Rod Garbutt clearly remembers the day in 1994 when he first saw evidence of a spruce bark beetle outbreak in the southwest Yukon. He was in a plane flying south over Kluane National Park when he looked across Kathleen Lake and saw a red glow on the horizon.

Garbutt, a forest ecologist with the Pacific Forestry Centre, knew that the red colour meant dead and dying spruce trees. He was on his way to map a spruce beetle outbreak on the Haines Road near the Alaskan border, and suspected that beetles were also killing the trees in Kluane.

When he had a chance to look at the trees themselves, he saw the bore holes and pitch tubes that are the telltale signs of a spruce bark beetle invasion. He estimates that the infestation had already spread over 32,000 hectares at that point.

He did not know then that this outbreak would eventually become the largest and most intense infestation of spruce bark beetles ever to wipe out trees in Canada. By 1999, the beetles had killed almost all of the mature white spruce over some 200,000 hectares in the Alsek River corridor in Kluane National Park and in the Shakwak Valley north of Haines Junction.

Spruce bark beetles are not new arrivals in this part of the world. While the Haines Road was being built in the 1940s, an outbreak caused quite a bit of damage in the bordering forest. In the 1920s, beetles infested more than 200,000 acres near McCarthy, Alaska.

The beetles are always holding their own somewhere within any given forest, surviving in trees that are weakened by stress. Garbutt thinks the resident beetles began spreading because drought has stressed trees in the southwest Yukon for most of the last decade.

But Garbutt says that this outbreak has not been a run-of-the-mill epidemic. "It is extremely unusual to have the coincident of conditions that you have here. It is unique in my experience."

For one it has lasted much longer than normal. In British Columbia, outbreaks typically last three or four years and then decline quickly. Garbutt estimates that this outbreak only started to slow down last year.

A series of mild winters and springs provided good breeding conditions for the beetles, allowing them to multiply rapidly. Also, for an insect with one preferred main course on its menu, the forests of the southwest Yukon provide a true banquet for the beetles.

Not only is white spruce the only conifer in this part of the world, the stands are almost all of the same age. That's because as recently as 1852, the waters of Glacial Lake Alsek covered more than 100 kilometres of valley bottom in this area. When the lake drained, it sent a wall of water down the Alsek River and up onto the surrounding slopes, flooding the trees growing there.

Wildfires help create a more mixed forest, encouraging the growth of other species such as lodgepole pine, which is more resistant to beetle attacks than spruce. But lightning strikes are infrequent in the area, as are fires, and no lodgepole pine breaks up this spruce forest.

When beetles invade trees, they bore into them, and lay their eggs underneath the bark. After larvae hatch from the eggs, they feed on the phloem, the tissue just beneath the bark that transports nutrients from leaves to roots.

A healthy tree will fight back by producing large amounts of pitch and flooding the beetles and their eggs out of the tree. But trees weakened by drought or other stresses do not have the energy to fight, and will eventually die as the larvae eat away the phloem.

Garbutt says he cannot predict what the forests in the Kluane area will look like in the future. A number of test plots have been set up that have not been severely infested, and spruce are starting to grow back in most of them.

If spruce also manages to reseed itself in the hard-hit areas, in another 100 years the forest might look much as it does today -- a blanket of spruce across the landscape. But if more aggressive species like willow or dwarf birch take over, it will be harder for spruce to take hold.

"No one knows the answer as to what will happen next. There could be many different scenarios," he says.

Rod Garbutt can be contacted at rgarbutt@pfc.forestry.ca.