Pining Away

A swath of dead whitebark pines on Hoyt Peak in eastern Yellowstone National Park in 2004. (Photo: ©Jeffrey Hicke, University of Idaho)

FOR many millennia, slow-growing whitebark pines have held a place of special influence in their rugged alpine communities. These hardy conifers often live for centuries, thriving in the rocky windswept environment near the tree line in the Rocky Mountains, Cascades, and Sierra Nevadas. In recent years, things have changed. Whitebark pine populations are now declining steeply throughout their range. Climate change, disease, and the mountain pine beetle (a native species whose numbers have exploded in recent years) are largely to blame.

“There’s no happy ending,” says Polly Buotte, a Northwest Climate Science Center Graduate Fellow at the University of Idaho who studies pine beetle outbreaks in whitebark pines. “There’s nothing that people can do on a large enough scale to prevent big change.”

That big change, says Buotte, is already here. Half of all whitebark pine trees are now dead or dying. Forest Service scientists estimate that without active management the tree will disappear from 97 percent of its current habitat by 2100. This change won’t affect just trees.

As whitebark pines develop, they help other plants establish and survive by maintaining soil and slowing spring runoff by trapping and shading snow. Mature whitebark pines also produce large, rich seeds that feed birds, squirrels, foxes, and grizzly bears (see sidebar). The loss of this resource may have a domino effect throughout the ecosystem. But resource managers are hoping to help whitebark pines recover. Buotte is among those providing the science aiding this effort.

With support from the Northwest Climate Science Center (NW CSC), Buotte and her advisor, Jeffrey Hicke, a professor of geography at the University of Idaho, spent the last year developing statistical models to better understand the relationship between climate change and mountain pine beetle outbreaks in whitebark pine forests. 

Native to western North America, mountain pine beetles occasionally infest pine stands, boring into trees to lay their eggs, a process that spreads a fungus and is often fatal. A tree infested with mountain pine beetles cannot be saved, and its needles eventually turn from green to red. 

In recent years, the U.S. West has been banded with red stands of dead trees. Yet in the past beetle outbreaks were less disruptive. For ages, the mountain pine beetle has been in a state of equilibrium with its environment, acting as what’s known as a “disturbance agent”—recycling trees weakened by drought, fire, and disease and opening clearings in mature stands that support different species and increase forest diversity. What’s more, past outbreaks occurred mostly in isolated pockets, while overall beetle numbers remained low. Healthy trees defended against invaders by producing pitch cones to physically eject the beetles from their bark. Cold weather, predators, parasites, and diseases also helped keep beetle numbers in check.

However, climate change is now altering the environment in ways that favor beetle epidemics. When beetle populations are high, large numbers of beetles can launch synchronized attacks that overwhelm the defenses of even healthy trees. In these cases, large-scale outbreaks can occur that kill vast swaths of forest. 

In order to better understand how environmental conditions such as climate change influence beetle outbreaks, Buotte and Hicke analyzed the relationships between the number of beetle-killed whitebark pines and data collected by weather stations. Their analysis revealed that fall temperatures, winter minimum temperatures, and cumulative summer rain over a two-year interval were important factors for predicting beetle outbreaks. The researchers went on to create a statistical tool—a “weather suitability model”—using these three variables. The model predicted historical patterns of beetle outbreak both spatially (across forests) and temporally (over time). Its accuracy was very high, explaining more than 85 percent of the natural variation in beetle outbreaks across the landscape.

Next Buotte and Hicke combined their statistical model with downscaled climate projections for the region. This allowed them to calculate future beetle outbreak probabilities under three different climate change scenarios (each depending on a different level of global greenhouse gas emissions). As their number crunching commenced, the two say it quickly became clear that future conditions will be much more conducive to beetle outbreaks. Warmer falls will speed beetle development, while warmer winters will kill fewer beetles and increases in summer drought will weaken tree defenses.

The best hope for whitebark pines, the researchers conclude, may be at the highest elevations, where winter temperatures still get low enough to kill off beetle larvae.

But, says Boutte, there are other reasons for optimism. Even in the areas hardest hit by beetles not all whitebark pines are killed. Some small and medium-sized trees persist (beetles favor large trees) along with a few mature seed-producing trees, whose survival is a bit of a mystery. It may be that these individuals have a rare resistance to beetles and can be used to propagate more beetle-resistant trees to be used in restoration.

Buotte and Hicke’s maps of future beetle outbreak probabilities are helping guide conservation strategies for whitebark pine stands. Their results will continue to help managers decide where to invest in measures to protect whitebark pines from beetles, such as releasing chemicals that disrupt the beetles’ ability to coordinate attacks, where to plant seedlings, and where to reintroduce fire as part of restoration efforts. 

Partners at the U.S. Fish and Wildlife Service, the U.S. Forest Service, the National Park Service, and the U.S. Geological Survey have been involved in the study since its inception and consider the research critical. All groups appreciate having tools that help them make as much impact as possible with their limited conservation resources. Buotte says she’s come to see the problem in terms of risk management, much in the same way as the financial sector.

“We need to take a portfolio approach and spread out our management efforts to spread out our risk of the unknown,” she says. “We can use science to decide what percentage of our efforts to put where, but we shouldn’t forget that we don’t know everything, and that there are going to be surprises.”

For more information, see the final report, watch Buotte’s webinar, or contact her at pbuotte@uidaho.edu

Polly Buotte (left) discusses mountain pine beetles with retired U.S. Department of Agriculture Forest Service entomologist Jesse Logan in the Absaroka-Beartooth Wilderness just north of Yellowstone. (Photo: Laura Lundquist, ©Bozeman Daily Chronicle)