More than 1.5 million acres of Colorado forest are impacted by beetle infestation. Photo: Marc Piscotty
Fires are writing smoky warnings across the Rocky Mountain sky. Dying, rust-hued trees cover thousands of acres in Colorado. The hallmark forests of the Rocky Mountains are in ecological distress. While some changes reflect natural cycles, other worrisome trends stem from human-caused disruptions.
Against this backdrop, University of Denver researchers are conducting studies that may help political leaders and the public better understand and manage the region’s native conifer forests.
Geography Assistant Professor J. Michael Daniels wants to know how deadfall and other debris left by mountain pine beetle epidemics affect stream channels and surface hydrology.
Hillary Hamann, a geography lecturer, focuses on how forest fires change the chemistry of ash and soils — information that could give towns a heads-up if trouble is flowing toward their drinking water supplies.
Biology Professor Robert “Buck” Sanford searches for clues to help government agencies decide where and whether to use fire as a tool to make forests healthy again.
Meet the beetles
Obviously, an individual tree has a life cycle, but so do entire forests. It’s natural — particularly in Colorado — for big stretches of woodlands to take root, thrive and die.
Every few decades, a native insect called the mountain pine beetle erupts in enormous numbers in Colorado’s conifers. The bugs — each smaller than a human fingernail — feast underneath a tree’s bark and deprive the pine of nutrients. The beetles especially flourish in the aftermath of intense droughts, like the one that gripped most of Colorado from 1996 until last year. (In the Clear Creek drainage west of Denver, 2002 was the driest year on record since Isaac Newton was alive, according to the National Oceanic and Atmospheric Administration.)
Colorado’s most recent, ongoing beetle infestation has spread amazingly fast: Of the 1.5 million acres of Colorado forest impacted so far, some 500,000 acres were infested just in 2007, the U.S. Forest Service estimates. Residents of mountain towns like Vail and Winter Park have complained to elected leaders that the dead and dying trees make communities vulnerable to fires. And, rust-brown forests look bad and don’t draw tourists.
Although the mountain pine beetle outbreak has primarily affected lodgepole pines, the insect also attacks other species, including the ponderosa pine, which dominates mid-elevation forests along the Front Range and parts of the Colorado Plateau. Most foresters expect the beetles to spread beyond the lodgepole stands in the middle of the state, putting forests close to metro Denver at risk.
Climate change may play a role in the outbreaks. Usually, pine beetles lay eggs just once in a season before cold weather arrives, Hamann explains. But climate change means longer warm seasons in the high country, which may enable the bugs to lay two sets of eggs per year, letting them spread farther, faster.
Beetle-killed trees will elevate fire hazards for the next few years until the dead trees drop their needles. Once the volatile needles are gone, the snags will be less flame-prone, Sanford and Hamann say.
Still, Daniels says, the widespread beetle infestation may cause important long-term changes. The standing dead trees eventually will fall, and logs and debris could clog or otherwise alter nearby streams — potentially affecting fish, wildlife and human communities.
Daniels studies the Fraser Experimental Forest in Grand County, where records have been kept on the forest’s small streams for many years. Daniels wants to compare the baseline shape of the area’s small creeks with how the waterways look after a beetle infestation, including how much sediment and woody debris winds up in stream channels.
That’s important, as national forests provide 75 percent of Colorado’s drinking water, including that used by Denver and its suburbs.
“When most people think of watersheds they think of the big streams like the South Platte or Colorado River,” Daniels says. Yet small streams are important, too. Individually they may be just a fraction of the total flow, but there are so many of them that, taken together, they account for a sizeable part of the entire watershed.
“What happens at the headwaters really does affect what happens downstream,” he explains.
Fire power
Many problems now besieging Rocky Mountain forests — devastating fires and even some of the beetle-kill — can be attributed to human disruption of normal fire cycles.
The most common native types of Colorado forests naturally burn every few decades, and the fires are generally moderate in size. In the ponderosa stands that cover the Front Range foothills, for example, flames should creep along the ground, clearing out underbrush and sick trees but leaving most healthy pines alive.
But during the 20th century, federal policies required all forest fires, even small beneficial blazes, to be extinguished. So today forests harbor an unnatural build-up of potential wildfire fuel, including dense underbrush and sick, scraggly trees — ripe conditions for huge conflagrations. Now when wildfires erupt in Rocky Mountain forests, they burn with unusual speed, intensity and size and kill even mature trees that would survive a moderate blaze.
“Fire-suppression is one of the most dramatic changes humans have made on the landscape,” says Hamann.
She is studying what happens to forests when fires release certain chemicals that have built up in the trees — not only natural nutrients but also the human-caused pollutants that conifer forests capture. Pine needles, for example, sequester mercury.
A particular concern is nitrogen pollution from cars and power plants. In the right amount, nitrogen can help plants grow, but if there’s too much, it can kill trees.
“We’re over-fertilizing our forests,” Hamann says.
For a decade, researchers have known that air pollution deposits nitrogen into high-altitude lakes, but Hamann hypothesizes that the nitrogen enters the ecosystems in another way. During or after a forest fire, large nitrogen deposits contained in trees may wind up in the ash, which washes into nearby streams and lakes.
As air pollution deposits more nitrogen into regional forests, and as those forests burn in big, hot fires, then unacceptably large amounts of nitrogen could enter municipal watersheds, she says.
Current nitrogen levels in Colorado’s forests don’t endanger human health, she notes, but public health laws limit how much nitrogen is allowed in drinking water. Hamann hopes her research can help small mountain communities prepare their municipal water treatment plants for increased levels of nitrogen and other contaminants that may follow big forest fires.
A big wild fire is an amazing phenomenon, but even moderate-sized fires leave evidence of their passing for centuries or longer. Sanford has traced evidence of forest fires around the globe and has found that many factors affect how forests re-grow after a natural or human-caused catastrophe.
Even tropical rain forests may re-generate if — and this point is important, Sanford emphasizes — people who set fire to the forests don’t disturb too much of the forests or ignite blazes in the wrong places.
Dating back thousands of years, native peoples in Central and South America burned patches of jungle to grow crops. But they burned only small areas and would put pinches of dirt in their mouths before deciding where to set the fires. If the soil tasted acidic, the natives knew not to set a blaze there because the soil wasn’t fertile and trees wouldn’t grow back, Sanford explains.
North America’s indigenous peoples also set fires, he adds.
“Their survival depended on picking the right place,” Sanford says. “It was very different from what’s happening today.”
Today, deliberately set but controlled fires are among the best tools for killing pine beetles and getting rid of the dangerous overload of potential wildfire fuel. But the tool must be carefully used, and modern land managers should heed lessons from indigenous populations.
Specifically, Sanford warns, there could be significant amounts of charcoal in the region’s forest floors and soils — material left behind both by natural fires and by blazes set by humans over the past 8,000 years. The quantities vary greatly by location. If controlled burns are ignited in areas with large amounts of charcoal in the forest’s soils, the carbon emissions could be disturbingly large.
“It has huge implications for forest management,” Sanford says. “In some areas we can create carbon sequestration in the forests. But if we burn other areas, we could get a large release of carbon dioxide.”
And carbon is the final wild card in forecasting the forests’ future.
Sanford initially was skeptical of climate change theories but became convinced as real-world observations increasingly matched the predictions. There’s already evidence that changes are occurring in Colorado, with earlier snowmelts and some tree species growing at higher and higher altitudes.
In fact, the International Panel of Climate Change predicts that by mid-century Colorado will be hotter — perfect conditions for pine beetle outbreaks — and more vulnerable to drought and forest fires — the very outcomes Rocky Mountain residents fear.
If that’s correct, then the changes affecting Rocky Mountain forests may be more than just part of a normal cycle. The question, which science must answer, is if the troubles now besieging the forests are harbingers of permanent transformations.
Read more about the human impacts of forest fires.