For the last decade, spruce bark beetles have been attacking trees across a wide swath of the North, all the way from south central Alaska to the Kluane region in the Yukon.

The severity of the attack has caught experts by surprise, and left them wondering whether similar outbreaks have ever happened in the past. While it is true that beetles have been munching on trees for thousands of years, this infestation is the largest spruce bark beetle attack ever recorded in North America.

The outbreak has also lasted much longer than normal. Typically after a few years of attacking older mature trees, younger healthier trees manage to oust the beetles and the infestation dies out.

But this time around, the beetles have just continued to spread. They have killed trees on more than a million hectares of forest on the Kenai Peninsula and another 250,000 hectares in the Kluane area. Large expanses of forest in the Wrangell-St. Elias area are also infected with the insects.

Tree rings have been providing researchers with some answers and they are pointing squarely at a warming climate as the trigger for this outbreak, particularly the warmer drier summers which occurred during the 1990s.

Tree rings have also shown different histories for the outbreaks in Alaska's Kenai Peninsula and the Yukon's Kluane region. While beetles attacked forests on the Kenai Peninsula three times in the nineteenth century and two times in the twentieth, the infestation in the Kluane area appears to be an isolated twentieth century phenomenon.

Dr. Ed Berg, an ecologist at the Kenai National Wildlife Refuge, began taking cores from spruce trees in that region about a decade ago, and has found a close match between warmer than normal summers and the number of dead trees.

In the Kluane area Berg collaborated with Dr. David Henry, an ecologist with Kluane National Park and Reserve, on a tree ring study. In 2001, the researchers drilled cores from about 100 infected trees in each of four different sites in the Kluane area.

They chose productive stands located high enough that they would have been above the high water mark of Neoglacial Lake Alsek, the massive body of water that last drained sometime between 1857 and 1891.

Cores from three of the sites showed a steady rate of growth dating all the way back to the 1500s. "That gets you back to the Little Ice Age, and it is very doubtful that a large beetle outbreak could have occurred during this cold period," says Henry.

But rings from the fourth site, located near Klukshu, showed that a beetle attack had occurred there in the past. Between 1934 and 1942 many trees died, and tree rings from the surviving ones showed that a major growth spurt had taken place.

"Starting in 1934, the rings were bigger, double in size and they stayed double for several decades. We call that a significant release in growth," says Henry.

Henry and Berg think that a beetle attack is the most likely explanation for this phenomenon. Beetles typically go after the more mature trees first, opening up the forest canopy so that more light, moisture and nutrients reach the younger trees, triggering a growth surge.

"The younger trees resist the attack better than the more mature trees, so you see this abrupt release in growth," says Henry.

The tree rings are scrutinized under a dissecting microscope and measured to within a fraction of a millimeter. Henry points out another benefit from this meticulous work.

"It was good documentation of how slowly trees grow in the Kluane region. These were productive sites and a white spruce grows at an average rate of about one millimeter per year. That is pretty slow growth, so it is good to know how long it takes to replace a tree."

In Kluane about half of the dead trees are in the Shakwak Trench between Kluane Lake and the Tatshenshini River; the other half are in the Alsek River drainage.

Bets are off on whether the infestation could change the very nature of the forest in the Kluane region. Both the Canadian Forest Service and the Kluane Ecological Monitoring Project are keeping track of the changes, but Henry says that it is too early to tell what trees or shrubs will regrow in the area.

In the pure spruce forests of the southern Kenai the beetles have little left to eat as they have already killed about 95 percent of the mature trees on this part of the peninsula. In the Kluane region no one is sure whether the infestation will continue to spread, but Henry says that a warming climate could fuel a spread of the beetles.

"If the summers continue to be warmer and drier, then I think that this outbreak will continue. My personal view is that this is one example of a global climate warming and it should increase our understanding of the nature of that problem and motivate us to address it. The change in the climate is affecting the North in very serious ways."

A copy of the report can be found on the web at http://kenai.fws.gov/biology. Dr. David Henry can be contacted at david.henry@pc.gc.ca.