Forest mortality declines across the U.S.

Pine beetles running out of food, spruce beetle infestation growing

Mountain pine beetle mortality is on the decline across the western U.S.
Spruce and fir mortality is on the increase in Colorado.

By Bob Berwyn

SUMMIT COUNTY — Tree mortality from insects and diseases has dropped dramatically in the past few years, mainly because mountain pine beetles are running out of food, according to a new report from the U.S. Forest Service.

But the next significant cycle of insect infestation has already reached epidemic proportions in the south-central Rockies, where spruce beetles are devastating stands of mature spruce trees. The spruce beetle outbreak has been especially intense in the San Juans, where the bugs have killed almost every single mature tree from the creek bottoms all the way up to high-elevation krummholz.

It will be interesting to see if the numbers go back up after this summer’s drought weakened trees across the region.

Tree mortality was reported across 6.4 million acres in 2011, down by nearly half from the 12-million acre peak in 2009, but still significantly higher than during the 1990s, when tree mortality stayed under 1 million acres per year between 1990 and 2001.

Acres of forests with dead trees due to the mountain pine beetle declined from 6.8 million acres in 2010 to 3.8 million acres in 2011 in western states, according to a report released by the U.S. Forest Service last week. Mountain pine beetles accounted for about 59 percent of the total damage, the agency said.

This marks the second straight year with reduced mortality rates after steady increases between 2006 and 2009. Although Forest Service surveyors attribute some of the reductions to fewer available lodgepole pines, ponderosa pine and high-elevation white bark pine are still at risk.

“Native insects and diseases run in cycles, and right now we are grateful the trend is downward,” said U.S. Forest Service Chief Tom Tidwell. “While the news is good, we are certain to continue to face challenges, such as the effects of climate change and the introduction of invasive species.”

Despite the decline, pine beetles still resulted in more than 3.8 million acres of mortality in 2011, with the biggest affected areas in Colorado, Montana, Idaho and Wyoming.

The most widespread pine beetles damage was in Montana, at nearly 1 million acres, with Colorado, Idaho and Wyoming all reporting between 700,000 and 800,ooo acres of pine beetle mortality in 2011.

In Colorado, mountain pine beetles continued to cause damage, with most of the mortality now reported east of the Continental Divide, including ornamental plantings in downtown Denver. The Forest Service said the insects are spreading readily into lower elevation ponderosa pine forests in Bouler and Larimer counties.

West of the Continental Divide, mortality continues to spread around Aspen and Vail. The bugs area also starting to attack limber pines and Rocky Mountain bristlecone pines.

Forest experts said drought-induced stress and wind-downed trees helped fuel the surge in spruce beetles in the region. Spruce beetle outbreaks in Colorado include the Grand Mesa, the Wet Mountains and especially the eastern San Juans, where the bugs have marched into the headwaters of the Rio Grande and continued into the southern portions of the Gunnison National Forest.

Nationwide, spruce beetles killed trees across 428,000 acres, while the fir engraver has caused tree deaths across 323,000 acres, mostly in California.

In the East, tree mortality due to insects and disease continue to remain low, with southern pine beetle-caused mortality at historically low levels. The southern pine beetle outbreak in New Jersey declined from 14,000 acres in 2010 to about 6,700 acres in 2011. However, that lower number of acres is still considered very high for New Jersey.

Invasive forest diseases and insects, such as the emerald ash borer and the Asian long-horned beetle remain a big threat to eastern forests.


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