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Climate change is transforming Western forests. And that could have big consequences far beyond wildfires.

The heat of a warming planet, like an artist’s palette knife on a canvas, etches its way across Western forests, slowly altering ecosystems that have flourished for centuries.

Camille Stevens-Rumann, left, a forestry researcher at Colorado State University, graduate assistant Zoe Schapira , and field technician Zane Dickson-Hunt gather data amid the burn scar of the 2018 Spring Creek Fire June 11, 2019, near La Veta. (Mike Sweeney, Special to The Colorado Sun)
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LA VETA — From her family’s summer cabin north of Walsenburg, Camille Stevens-Rumann could see the glow of the Spring Creek Fire on the ridge to the south in the summer of 2018. “It was pretty spectacular, my 4-year-old was excited,” she said

This past June, Stevens-Rumann walked a burnt slope near the town of La Veta, a piece of the more than 100,000 acres ravaged by the fire that raged for more than two months. No tree survived on this hill, a vista of scarred black pillars that stretched in all directions.

New growth of aspen sprouts from the forest floor in the Spring Creek Fire burn scar near La Veta, Colo. CSU field technician Zane Dickson-Hunt works in the background. (Mike Sweeney, Special to The Colorado Sun)

Stevens-Rumann, a 33-year-old assistant forestry professor at Colorado State University, was there to measure and mark what comes next. In all likelihood, the ponderosa pine forest that had been there would not return.

Aspen and scrub oak have already sprouted, but all the pine trees and their cones were destroyed. No pine saplings poke through the charred soil.

Across the Rockies and even into the Sierra Nevada and the Pacific Northwest’s Cascades, forests are changing or simply vanishing. Wildfire has played a big role. Insect infestations have also had a hand, as has drought.

Behind it all is one driving force — climate change. Scientists charting the fate of forests see it, whether they are entomologists or botanists or wildfire ecologists like Stevens-Rumann. The heat of a warming planet, like an artist’s palette knife on a canvas, etches its way across Western forests, slowly altering ecosystems that have flourished for centuries.

“We are really moving out of a climate that is suitable for forests,” Stevens-Rumann said. “Old trees can persist, but when change comes in a disturbance like a wildfire and the ecosystem resets, the forests don’t come back.”

The transformation isn’t quite that simple. Lower elevation forests, like those along the Front Range, are most at risk, but as the forest rises into the mountains, the nature of the woods may change with spruce, fir and pine competing for survival even as new pests push into those higher, and now warmer and drier, mountain reaches.

“As ecosystems change, there are going to be winners and losers,” said Thomas Veblen, a biogeographer and distinguished professor at the University of Colorado. “The regulator function of the forest could diminish … leading to more runoff and flash floods. With a reduction of the forest canopy, we are going to see the potential for greater erosion. The question is how much of the forest will fail to regenerate.”

Fire changes the forest’s composition

Colorado’s Front Range has had five ecotones — shifts in plant and animal communities — from grasslands at 5,500 feet above sea level to alpine tundra at 11,300 feet.

“When we go to higher elevations under warming temperatures, we do expect the species from lower elevations to do better after a fire or other disturbance,” Veblen said.

After six years as a forest firefighter in an elite hotshot crew, Stevens-Rumann, curious about what happens after the fire is out, became a wildfire ecologist.

In a study of 1,485 sites that burned in 52 wildfires in forests from Colorado to northern Idaho, a team led by Stevens-Rumann found tree regeneration was significantly reduced at the sites that burned after 2000.

Fewer than half the spots had signs of growing back with a density of trees similar to the pre-fire forest, and nearly one-third of the sites had no trees at all.

Colorado State University graduate student assistant Jesse Wooten measures the diameter of a burned tree near La Veta. The measurements help determine how much area a stand of trees occupied. (Mike Sweeney, Special to The Colorado Sun)

These forests ranged from lower elevation dry conifer forests, containing ponderosa pine and Douglas fir, to moister conifer forests of Engelmann spruce and lodgepole pine. The highest elevation forests in the study were around 9,000 feet.

The researchers measured the site temperatures and moisture, and classified the areas by the severity of the burn.

It appeared that the hotter and drier the site, the less chance of a forest coming back. “There is an ecotone shift already underway,” Stevens-Rumann said. “We may see aspen and scrub oak replace pine and at higher elevations, maybe pine replace fir.”

This is happening across the Front Range. An analysis of five Front Range forest fires between 1996 and 2003 — Bobcat Gulch, Overland, High Meadow, Buffalo Creek and Hayman — found that 23% of the forest cover has been lost.

“Below 8,200 feet, we saw little generation; above 8,200 feet, where it tends to be cooler and moister, we saw more,” said Marin Chambers, a researcher at the Colorado Forest Restoration Institute and the study’s lead author.

Savage wildfires disrupt the trees’ lifecycle

At the site of the 2002 Hayman Fire — the largest in the state’s history, consuming 135,114 acres northwest of Colorado Springs — the most intensely burned areas have come back as grasslands.

The problem, Chambers explained, is that while fire is part of a ponderosa pine forest’s life cycle, the pine’s seeds are heavy and not easily dispersed. That, coupled with heat and a water deficit, make it harder for the lower elevation forest to regenerate.

Fire has also been an essential component for the lodgepole pine, which is “serontinous” — depending on fire to release their seeds and simultaneously clear an ashy, nutrient-rich bed for new seedlings.

Two things, however, have altered the natural cycle. First, a century of fire suppression — think Smokey the Bear — has prevented regeneration, creating forests of mostly large, old trees. Additionally, it has built up dead wood on the forest floor that aids fires to burn more intensely when they do happen.

Catastrophic wildfires have destroyed more acreage in Colorado since 2000 than in the previous three decades, including the Black Forest Fire that consumed 14,280 acres and destroyed more than 500 structures in a nine-day period of 2013. (John Ingold, The Colorado Sun)

And now those fires are coming more quickly and more savagely. Since 2000, there has been vastly more acreage burned in Colorado than in the three previous decades, with peaks of more than 300,000 acres scorched in 2003 and about 160,000 acres destroyed in 2013.

Across the West, about 20 million acres burned between 1979 and 2015. The average fire season grew by 26 days, a 41% increase, and high-fire-potential days increased by 17, according to a study by John Abatzoglou, a University of Idaho geographer.

Abatzoglou measured drought conditions and water availability, as well as temperature, and estimated that climate change contributed to about half the forest fire acreage as heat parched the forests, creating more dry fuel.

The analysis also found that significant declines in spring rains in the southwestern U.S. during the period from 1979-2015 and in summer precipitation in the Northwest add to the fire problem.

Another Abatzoglou study projects the shortening of the snowpack season except for in the high Rockies and parts of the Uinta and Bighorn ranges in Utah and Wyoming, as well as more precipitation falling as rain rather than snow.

How much hotter has it been? The average observed summer temperature in Colorado between 2005 and 2009 was nearly 67 degrees Fahrenheit — the hottest it has been in a century, up almost 2.5 degrees since 1989, according to the National Oceanic and Atmospheric Administration.

The temperature itself poses an ecological Rubicon. A study of 177 burn sites from 21 forest fires in the northern Rockies documented the same phenomenon Stevens-Rumann saw: fewer trees growing in the lower elevation patches and no trees at about one-third of the sites, with grasses, sedges and a wild, purple evening primrose called fireweed taking root.

The study also calculated that at summer average temperatures above 63 degrees, fir tree regeneration would be “minimal.” Ponderosa pine is slightly more heat tolerant at temperatures up to 66 degrees, the study said.

“Ponderosa pine might compete a little better at higher elevations,” said Kerry Kemp, head of the study and a forest ecologist for The Nature Conservancy, an environmental group. Still, by midcentury, 82% of the sites in the study are forecast to have average summer temperatures above 63 degrees.

“There are always going to be favorable opportunities for establishment,” Kemp said. “But as the mean temperature climbs over time, there are going to be fewer and fewer of those windows.”

20 years of watching and the ponderosa have not returned

And it isn’t just trees that are lost. In 2010, the Church Park fire destroyed 450 acres on a steep hillside near Fraser. The area had previously been infested by pine bark beetles, but in the still-forested areas after the fire, whortleberry bushes — a wild huckleberry that flourishes in the shade of pine forests — went from 30% of ground cover to 2%. “Whortleberry took it in the shorts,” said Charles Rhoades, a U.S. Forest Service researcher, who studied the site.

Aspen grew back in some places, as that tree’s roots are not destroyed by fire, and grasses and sedges came in. But because it was such a steep slope, almost half the site remains bare rock, Rhoades said.

One of the first signs of the future came on the hills above Boulder almost two decades ago. The Walker Ranch Fire consumed about 1,100 acres around a popular hiking trail in September 2000.

When Veblen visited the site the following spring, he saw something surprising. “The ponderosa was not regenerating the way you’d expect,” he said. 

Veblen returned year after year “waiting to see ponderosa regeneration.” It never came.

In 2003, Veblen, 71, who has been studying Western forests for nearly four decades, put together a presentation for the U.S. Forest Service offering the idea that there could be a shift in wildfires and ecotones likely related to climate change.

“The reaction was, ‘Oh no, this is not unusual,’” Veblen said. “The idea that wildfire and forests were linked to climate change was not widely accepted.”

“Our forests have been resilient, and based on tree-ring studies of the past 500 years, we have a very good understanding of fire history,” he added. “A lot of areas, such as the Rocky Mountain spruce-fir forests, haven’t burned in 400 years, and we are really concerned those old-growth forests stands are going to be really susceptible to fire.”

“Now the forest is not recovering from fire, and it is a real change in the game,” Veblen said.

A tuft of corydalis aurea , commonly known as scrambled eggs, blooms next to a charred tree in the Spring Fire burn scar near La Veta. (Mike Sweeney, Special to The Colorado Sun)

Stevens-Rumann is marking the change foot by foot. On the blackened hillside, she and her team used tape measures to mark off a 10th-of-an-acre area and then recorded everything from the width of dead trees to the number of little Gambel oak bushes, and the presence of a native ground cover with delicate green leaves and bright yellow flowers commonly called “scrambled eggs.”

She is laying out 40 such plots along the slope from an elevation of 7,500 feet to 9,500 feet and patiently waiting to see what happens. “Fire is an amazing catalyst of change, and we have this huge inertia in these very old forests,” Stevens-Rumann said. “This will be a glimpse of how things are actually changing.”

The view from above

On a bright July morning, Dan West climbed into a four-seat Cessna at the Durango Airport and headed out to search for forest areas under attack.

As the chief entomologist for the Colorado State Forest Service, West spends part of his summer flying over the state’s 24.4 million acres of native forest looking for damage caused by pests and disease.

As the survey crew flew about 1,000 feet above the forests of the San Juan Mountains, West could see defoliated aspen groves where a fungus, taking advantage of the wet spring, was at work. There were patches of dull brownish spruce in the drainages hit by spruce beetle and stands of pines turning from off-green to yellow as pine beetles bore into the trees. “It is an early sign of death,” he said.

Thousands of lodgepole pine trees in the South San Juan Wilderness on Piedra Pass show the damage done in Colorado’s forests by pine beetles. (Dean Krakel, Special to The Colorado Sun)

By late afternoon, with thunderstorms and turbulence building and making it hard to even mark maps to note outbreaks, the plane headed home.

In the four years he has served as chief entomologist, the 46-year-old West says he has watched “a forest in flux as we go from a dry year to a wet year, but there has always been the epidemic of pine bark beetles.”

Starting in the mid-1990s, an outbreak of mountain pine bark beetles swept through the forests at an unmatched scale. Since 1996, more than 3.4 million acres of forests in Colorado have been laid waste as once green mountainsides of ponderosa and lodgepole pine turned reddish-orange and then gray and dead as all the needles fell from the trees.

“Forest have coevolved with pests,” West said. Indeed, pests, like fire, help to clear the forest land, taking out the biggest trees, opening the canopy and allowing for new seedlings to grow. “Those forests have some plasticity, some ability to withstand some disturbance.”

The mountain pine beetle outbreak of the past two decades, however, is without precedent. “It is 10 times bigger than anything recorded, and it’s on the move,” said Diana Six, an entomologist at the University of Montana. “They’ve jumped the northern Rockies and are heading to the Yukon. They are heading east. They’ve also moved up in elevation, wiping out whitebark pine.”

In Yellowstone National Park and the surrounding country, bark beetles have killed half the whitebark pines in areas that have recorded average increased temperatures.

And just as a century of fire suppression is playing a role in creating more wildfires, it has also fed into the pine beetle outbreak. “We don’t have the variety of trees,” West said.

The beetles attack only the largest trees, which provide the most food. Any attack is a risk for the beetles since trees have chemical defenses, so the insects focus on the high-reward trees.

In a forest with trees of different ages, the insects attack the biggest trees while ignoring the younger, smaller ones. Colorado forests, however, are populated primarily by older trees, so each one is a prime target for an infestation.

“Beetles have to build their numbers very quickly to attack a tree, and the pest moves in waves,” West said.

Drought weakens the natural defenses of healthy trees

Healthy trees have a range of defense against pest attacks. Ponderosa and lodgepole pines can release an oily resin that flushes and seals wounds created by the insects, and it also suppresses a fungus the beetle uses to make a pheromone to lead other beetles to the tree.

If a tree is stressed by drought, it has more trouble making these chemical weapons and becomes less able to defend itself, to flush out the invaders.

The mountain soils are mostly thin and granite based, unable to retain much water. “That is why precipitation is so important to tree defenses,” West said. “They are already working on thin resources, and then you get climate variability.”

The water the trees need to flourish comes mainly from snowpack, which has been dwindling. “The snowpack is coming off two or three weeks earlier,” West said. Then come summer’s higher temperatures.

The “water year” October 2017 through September 2018 experienced the warmest annual temperatures since records began being kept in 1895, according to NOAA. It was also the second driest year on record.

Those temperatures also have an impact on the insects. Winter temperatures of minus-35 F for two days will knock back beetle populations, but West said we haven’t seen those temperatures since 1985.

“Warmer temperatures mean a greater number of beetles and more survival over the winter,” Six said. “In cooler, wetter weather, beetle populations go down in strength, and the strength of trees goes up.”

The present climate enables the insects to rapidly build their numbers “to get to a tipping point where the number of bark beetles just explodes,” West said.

And the mountain pine bark beetle is just one of several bugs that make up what the state forest service calls the “bark beetle complex,” an army of different species that attacks different parts of the tree at different times of the year.

Glowing in the light of a setting sun, lodgepole pine trees below Snow Mesa near Lake City show the damage done by pine beetles. (Dean Krakel, Special to The Colorado Sun)

There are roundhead pine bark beetles, Western pine beetles, pine engraver beetles, Ips beetles, as well as mountain pine beetles. “It’s a brilliant strategy,” West said. “They partition their resources.”

Impacts from the bark beetle complex have “significantly increased since 2012, according to the report.

As big and as bad as the infestation has been, Six said, “the current outbreaks are a harbinger of things to come.”

The effects of changing climate are working their way up

In 2017, CSU graduate student Isaac Dell lugged insect traps, collection cups and temperature data loggers to 15 high-elevation sites, including Wolf Creek Pass, Monarch Pass and Guanella Pass, with altitudes of up to 11,600 feet. Here the forests are dominated by Engelmann spruce with a mix of subalpine fir, lodgepole pines and quaking aspen.

The prey Dell and Seth Davis, a CSU assistant professor who directed the study, were after was the spruce beetle, which has become the biggest scourge of Colorado forests.

In 2018, mountain pine beetles hit just 500 acres of new forest, the lowest acreage in two decades, according to the CSFS annual report. The spruce beetle consumed 59,000 new acres of forest the same year, and was the most widespread and destructive forest pest in the state for the seventh consecutive year, with some 178,000 acres currently infested.

In his aerial surveys, West said, he has watched as the spruce beetle outbreak in southwestern Colorado moved over South Park and north into Rocky Mountain National Park.

“Engelmann spruce has been around for a long time,” Davis said. “It’s one of the most ancient species and most widely distributed.” The oldest known Engelmann spruce in Colorado was nearly a thousand years old. The trees’ days, however, could be numbered.

Since 2000, 1.8 million acres — 40% of Colorado’s spruce forests — have been infested by the beetle, an area larger than the states of Delaware and Rhode Island combined.

Like pine trees and pine beetles, the spruce and the spruce beetle have coevolved with periodic outbreaks of the insect. But the scale and duration of the current outbreak is new, and Colorado’s forests are particularly vulnerable. 

“We’ve periodically experienced these large outbreaks and 30% or 40% of the trees will survive,” Davis said. “At lower elevations, the species regenerates.

“In Colorado the problem is a little more complex,” he added. “The species live in high-elevation bands.” 

In this terrain, there may be too little room to maneuver, and they could be out-competed by fir and lodgepole pine, which the beetle cannot attack. “Think of it as them being pushed off the mountain,” Davis said.

A hiker crosses an area of fallen trees on the Encampment River trail on the in the Mount Zirkel Wilderness in Routt National Forest on the Wyoming-Colorado border. Lodgepole pine trees killed by pine beetles are creating havoc on many trails in Colorado. (Dean Krakel, Special to The Colorado Sun)

Two years of data, which included collecting 70,000 beetles, showed warmer winters lead to the beetles taking wing earlier, and warmer summers keep them flying longer.

The data also found the beetles growing bigger, although that effect diminished with elevation, and that there were more females, which could lead to a more eruptive population.

Using climate models, the researchers tried to predict future trends. “There is no best-case scenario,” Davis said. Even a 1.8-degree Fahrenheit increase would lead to higher rates of beetle reproduction.

So much is at stake. And it’s not about the view.

There is much more at stake in the fate of the high-country forests than just a majestic view. The snowpack that falls in the woods, and is essential to nourishing the forest, and it is also the main source of drinking water for the state.

“Every person in Colorado gets a touch of the forest ecosystem every day when they open up the tap,” West said. But thinner forests would lead the dwindling snowpack to run off more quickly.

Even without the spruce beetle, the high-elevation forests are under threat. In a study of Colorado Front Range forests between 9,500 feet and 11,150 feet, researchers found a decrease in new spruce and fir as a result of declining snowpack and rising summer temperatures.

Above-average snowpack was found  to be a key in the establishment of new Engelmann spruce and subalpine fir, according to one of Veblen’s studies. Conversely, declining snowpack along with cooler, wetter summers was related to a decrease in the number of fir and spruce establishment events from 1975. 

A study of high-elevation areas in Rocky Mountain National Park warned that these ecosystems were “at higher risk of species redistribution as they are more insular and experience more rapid changes than environments at lower elevations.”

In some places, climate change is pushing forests higher or farther. In Alaska’s Noatak National Preserve, boreal forests have moved as much as 300 feet north onto what was formerly treeless tundra.

In Yosemite National Park researchers have found whitebark and lodgepole pines pushing into montane meadows as high at 10,000 feet.

“We are seeing these biome shifts all over the world,” said Patrick Gonzalez, a forest ecologist at the University of California, Berkeley. “It is a global phenomenon.”

In his work in Africa, Gonzalez has documented forestland in the Sahel, the region immediately south of the Sahara, shifting more than 20 miles south toward the wetter equatorial region over 50 years, with one out of three species being lost along the way.

Colorado high-country forests, however, have no place to go. “The tundra is too rocky,” Veblen said. “The lower limit may be moving up slope, but the upper end is not moving. So we are having a contraction of the forest. It is subtle, but it is happening.”

The forager sees changes, too

Erica Davis had heard that the porcini mushroom hunting was good on Monarch Pass, but when she went there last year, she faced a grim vista. “There were a lot of standing dead spruce and patches of trees with yellowing needles. It seemed ghostly.”

Erica Davis forages for wild edible plants, like carduus nutans, or nodding thistle, that she found along the Brookside trail in the Pike National Forest near Bailey. (Mike Sweeney, Special to The Colorado Sun)

Davis, 44, is no scientist, but she spends a lot of time in the woods foraging for wild foods. Under the moniker “Wild Food Girl,” Davis blogs about her hunts in the Colorado woods for greens, mustards, cow parsnip, wild onions, a suite of berries — including whortleberry — and of course, mushrooms.

On a recent walk through the open and dry conifer woods of the Pike National Forest at about 7,700 feet, just the kind of place potentially at risk of vanishing in a fire, Davis pointed out a wild spinach called goosefoot, and yellow stonecrop, a native succulent.

The National Forest districts have incidental use policies permitting the taking of a small amount of plants — say, enough for a meal while backpacking — and some issue permits for mushroom collecting for personal use.

Davis did stop to harvest a nonnative, invasive thistle, cutting away the leaves and barbs, revealing the stalk to be something akin to a cucumber.

Erica Davis pinches a sprig of yellow stonecrop near the base of a ponderosa pine. (Mike Sweeney, Special to The Colorado Sun)

For all her time in the woods, she isn’t sure what’s afoot, and the scientists studying those forests caution that there is still much they don’t know.

Studies have been patchy and the West is so big that what’s happening in Southern Colorado may be different than what’s happening in northern Idaho, Gonzales and Veblen both caution.

Still, after her day on Monarch Pass, Davis keeps a watchful eye. “I am always looking for bore holes of beetles in trees,” she said. “Sometimes I’ll find a plant at a higher elevation than the guidebook has it, and I’m not sure what it means. I have the worry that these forests that I love will look like Monarch Pass.”


UPDATED: This story was updated Aug. 1, 2019, at 12:56 p.m. to clarify the role fire plays in the life cycle of ponderosa and lodgepole pines.