How rising temperatures will change forests in the Rocky Mountains
And what remains uncertain
by Allen Best
Runoff has begun, the streams and creeks soon to roar as the deep snowpacks of this year’s full-throated winter melt in Jackson Hole, Ketchum, and other mountain towns of the West.
But runoff this year, if it hews to trends of recent decades, will peak earlier, leaving a longer and probably hotter summer. Temperatures have been rising globally, with the three hottest years on record occurring in the last three years.
This warmth is driving changes in trees, and by enabling more rapid infestations by bark beetles and other pathogens. During the longer dry season, this also makes forests more vulnerable to fire, according to experts at a recent forum in Colorado sponsored by a group called Carpe Diem West.
In the last 20 years, Denver Water and other municipal water providers along Colorado’s urban Front Range corridor, have become more aware of their vulnerability to fire. The hot-burning 1996 Buffalo Creek Fire in the South Platte River drainage upstream of Denver resulted in the granitic soils fusing together, making them unable to absorb moisture from storms. The result has been a dramatic increase in silting of Denver Water’s Cheesman Reservoir. To dredge it has cost the agency $28 million, but without total success.
The general trend described by Tony Cheng, director of the Colorado Forest Restoration Institute, is of a longer fire season, now beginning some years in March. Fires have been burning hotter. Even now, 15 years after the Hayman Fire of 2002, the largest fire in Colorado’s recorded history, which also occurred in the foothills southwest of Denver, lands have been slow to regenerate.
“This is not what we might have expected under historic fire regimes, indicating a likely loss of forest resilience,” he said. Some research suggests it will take the forest 900 years to regain its resilience, he added.
The event at which Cheng spoke featured both water providers and environmental organizations, all focused on the idea of creating partnerships for forest management. Most if not all concur with the idea that forests in key areas need to be thinned, to reduce risk of fire impacts to streams and rivers, after a century or more of fire suppression.
Cheng warned against expecting too much. “We can’t necessarily stop these types of fires, but we can influence the impacts,” he said.
A group of water providers and land management agencies called the Front Range Roundtable was formed to serve as a focal point for coalescing efforts to improve what is often called forest health. In other words, in most cases they want to thin forests.
In one such project, Denver Water in 2010 put in $16.5 million toward forest management, with that amount matched by the U.S. Forest Service and other state and federal agencies. Recently it renewed that commitment for another five-year run, with work planned for both Summit and Grand counties at the headwaters of the Colorado River. If located across the Continental Divide, Denver draws water from both places.
“You can’t treat everywhere,” said Cheng. “You have to be realistic about what you are trying to do.”
He also warned against expecting that fire can be stopped altogether. “There is no published paper that says that treatment will stop fire, you can’t stop fire. You can only modify fire behavior and the effects up to a certain point.”
If fire is natural on the landscape, and climate change, too, most of the climate change now underway is unnatural, the result of greenhouse gases and other human activities.
Jeff Lukas, a scientist from the Western Water Assessment in Boulder, Colo., outlined the evidence for rapid change: the last three years have been the warmest on record and, in Western states, the temperatures have increased 2 degrees Fahrenheit in the last three decades. Heat waves have been longer, and the same is true of frost-free season—no doubt a welcome respite in many mountain towns.
Snowmelt has come earlier. “Peak runoff is earlier virtually everywhere, and in particular in the more coastal regimes,” he said, referring to both the Sierra Nevada and to the Cascade Range.
But annual streamflow is declining in most areas, as more precipitation is lost to the atmosphere as a result of both evaporation and transpiration.
“The atmosphere giveth, and the atmosphere taketh away,” explained Lukas. This thirsty atmosphere is called the vapor-pressure deficit. What’s important is that the thirst that causes soil to dry out and streams to evaporate increases exponentially with temperature. Generally, the atmospheric thirst is projected to increase 10 to 25 percent by mid-century.
The upshot of this progression of warmth is more rain in place of snow, reduced late-summer flows, and more severe droughts. Droughts, in turn, stress trees leading to more vigorous insect infestations and disease outbreaks.
There’s also this: more frequent and destructive wildfires. And finally: over time, changes in the trees and plants you will see outside your mountain town window.
Lukas emphasized uncertainties. Climate models use different approaches, and although many concur in key predictions, there is no universal certainty. “Don’t let them give you just one number,” he said. “There should be a range.”
For example, the models concur about increased warming. There is high confidence of this. But there is a range of predictions about how much: 2.5 to 6.5 degrees Fahrenheit by 2050. Part of this depends upon to what extent the emissions of greenhouse gases can be slowed or even withdrawn from the atmosphere.
Physics of clouds continue to confound scientists. There is no clear understanding of how much increased water vapor—itself a greenhouse gas—in the atmosphere will in turn cause increased heating.
For that matter, it’s unclear whether warming temperatures will cause more or less precipitation. Most models suggest less moisture in places like Mammoth and Telluride, but with more precipitation in Ketchum, and almost certainly so in Canada. But again, this is likely to come increasingly as rain, not snow.
Climate models show warming as inevitable. Absent the moderating effects of a nearby ocean, Colorado and other states in the continent’s interior are expected to see temperatures rising more rapidly. It might already be happening, but the effect is not as obvious we might be expected, says Lukas.
Higher-elevation mountains are expected to warm even more as the snow gives way to bare ground for more of the year, which absorbs solar radiation instead of reflecting it, as snow does. This more rapid elevation gradient warming has been detected in the Himalayas, but there are too few high-quality temperature mountains in the mountains of Colorado to confirm it here.
Heat has a muscular effect among these changes. For example, If precipitation increases 100 percent but evaporation and transpiration increase 50 to 90 percent, that means 50 percent less runoff.
If drought obviously stresses trees, scientists remain unclear about the exact mechanisms by which trees die.
Further, not all forests turned red by bark beetles can be blamed solely on drought and warmer temperatures. But they have played a factor in the 40 million acres in western North America affected since the late1990s.
Frequency and scale of fires in the West has increased, owing to earlier snowmelt and drier fuels. One new study concludes that roughly 50 percent of the total burned area in the West since 1984 is due to climate change—a figure that Lukas said he doubts. Also, it’s important to remember that even bigger fires occurred naturally before the 1800s, when Euro-American settlers arrived. Fire is a component of the landscape.
But warming will produce changes. Trees will want to shift upslope or, more slowly, to more northerly latitudes For example, the Engelmann spruce and subalpine fire forests found today in Colorado at 10,000 feet might be replaced by lodgepole pine, and lodgepole might be replaced by ponderosa.
At least in Colorado, the vast majority of moisture falls in the form of snow in the elevation band of 9,000 to 11,000 feet, mostly spruce-fir forests. “We need to keep our eye on the spruce-fir zone in terms of watershed impacts.” said Lukas. “That’s where most of our water comes from in the West.”
PowerPoint slides courtesy of Jeff Lukas and Tony Cheng.