Destructive insects in unprecedented
numbers are finding Alaskan
forests to be a more comfortable
home, year by year, and climate
change could well be the cause.
Warmer winters kill fewer insects, and longer, warmer summers
let insects complete a life cycle and reproduce in one year instead
Warmer winters can also damage trees and make them less able
to fend off insect attacks by changing the nature of snow. Instead
of light, fluffy snow formed at extreme cold temperatures,
warm winters produce wet, heavy snow more likely to break the
limbs of spruce and other trees.
Since 1980, aerial surveys indicate spruce bark beetles have
killed mature white spruce trees on 4.4 million acres, including
more than a million acres on the Kenai Peninsula.
After two successive warm winters, Alaska had some really warm
summers, and the beetle numbers increased dramatically. At the
same time, reduced rainfall meant that the resistance of the
trees was down everywhere. One major outbreak marched
across the landscape killing most of the mature trees in a whole
region of the state.
Spruce bark beetles bore into trunks and feed on the live cambium
layer, a thin strip of tissue under the bark. Trees resist beetles
with pitch, the substance from which turpentine is distilled.
If the tree is healthy, beetles try to avoid it; too much pitch pressure
makes boring difficult. When a tree is injured, bark beetles
can detect the change in pitch composition and home in for the
kill. When it comes to climate, warmer is not necessarily better.