Rocky Mountain forests won’t look anything like they do now if the region’s temperature and water levels continue their path toward hotter and drier.
Suitable habitat for lodgepole pine could drop 90 percent by 2060, assuming medium-high levels of heat-trapping emissions, according to a report by the Union of Concerned Scientists released on Wednesday. Ponderosa pine habitat would shrink by 80 percent, while Englemann spruce would lose two-thirds of its territory. Douglas fir would decline by 58 percent.
The pattern shows up on U.S. Forest Service maps of current and predicted forest ranges for common Rocky Mountain tree species, according to Stephen Saunders, a researcher with the Rocky Mountain Climate Organization who was co-lead author of the report. They pick the middle of three possible climate-change scenarios that assume average temperatures would rise 1.6 to 2 degrees Fahrenheit.
“We don’t want to make too much of the precise numbers, but the point is if we keep changing the climate the way we are, we’re fundamentally changing the nature and composition of the Rocky Mountain forests,” Saunders said. “Think of much-reduced forest cover. We have a whole lot at stake, if we continue to change the climate at the pace we’re doing so now.”
The change would also affect aspen groves, eliminating possibly 71 percent of the stands in Montana and 41 percent of those in Colorado, Saunders said.
The problem is not just increasing temperatures, according to University of Montana entomology professor Diana Six, who also participated in the report. Heat changes the runoff patterns of winter snowmelt and improves conditions for pests like mountain pine bark beetle.
“What we’re seeing is unprecedented,” Six said. “We’ve had bark beetle outbreaks in the past, but these are different, and they’re directly attributed to increases in temperature and drought. The current outbreak is 10 times bigger than any outbreak recorded in the past. That practically screams something is really different and very, very wrong.”
Six said the most convincing evidence was the beetles’ appearance in places previously too cold for it to survive, such as the Yukon, northern Alberta and Saskatchewan. Beetle infestations are also taking a toll on whitebark pine, whose high-altitude, subalpine habitat also used to be too cold to support the bugs.
Princeton University researcher William Anderegg said his studies of aspen trees show one-fifth of Colorado’s groves have already been damaged by drought.
“The early 2000s drought wasn’t as severe as the Dust Bowl or the 1950s,” Anderegg said. “More rain fell in this one. But what was critical was the temperature. The early 2000s drought was several degrees hotter, less snow fell, it melted sooner and the summer was blistering hot. Hot atmosphere sucks the water out of leaves.”
Jason Funk, the senior climate scientist for Union of Concerned Scientists and co-lead author of the report, said helping forests hold their ground wouldn’t be easy.
“In the Rockies, people haven’t done the forests any favors by suppressing wildfire for last 100 years,” Funk said. “They’re overly dense. Many forests are in wildfire deficit, with too many trees on landscape. One way we can work to mitigate changes in the future is to take a more active management stance. We acknowledge we’ll see a ramp-up of wildfire in future. So it’s better to prepare forests for that to happen before, rather than after by trying to put the fires out.”