damage to the trees at the same moment.

Bark insects kill trees by eating through bark and putting their larvae inside the bark. An increasing number of beetle outbreaks in the past two decades have devastated forests across the American West, including New Mexico, striking nearly 11 million acres nationwide and threatening the basic structure and ecological processes of some forests.

The beetles profit from the West’s warming and dry climate. Trees can protect themselves against infestation if the temperature and precipitation are at their historic levels. However, bark beetles can often spread to trees when there is drought. This is because trees that are water stressed suppress their photosynthesis, close the stoma and grow slower, which can lead to a decrease in their carbon storage. The temperature in the air and under the bark determine the life cycle of the beetle. Warmer temperatures decrease the beetles that are killed by the deep winter cold, and speed up and extend the breeding season. When bark beetles exhaust their supply of trees that are susceptible, outbreaks occur. Acutely cold temperatures kill the beetles and predators or parasites decimate the bark beetle population.

The study examined historical and current temperature trends across a wide area of the Sierra Nevada. This included several national forests, Kings Canyon, Sequoia and Yosemite National Parks.

The team developed a model that simulated the breeding cycles of bark beetles and their population dynamics. This was a novel approach. This model was incorporated into a tree death and insect attack model. It accounts for the number bark beetles flying, the size and number of trees that are available as hosts and the drought. These models were validated using field data.

The Paper: “Warming increased bark beetle-induced tree mortality by 30% during an extreme drought in California,” by Zachary J. Robbins, Chonggang Xu, Brian H. Aukema, Polly C. Buotte, Rutuja Chitra-Tarak, Christopher J. Fettig, Michael L. Goulden, Devin W. Goodsman, Alexander D. Hall, Charles D. Koven, Lara M. Kueppers, Gavin D. Madakumbura, Leif A. Mortenson, James A. Powell, Robert M. Scheller, in Global Change Biology. DOI: 10.1111/gcb.15927.

The Funding: University of California National Laboratory Fees Research Program Los Alamos National Laboratory.

About Los Alamos National Laboratory

Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is managed by Triad, a public service oriented, national security science organization equally owned by its three founding members: Battelle Memorial Institute (Battelle), the Texas A&M University System (TAMUS), and the Regents of the University of California (UC) for the Department of Energy’s National Nuclear Security Administration.Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.

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