After suffering more than 15 years of a mountain pine beetle outbreak, Colorado’s forests are now facing another bark beetle epidemic. Last year, 183,000 acres of Colorado’s forests were infested with the spruce beetle, bringing the total acreage affected by spruce beetles to just under 1 million since the initial outbreak in 1996. The area affected by spruce beetles is still smaller than pine beetles, who affect some 3.4 million acres of forests in Colorado, according to the U.S. Forest Service. But 2012 was the first year in recent history the spruce beetle affected more trees than the pine beetle, which infested a new 140,000 acres last year, according to a 2012 report from the National Forest Service.
With the spruce beetle outbreak comes a renewed debate between Colorado foresters and researchers: Is global warming the most important variable in the bark beetle equation, or are other forest management techniques also responsible and therefore capable of helping to slow the spread? While forest researchers disagree on the importance of climate change and beetle infestations, they concur that Coloradans should be concerned about the impact that climate change and high-elevation beetle outbreaks will have on watersheds.
“There’s been a long history of research by forest entomologists documenting that warming temperatures are favorable for the survival of bark beetles,” says Tom Veblen, a professor in biogeography and conservation at the University of Colorado Boulder. “There is no debate whether or not warming is contributing to the increase in spruce beetle activity.”
But warming is just one part of the equation for Allen Owen, Boulder district forester for the Colorado State Forest Service (CSFS), and Sky Stephens, forest entomologist for the CSFS. They argue that lack of management and large, over-mature tree stands play equally important roles.
“Climate change is certainly one of the many factors impacting the landscape,” Stephens says. “But even if we remove the impact of climate change, we would still see significant insect activity. Lack of management, drought conditions and tree stocking densities are all making trees even more susceptible to spruce beetles.”
Spruce forests are typically higher-elevation forests, according to Owen, which is part of the reason those forests have seen less management.
“High-elevation forests are usually inaccessible,” Owen says. “They’re more recreation- and wildlife-oriented, so intentionally, we don’t manage these forests as intently as lower-elevation forests like ponderosa or lodgepole pine. They just remain a lower priority as far as forest management goes.”
High-elevation spruce forests are usually under U.S. Forest Service (USFS) jurisdiction and are not man- aged locally, according to Owen. The mountain pine beetle has been more of a priority for the USFS; thus, man- agement of spruce forests has suffered and beetles have spread, Owen says. But jurisdiction and lack of management aren’t the whole picture for Stephens.
“Much of the spruce beetle activity in southwestern Colorado has started in a wilderness area — by definition, areas in which we’ve elected not to do any management, to let whatever happens, happen,” Stephens says.
Warming temperatures outweigh lack of management and the number of readily available host trees in spruce forests, says Veblen, who recently won a $300,000 grant from the National Science Foundation to study spruce beetle outbreaks in the context of climate change and wildfires.
“Climate change would override those more local factors like tree size and stand position,” Veblen says. “Historical factors like logging and for- est management are definitely a secondary influence.”
Spruce beetle outbreaks in Colorado aren’t new, nor is the science linking beetle outbreaks to warmer temperatures, he says. Similar spruce beetle out- breaks were recorded in Colorado forests in the 1800s and 1940s. By coring trees and examining tree rings that date back hundreds of years, Veblen and his team have reconstructed historical beetle outbreaks.
"The dates of those outbreaks correspond with warmer and drier conditions,” Veblen says.
Warmer temperatures accelerate the life cycle of the spruce beetle, while drier conditions put an added stress on trees, leaving them more susceptible to infestation.
“The fact that we have warmer temperatures during the winter is important in favoring the survival of larvae,” Veblen says. “In habitats and elevations where formerly the life cycle of the spruce beetle required two years, it now requires only one. … The drier conditions associated with warmer temperatures and natural variability reduce the resistance of trees to attack.”
“I don’t know that drought or weather were a factor in the past,” Stephens says. “Forests were just in a condition that lent them to being susceptible, much like today.”
Researchers need to caution against overreacting to the spruce beetle outbreak as an ecological disaster in and of itself, according to Veblen. The spruce beetle is a native insect, and we know that large-scale outbreaks have occurred in the past.
“Instead, we have to look objectively at what the potential changes in ecosystem services that we derive from forests are,” Veblen says. “The big question is, what might be the impact on water resources?”
Veblen, Stephens and Owen agree that people should be concerned about the impact that climate change and spruce beetle outbreaks can have on Colorado’s water supply.
Tree cover and snowpack are critical to watersheds in Colorado. When there is denser tree cover in high altitude forests, the spring snowmelt is more gradual, which means there is less stress on waterways when melt occurs. Without trees, the melt is much faster and thus more stressful on watersheds.
As spruce beetle infestation continues to threaten Colorado’s higher-elevation forests, killing mature trees and thinning overall forest composition, there will be less vegetation to slow snowmelt and to keep debris out of waterways. Forests that have suffered wildfires are extreme examples of what could happen to Colorado’s waterways, according to Stephens.
“The Poudre River right now is still black and disgusting and full of junk from fires this summer,” Stephens says. “And it’s gonna be that way for a while. All that vegetation to slow the water going downhill is gone.”