The main evergreen and aspen tree species making up Rocky Mountain forests are dwindling and likely will die out dramatically by 2060, according to a report by science advocacy groups.
It's not clear what will replace them.
The Union of Concerned Scientists and Rocky Mountain Climate Organization report, unveiled Wednesday, draws on U.S. Forest Service data documenting tree deaths and projecting future growth based on climate.
It found that aspens, already seen to be vulnerable to drought, will decrease by 45 percent in Colorado and 61 percent across a six-state region. It found that the amount of land suitable to sustain conifer species ? lodgepole pines, Douglas fir, Engelmann spruce, ponderosa pine ? will decrease by at least 50 percent.
The groups blame climate change driven by human emissions of heat-trapping greenhouse gases, which has hit forests with droughts, higher temperatures, intensifying fire and insect infestations.
"These aren't tiny, rare tree species on the brink. They are massive, widespread species, the characteristic species of Rocky Mountain forests," the union's chief climate scientist Jason Funk said.
Forest survival "really is up to us," Rocky Mountain Climate Organization president Stephen Saunders said. "The extent of changes depends, in very large part, on whether we reduce emissions."
Colorado natural resources assistant director Lisa Dale, acknowledging "dramatic changes in forest composition," cautioned that individual species dying "doesn't mean entire forests are dying."
Among the findings:
? The rate of trees dying from no obvious cause has doubled over the past few decades.
? If emissions of heat-trapping gases keep increasing, the land around the region that is suitable for lodgepole pines could decrease by 90 percent, for ponderosa pine by 80 percent, for Engelmann spruce by 66 percent and for Douglas fir by 58 percent.
? Bark beetle outbreaks over the past 15 years killed more trees than ever recorded over 100 years ? across 46 million acres.
? The average annual frequency of wildfires burning more than 1,000 acres increased by 73 percent between 1984 and 2011.
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