But this is a forest under siege. Among the green foliage of the healthy spruce are the orange-red needles of the sick and the dead, victims of a beetle infestation closely related to one that has already laid waste to millions of acres (hectares) of pine forest in North America.

"The gravity of the situation is very real," said Rolf Skar, a forest campaigner with Greenpeace.

The plague has cost billions of dollars in lost timber and land values and may thwart efforts to combat climate change, as forests are major storing houses of carbon, the main greenhouse gas blamed for global warming.

The beetle outbreak, which has taken a lesser, but mounting, toll on spruce trees, could make it that much tougher to meet the ambitious target to reduce U.S. carbon emissions by 17 percent of 2005 levels by 2020 and 83 percent by 2050.

That is laid out in a climate bill that narrowly passed in the U.S. House of Representatives and waits Senate debate.

Many researchers have also linked the infestation in the U.S. and Canadian West to climate change, notably a dearth of winters cold enough to kill the voracious little bugs.

"Pine beetle infestations are cyclical in nature and have been occurring for thousands of years but what is making things worse now is the effects of global warming," said Skar.

"If you don't have the real cold extremes to kill off the larvae under the bark you are going to have extreme infestation events," he said.

In the Medicine Bow National Forest, scientists are getting a first-hand look at the carbon implications.

The forest is home to the U.S. Forest Service's Glacier Lakes Ecosystem Experiments site in a tower with gadgets that, among other things, examine the "carbon flux" of the forest.

The site was established a decade ago, before the spruce beetle infestation, and gives scientists a unique chance to measure the changes to carbon storage wrought by the insects.

"We are getting readings here every half hour," said Colorado-based U.S. Forest Service scientist Mike Ryan, shouting above the wind as he pointed to an instrument that measures carbon. This gas analyzer resembles a small space capsule on the end of horizontal a metal pole.

In the terminology of trees and carbon, a healthy forest is a net "sink," with trees storing carbon as they grow. When they die and rot they "emit" carbon back into the atmosphere, and so a dead or dying forest becomes a "net source" of greenhouse gas, meaning it emits more carbon dioxide than it stores.

Ryan said the net carbon storage in this patch of woods is about half of what it was three or four years ago. In another three or four years, he believes it will become a net source.

This scenario is being replayed across the West. In Colorado, aerial surveys show that from 1996 to 2008 Colorado lost almost 2.5 million acres (1 million hectares) of pine forest to the beetle outbreak, Wyoming 677,000 acres (274,000 hectares) and South Dakota 354,000 acres (143,000 hectares).

Over the same period of time, the spruce beetle, which has also ravaged forests as far north as Alaska, took out 374,000 acres of spruce trees in Colorado and 340,000 in Wyoming.

That cumulative total of over 6 million acres (2.5 million hectares) is an area larger than Israel or South Africa's Kruger National Park.

Farther north in Canada, the pine beetle has attacked trees over an area of about 39 million acres (14.5 million hectares) in British Columbia since the the 1990s.

The sheer scale of the damage can be seen northwest of Denver in Colorado's Yampa Valley. Vast tracts of formerly evergreen forest now have huge splashes of orange running through them.

According to the U.N. Food and Agriculture Organization, a third of the United States' land area is covered in forest but it is only expanding at a rate of about 0.1 percent per year.

Under "cap and trade" provisions in the U.S. climate bill, additional forest growth may be encouraged through a market mechanism that will allow reforestation efforts by landowners and other groups to be counted as "carbon offsets."

Such projects could generate cash through "carbon credits" paid by polluters who want to exceed their own emissions caps.

A forest can recover, but that can take decades. "Most forests will recover the carbon they lose but if the next 50 to 100 years is important we may not have that much time. It's setting back carbon storage efforts," said Ryan.

Forest growth in the United States currently sucks up about 12 percent of the country's greenhouse gas emissions. "That's a big number. To get another 10 percent you would have to convert a third of U.S. agriculture land to forest," said Ryan.

The outbreak has other consequences. It is creating huge fire hazards as it leaves mountains of combustible wood in its wake. In a worrying trend, it also has spread from lodgepole pine to ponderosa pine.

There are expenses for landowners as well. On his ranch in northern Colorado, mountain realtor Bill McClelland points to a dying tree and says: "A week ago that tree was green. I've lost another one."

In May, he had to cut 476 pines on his property and then have them ground into wood chips -- an expensive operation that is one of the few ways to contain the outbreak. He reckons an infestation will generally shave about 20 percent of the value off a private wood lot or ranch.

Past beetle outbreaks have been stopped by very cold winters but recent winters have not been cold enough.

Another factor scientists attribute to the outbreak is past forest clearance and fires that saw large areas cleared.

Often when this happens, the forest that regrows in its place will have huge patches of trees the same age and this makes them susceptible to a collective attack when they mature at the same time into the older trees that the bugs favor.

The beetles may collectively wreak havoc by nesting and feeding in the trees but they look harmless enough as individuals, not least because they are so tiny.

At Medicine Bow, Ryan points to a few writhing in a glass jar that have been trapped on the trunk of a spruce tree.

"Until we get a big cold spell they are going to go on until they have nothing to eat," he said.