Temperature increases caused by climate change were a major factor driving the outbreak of mountain pine beetles that killed whitebark pine trees across millions of acres in and around Yellowstone National Park in the early 2000s, according to a new study by University of Idaho researchers and their partners.



The study is the first to pinpoint the multiple climate factors behind these outbreaks, with the goal of helping forest managers protect whitebark pines in the future, as the influence of climate change pushes temperatures up.

“By the middle of this century, most years will be as suitable for beetle outbreaks as in the 2000s. It’s where we’re headed,” said Polly Buotte, a postdoctoral researcher in the UI College of Science Department of Geography who led the study, which was supported by the Northwest Climate Science Center and also included researchers from the USDA Forest Service and the University of Wisconsin, Madison.

Whitebark pine is an iconic and ecologically important tree to the Greater Yellowstone Area. It’s an important source of food for grizzly bears, squirrels and seed-eating birds called Clark’s nutcrackers, and it provides shelter for other trees at high elevations.



Mountain pine beetle epidemics in whitebark pine are no surprise. Models developed in the 1990s by study co-author Jesse Logan, a retired Forest Service researcher, predicted that warming temperatures would lead to more outbreaks. However, the UI-led project sought to identify exactly why.



The researchers created a statistical model with data about whitebark pines, mountain pine beetles and climate in the Greater Yellowstone Area during 1985-2009, including the outbreak years 2000-2009.



They found three important factors were tied to climate:

• - The strongest influence was warmer winter minimum temperatures. In previous decades, researchers thought whitebark pines grew at high enough elevations for beetles to freeze and die every winter, Buotte said. But with the absence of very low winter temperatures after 1992, more beetles survived. This effect was a result of climate change that occurred in recent decades.

• - The second-most important influence was higher average fall temperatures. Warmer falls synchronize beetle populations, so they all hatch and emerge to infest other trees at the same time. These temperatures were not tied to climate change in recent decades.

• - Finally, lower summer precipitation levels and warming led to drought-stressed trees that were more susceptible to beetle infestation.

• - The researchers also created a model showing the potential for beetle attacks in coming decades and found that projected future climate change will continue to lead to conditions favorable to outbreaks.

“This adds pretty strong evidence for the need to include climate as a consideration for managing whitebark pine,” said co-author Jeffrey Hicke, an associate professor of geography at UI.



Planning for potential beetle outbreaks is vital to the success of efforts to preserve the whitebark pine in the Greater Yellowstone Area, Hicke and Buotte said. For example, Forest Service researchers have bred whitebark pines that resist blister rust, another threat to the species. But managers should consider where beetle outbreaks may soon occur when deciding where to plant them, Buotte said. Forest managers should look for cold microclimates, such as in cold air drainages, which could be a prime spot for replanting because they may remain cold enough to kill the beetles, at least for many years.



Buotte and Hicke are continuing their research into the interplay of climate change, whitebark pines and mountain pine beetles. They are working to identify the ripple effects of beetle outbreaks, identify genetic differences in trees that survive infestation, and quantify how much human-caused climate change has contributed to beetle outbreaks.



UI associate professor of geography John Abatzoglou also contributed to this research. The study was published in the July 26, 2016, issue of Ecological Applications.

This press release was originally posted by the University of Idaho. The contents of this article are solely the responsibility of the authors and do not necessarily represent the views of the USGS.