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Drought amplifies beetle damage to Colorado’s forests

State Forest Service report highlights trees’ fragility as insects spread
Mark Loveall, supervisory forester with the Colorado State Forest Service, walks through Overend Park on Tuesday looking at the health of trees in the park. The Colorado State Forest Service released its annual forest health report March 2, showing that bark beetles and drought continue to harm the state’s forests. (Jerry McBride/Durango Herald)

Insects and drought are taking an increasing toll on Colorado’s forests, challenging the long-term sustainability and resiliency of the state’s roughly 24.5 million acres of forest, a new report says.

The Colorado State Forest Service released its “2021 Report on the Health of Colorado’s Forests” on March 2. In the report, the state agency details the fragility of the state’s forests as drought compounds the damage done by beetles. Both beetles and drought threaten the future of Southwest Colorado’s forests, as CSFS partners with the U.S. Forest Service to mitigate their impacts and sketch a future for the region’s forests.

“Without rain, trees don’t have any defense. That’s the key,” said Mark Loveall, supervisory forester with CSFS’ Durango field office. “For all trees with these bark beetles, moisture really helps them defend themselves. When they’re dry, they just don’t have the sap (and) the resources to mount an effective defense.”

Teetering forest health: Bark beetles

CSFS’ annual forest health report paints a grim picture of Colorado’s forests.

Spruce beetle remains the most deadly forest pest in Colorado, affecting about 53,400 acres statewide in 2021. A list of other beetles, including the Douglas-fir beetle, western spruce budworm, piñon ips and western balsam bark beetle, collectively consumed more than 100,000 acres of forest in Colorado in 2021.

In Southwest Colorado, Douglas-fir beetle, roundheaded pine beetle and mountain pine beetle pose the most prolific threats to the region’s forests. However, many of the other beetles seen across the state also impact the region’s forests, according to a forest health map by CSFS Forest Entomologist Dan West released along with the report.

Roundheaded pine beetles affect ponderosa pine forests in La Plata, Montezuma and Dolores counties, while CSFS foresters also discovered new infestations in San Miguel County last year. The insects accounted for nearly 5,500 acres of forest damage, the most in the region.

Mark Loveall, supervisory forester with the Colorado State Forest Service, examines a piñon pine in Overend Park on Tuesday that is slowly dying because of a possible beetle attack and fungus. In Southwest Colorado, Douglas-fir beetle, roundheaded pine beetle and mountain pine beetle pose the most prolific threats to the region’s forests, according to the Colorado State Forest Service’s recently released 2021 forest health report. (Jerry McBride/Durango Herald)

“Roundheaded is definitely in the area,” Loveall said. “A lot of it is in Dolores County (and) western Montezuma County, but we also have some outbreaks going in western La Plata County mainly in the Cherry Creek area (near Hesperus).”

The species is particularly dangerous because it creates what the report called “a bark beetle complex,” where it works with other native bark beetle species to weaken trees.

“The roundheaded attacks trees later in the year. Some of the other ones attack earlier,” Loveall said. “Unfortunately, the trees are under attack all year long. That’s where they’re really a problem – they all kind of intensify each other’s effect.”

Mountain pine beetle persist in lodgepole pine stands north toward Gunnison County, rarely making their way down to the southwest corner of the state, but Tim Leishman, a silviculturist with San Juan National Forest, has seen a proliferation of Douglas-fir beetles in Southwest Colorado in recent years.

“I’ve been down here the last eight years and what I’ve seen is a year-by-year perpetuation of Douglas-fir bark beetle that’s being driven heavily by changing climate and by perpetual drought,” he said.

CSFS foresters have noticed an increase in Douglas-fir beetle activity in dense forests stressed by drought throughout Southwest Colorado, but especially in La Plata, Archuleta, Costilla, Rio Grande, Saguache, Gunnison and Montrose counties, according to the forest health report.

While the species is native to the region’s forest, Leishman has begun noticing a concerning and unique trend in which Douglas-fir beetles kill mature trees.

“The pattern that we’ve seen has not been what you would typically see for an epidemic event like we have seen (with) the spruce beetle and the roundheaded pine beetle,” he said. “Older, dominant Douglas fir have been succumbing to bark beetles … In the last few years, it’s become much more vivid.”

Mark Loveall, a supervisory forester in the Colorado State Forest Service’s Durango field office, looks at a dying piñon pine in Overend Park on Tuesday. Loveall said roundheaded pine beetles are particularly devastating for Southwest Colorado’s forests. The species is dangerous because it creates what the Colorado State Forest Service’s annual forest health report called “a bark beetle complex.” The roundheaded pine beetle attacks trees later in the year, working with other native bark beetle species to weaken trees. (Jerry McBride/Durango Herald)

Douglas-fir beetles have transformed forests into husks of what they once were.

“This has been a slower aggregation of beetle mortality for the Douglas-fir bark beetle that has now led to a large-scale landscape level of mortality event over a decade or so,” Leishman said.

Drought, a contributor

Leishman directly correlated the expansion of Douglas-fir beetle with climate change-induced drought. CSFS’ annual report echoed the threat climate-driven drought poses to Colorado’s forests, which was also reflected in the report’s title: “Managing Colorado’s Forests During Drought.”

“This is a unique moment for Colorado’s forests. This report highlights how years of persistent drought, an indicator of a warming climate, have stressed our state’s forests, creating ideal conditions for insect and disease outbreaks and large-scale wildfires,” wrote State Forester and CSFS Director Matthew McCombs in the report’s introduction. “These conditions, combined with a growing population in areas where wildlands intermingle with neighborhoods, warrant bold action to protect our forests and the communities that depend on them.”

Climate-induced drought has a wide range of harmful effects for Southwest Colorado’s forests, many of which are also tied to insect populations, according to both Loveall and Leishman.

At its most basic, drought deprives trees of critical resources. Trees use water to transport nutrients, so a lack of water weakens them and makes them more susceptible to insects.

Without water and the nutrients they need, trees are unable to fend off pests once they do attack. Ordinarily, trees “pitch out” when a beetle attempts to bore, sending sap to the site of the attack and killing the bug. But drought stifles that process.

A pine beetle taken from a ponderosa pine tree near Rockwood north of Durango in 2017. Drought in Southwest Colorado has made trees more susceptible to bark beetles. When trees are attacked, drought prevents them from defending themselves with sap. (Jerry McBride/Durango Herald file)

“Drought is the real key because it’s kind of a double hit on trees,” Loveall said. “If they’re not getting enough moisture, they’re weakened, but they also can mount a defense. Drought is really a big problem.”

In what scientists call a “positive feedback loop,” drought-weakened trees unable to defend themselves serve as hosts for beetles, spawning more bugs and amplifying the effects of bark beetles on forests.

“They more successfully reproduce in those trees and then bug populations start expanding,” Loveall said. “It gets turned into a bad cycle for trees.”

Even healthy trees are no match for bark beetles if their populations are high enough.

“When the populations get large, it’s tough. It’s kind of like viral loading,” he said.

The effects can be devastating. Not only do beetles kill more trees, but they also end up killing the mature trees that serve as seed banks and can help replenish dying forest stands.

“Bark beetles are killing your overstory trees and your seed source,” Leishman said. “If those (seeds that are left) aren’t able to germinate because there’s no moisture available (in the soil from drought), then we’re kind of left with a situation where we don’t have a whole lot of prospects for future germination and the retention of a conifer forest.”

Leishman said climate change only exacerbates the effects that drought and beetle infestations have on forests.

In March 2012, researchers with the University of Colorado Boulder published a study in The American Naturalist showing that warming temperatures were allowing mountain pine beetles to produce two generations annually instead of one.

There is also a growing body of research that suggests climate change is allowing earlier and longer beetle flights, which are when beetles disperse and attack trees, Leishman said.

A 2021 study by researchers with Los Alamos National Laboratory in New Mexico found that climate change increased the number of trees bark beetles killed during drought by 30%.

Ponderosa pines that did not survive a pine beetle attack near Rockwood north of Durango in 2017. Climate change is worsening forest health in Southwest Colorado by making forests more vulnerable to beetles and preventing trees from re-establishing themselves, said Tim Leishman, a silviculturist with the San Juan National Forest. (Jerry McBride/Durango Herald file)

Winter temperatures never get cold enough to kill the bugs either, which die at sustained temperatures of minus 30 to minus 40 degrees Fahrenheit, Leishman said.

CSFS’ report highlighted poor snowpack in 2021 in the San Miguel, Dolores, Animas and San Juan basins that further stressed trees and contributed to the effects drought is having on Southwest Colorado’s forests.

However, even a few good years of snowpack in Southwest Colorado would not likely be enough to reverse the effects of sustained drought and hamper bark beetle populations, Leishman said.

“I think we would need more like a decade of average winters with average monsoonal events for us to see any kind of slowing of what we’re seeing with these different damage agents,” he said. “We need years of great winters and we need some of those temperatures in the springtime to stay colder.”

CSFS management, partnerships

Unlike the U.S. Forest Service, which manages wide swaths of public forest in Southwest Colorado, CSFS’ work is geared more toward private lands and forest education.

“Part of what we do is we provide the publications, the consultation, the educational materials and the outreach for folks when they have questions. That’s the main mission of what we do,” Loveall said.

Foresters with CSFS’ Durango field office will also conduct property visits and forest health assessments, and they work with private landowners to develop forest management plans for their properties and remove diseased trees.

The roundheaded pine beetle has destroyed forest stands across Southwest Colorado as drought and other climate factors have exacerbated outbreaks. A few years of good snowpack will not mitigate the effects of many years of drought, said Tim Leishman, a silviculturist with the U.S. Forest Service’s San Juan National Forest. “I think we would need more like a decade of average winters with average monsoonal events for us to see any kind of slowing of what we’re seeing with these different damage agents,” he said. “We need years of great winters and we need some of those temperatures in the springtime to stay colder.” (Courtesy of Colorado State Forest Service file)

“Really what we’re doing is working with landowners if they’ve got diseased trees and helping them get those trees out of there,” Loveall said.

While beetle outbreaks that CSFS foresters usually tackle are only a few acres in size or a few trees, the work is critical. According to CSFS, private landowners own about 30% of Colorado’s forests.

“Private forest lands are generally more where we live,” Loveall said. “… Those are where we spend our time and we see the effects of those quite readily.”

Private forests are often at lower elevations with infrastructure in place that allows foresters to more actively combat declining forest health. They are also at the wildland urban interface, where communities are most susceptible to wildfires.

“When these insect problems and drought combine and all of a sudden then we have forest fires, (private land) is where the most hazards are. That’s where we have our homes,” Loveall said.

To help private landowners address declining forest health on their lands, CSFS helps to coordinate and conduct tree removals. If there is no market for the few trees that come off a private property, CSFS will assist landowners with other techniques to kill bark beetles, including covering downed trees with black plastic to cook the beetles.

In addition to working with private landowners, CSFS also partners with the U.S. Forest Service through the U.S. Forest Service’s Good Neighbor Authority, which allows the federal agency to enter agreements with state forestry agencies to carry out forest management.

In partnering, the two agencies can take a holistic approach where they use “cross-boundary treatments” to address forest health on both public and private lands.

“When you talk about landscape-level management, it’s not truly landscape level unless all public and private lands are managed in concert with each other,” Leishman said.

Mark Loveall, a supervisory forester with the Durango field office of the Colorado State Forest Service, inspects a piñon pine in Overend Park on Tuesday. While CSFS’ annual forest health report paints a grim picture for Colorado’s forests, Loveall shares confidence that forest health efforts by CSFS and the U.S. Forest Service will help mitigate the impacts of bark beetles and drought. (Jerry McBride/Durango Herald)

He said the continued partnership between private landowners, CSFS and the U.S. Forest Service, as well as tribes and other entities, is the only way to address the forest health challenges that Southwest Colorado faces.

“Where we really start doing good things is when we start looking at this shared stewardship, where we can look at entire homeowners associations that are abutting the U.S. Forest Service and work with (CSFS) to treat on both sides of the fence,” he said. “Then we started having a much larger impact. It slowly starts becoming more of a watershed or landscape-level impact.”

While CSFS’ 2021 forest health report portrays a fragile future for the state’s forests, Loveall shared optimism that Southwest Colorado’s forests will weather the risk posed by bark beetles and drought.

“I think the state of (Colorado’s) forest health, even though it sounds grim, it isn’t as bad as sometimes it seems,” Loveall said. “There’s nothing new and we generally know what kind of techniques work. The big wildcard is drought.”

ahannon@durangoherald.com