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Global warming results in more bark beetles and dead pines than drought alone

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Pine trees killed by bark beetles. Photo by Ethan Miller.

In Sierra Nevada, California, mountain pine beetle infestations, magnified by global warming, kill 30% more mountain pine trees than mountain pine beetles in times of drought. A new supercomputer modeling study hints at the grim prospect of catastrophic future tree mortalities and offers ideas for mitigating the combined risk of forest fires and insect outbreaks.

“Forests represent a crucial buffer against global warming and are often touted as a low-cost climate change mitigation strategy,” said Zachary Robbins, researcher at Los Alamos National Laboratory, graduate student at State University from North Carolina and lead author of the beetle and ponderosa pine mortality article. “Our research shows that warming shortens the interval between beetle generations, thereby fueling the growth of the beetle population. This can then cause catastrophic mortality in forest systems during drought in the Sierra Nevada and throughout the western United States. “

In the study recently published in Global Change Biology, Robbins and colleagues developed a new modeling framework to assess the risk that mountain pine beetles, or bark beetles, pose in many forest ecosystems due to climate change. If the effects of compromised tree defenses (15-20%) and increased bark beetle populations (20%) add up, the team determined that 35-40% more ponderosa pines would die from ponderosa pines. beetles for every degree Celsius of warming.

“Our study is the first to attribute a level of tree mortality to the direct effect of warming on bark beetles, using a model that captures both beetle reproduction and development rates and host stress. “said Robbins. “We have found that even small increases in the number of annual generations of bark beetles due to warming can dramatically increase tree mortality during drought. “

Using Los Alamos supercomputers, the team modeled bark beetle dynamics and tree death during the extreme drought of 2012-2015 and earlier periods. Then, they studied these results using field observations of maximum and minimum temperature, precipitation, tree density, tree mortality, and the onset of beetle flight (when fully developed beetles leave their homes. original tree) as well as laboratory studies of the rate of development of beetles.

They found that a faster rate of production of new generations of offspring contributed more to deadly infestations than surviving the winter in the absence of deadly cold temperatures for the beetle, but, surprisingly, increasing the number of generations. was not very important. .

“In the Sierra we only saw about a third of more generations per year, but it really magnified the mortality,” Robbins said. “This shows that a small impact on the success of these populations can have a large impact on tree mortality, when we previously thought that the beetle needed an entire generation to have a huge impact on mortality.

“These results should generally apply to many species of pine forests around the West, although the beetle species may be different,” said Chonggang Xu, co-author of the article. Senior scientist in Los Alamos, Xu simulates forest-vegetation dynamics in his research.

“Death from beetles can cause forests to act as sources of carbon in the atmosphere for decades,” Xu said. “Dead trees do not absorb CO2 but release carbon into the atmosphere. This could potentially increase global atmospheric carbon predictions, which has not yet been explicitly accounted for in current generation Earth system models. “

The research also has implications for forest management under climate change.

“A mechanistic understanding of the interactions between climate, forests and disturbances can improve planning of forest management actions and better predict the effects of climate change on biological systems,” said Robbins.

Older and larger beetles are particularly vulnerable to beetle attack because their size supports large infestations, Xu said, while younger and smaller trees can survive.

“A diverse forest that also combines small and large trees and a diversity of species is also more resilient,” said Xu. He pointed out that forest management to minimize the risk of forest fires often removes small trees and preserves larger ones, “which creates a forest of large trees.” Then the beetle arrives and the trees could be devastated at the same time. “

Bark beetles kill trees all over the world by chewing the bark and depositing their larvae in the inner bark. A growing number of beetle epidemics over the past two decades have devastated the forests of the American West, including New Mexico, striking nearly 11 million acres nationwide and threatening the basic structure and the ecological processes of some forests.

Beetles exploit the warming and drying climate of the West. When rainfall and temperature remain at historic levels, trees can defend themselves against infestations, but drought often triggers bark beetle outbreaks. Indeed, trees subjected to water stress suppress their photosynthesis, close their stoma and grow more slowly, depleting their carbon storage, which can weaken their defenses.

The life cycle of the beetle depends on the temperature under the bark and in the air. Warmer temperatures reduce the number of beetles killed by the deep winter cold and accelerate and extend the breeding season. Epidemics eventually collapse when bark beetles deplete the supply of susceptible trees, extremely cold temperatures kill bark beetles or predators, and parasites decimate bark beetle populations.

The study examined historical and contemporary temperature trends across a wide swath of the Sierra Nevada, including several national forests and the Kings Canyon, Sequoia, and Yosemite National Parks.

In a new approach, the team used a model of breeding cycles and bark beetle population dynamics. The team incorporated this model into a model of tree death and insect attack, which takes into account the number of bark beetles in flight, the number and size of trees available as hosts, and drought. The models were validated against data from observations in the field.

Paper: “Warming increased bark beetle-induced tree mortality by 30% during 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.

Funding: University of California National Laboratory Fee Research Program at Los Alamos National Laboratory.


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Author: Bill Gabbert

After working full time in wildfires for 33 years, he continues to learn and strives to be a fire student. View all articles by Bill Gabbert


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