Forests are not forever
Report describes brisk changes in aspen, lodgepole, and other species as temperatures continue to rise
by Allen Best
Dramatic changes have occurred during the last 15 years in forests of the Rocky Mountains: huge fires, massive die-offs of aspen and piñon pine, plus the historically unprecedented bark beetle epidemic in stands of lodgepole pine.
The beetles, having mostly run out of host trees, have left enough standing dead that the U.S. Forest Service estimates 100,000 trees fall daily in northern Colorado and southern Wyoming.
Buckle up, because the changes are almost certain to accelerate. That’s the bottom line of a new report called “Rocky Mountain Forests at Risk: Confronting Climate-driven Impacts from Insects, Wildfires, Heat, and Drought.”
The report synthesizes dozens of existing studies by scientists for the U.S. Forest Service and other agencies.
Taken together, the studies paint a picture of great changes in coming decades because of increased heat and a tendency toward greater dryness.
“If climate change continues unchecked, scientists expect the region to become even hotter and drier—and the impacts on its forests even more severe,” say the report, which was issued jointly by the Union of Concerned Scientists and Rocky Mountain Climate Organization.
Even with relatively low carbon emissions going forward, temperatures in the six Rocky Mountain States could rise to about 3°F above 1971-2000 levels by mid-century and remain that high into the last decades of the century, the study points out.
Of course, if emissions continue unchecked, average temperatures could rise by about 6°F by mid-century—and by 10°F in the last decades of the century.
The result? More bark beetle epidemics and larger, intense, and more frequent fires.
What is most unsettling are the modeling projections that suggest the changing climate will result in several iconic species—whitebark pine, aspen, and piñon pine—being eliminated from much of their current ranges.
Existing conifers—lodgepole pine, Engelmann spruce, ponderosa pine, and Douglas fir – could have dramatically reduced land suitable to them. Mountain valleys and slopes could look very different.
Stephen Saunders, director of the Rocky Mountain Climate Organization and a co-author of the report, says he has followed the studies closely over the years but nonetheless was surprised with the one key finding.
“I was shocked to discover the extent to which U.S. Forest Service scientists project future declines in tree stands in the Rocky Mountains, especially if emissions continue at a high level,” says Saunders. “That was the only thing that was really new to me, but it was beyond shocking.”
The most riveting table in the report describes those declines in terms of lands suitable for various species of trees in the recent past as compared to 2060. Acreage suitable for lodgepole pine declines 90 percent, ponderosa pine 80 percent, Engelmann spruce 66 percent and Douglas fir 58 percent.
A footnote points out that this is based on the Intergovernmental Panel on Climate Change’s medium-high A2 scenario, in which carbon dioxide emissions grow from 400 parts per million now to near 550 ppm by 2060.
Models have a great deal of uncertainty, but they do “strongly suggest the direction of changes and potentially suggest the size of changes,” says Saunders, an Interior Department undersecretary for water in the Clinton administration.
“If we want to preserve these forests in any way that we recognize them, we have to sharply reduce emissions.”
The study singled out three species of trees in the six states—Idaho and Montana, Wyoming and Utah, and Colorado and New Mexico—for special attention.
Whitebark pine, found in the northern Rockies, have already been hammered by an epidemic of mountain pine beetles, who have survived because of milder winters and warmer summers. A record cold snap in early October 2009—sending temperatures to 20 below in the Greater Yellowstone Ecosystem—slowed but did not halt the epidemic. The species is likely to remain, but not to the extent that exists even now.
The story of aspen
Aspen trees have also died, in some cases giant stands of them. Tree rings and other evidence point to major droughts and die-offs of the past, but those that were noticed beginning in 2002 were different: broader and without regeneration. The U.S. Forest Service found aspen mortality in Arizona of up to 95 percent. In places, agency personnel found that mature aspens had not resulted in a surge of new shoots, which is the common method for propagating aspen.
In Colorado, according to one 2008 study, the affected aspen stands were usually in locations most vulnerable to high temperatures, such as lower elevations and south-facing and west-facing slopes.
What will happen with aspen as temperatures rise? Despite the stresses, “widespread, long-term aspen decline is not a foregone conclusion,” the report says. Forest disturbances usually favor aspens, which take advantage of access to sunlight to colonize burn areas and those in which beetles have killed trees.
But the links identified between the recent aspen decline and the stress of drought, “suggest that mortality in forests that are already nearly as dry as aspens can tolerate will be substantial as the climate gets hotter and drier.”
This could happen soon, by 2030. Researchers for the Forest Service at their laboratory in Moscow, Idaho, modeled suitable range for aspens under two scenarios of heat-trapping emissions. One, called B1, sees only a modest increase in greenhouse gas emissions going forward. But the previously A2 scenario sees a sharp increase in gases.
In both scenarios, the Forest Service models project significant changes in the areas climatically suitable for aspens by 2030. By 2060, the changes become more dramatic under the high-emissions scenario: a 45 percent loss in Colorado, 85 percent in Idaho, 71 percent in New Mexico and 72 percent loss in Utah. A gain, there are great uncertainties in the modeling.
The third species, piñon pine, has had mass die-off in the drought of the last decade, on land nearly half the size of New Hampshire. The report notes that while the Southwest has always known drought, the drought of the 1950s was even drier in the southwest but killed far fewer piñons.
“Exceptional heat during the more recent drought made the critical difference” as up to 90 percent of piñons died at some sites in Mesa Verde National Park, near Los Alamos, N.M., and near Flagstaff, Ariz.