TAHOE NATIONAL FOREST “-If arid conditions continue this year, trees will likely begin dying in large numbers on the Tahoe National Forest, silviculturist Mark Brown said.

Despite the latest rain and snow, January has been among the warmest and driest on record, part of a recent dry trend in the northern Sierra noted by officials of the Nevada Irrigation District.

Regional warming has been pinpointed as the most probable cause of an increase in tree deaths in old growth forests throughout the West in recent decades, according to a new study by the U.S. Geological Survey.

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“Tree death rates have more than doubled over the last few decades in old-growth forests,” the USGS reported.

“If this year is another dry year, I would not be surprised to see a lot of dying trees on the Tahoe National Forest,” Brown said.

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Locally, the past two summers have produced no increase in insect and disease-related deaths among trees, said Brown.

But historical patterns show that a succession of dry years usually creates stress for trees, making them more vulnerable to fatal attacks by bark beetles and disease, he said.

Authors of the USGS study ruled out air pollution, long-term effects of fire suppression and other normal forest dynamics as the cause of the hike in tree mortality. “Average temperature in the West rose by more than 1 degree Fahrenheit over the last few decades,” said Phil van Mantgem, a USGS scientist and co-leader of the research team.

“While this may not sound like much, it has been enough to reduce winter snowpack, cause earlier snowmelt and lengthen the summer drought,” van Mantgem said.

They also found the increase of dying trees affects a wide variety of forest types, occurs at all elevations, is seen in trees of all sizes and in pines, firs, hemlocks and others.

Researchers also found that trees are dying faster than nature can replace them, wildlife will have to adapt to a rapidly changing environment, and forests could become sources of carbon dioxide to the atmosphere.

Another negative impact is the increase of wildfire risk in areas where neighborhoods meet the woods, Brown said.

From 1990 to 1992, one-third of the trees on the Tahoe National Forest died from stress related to dry conditions, Brown said.

He recalled helping mark thousands of dead red and white fir for harvest across thousands of acres.

“It sounds like a lot. It was very apparent to the eye.

“Even with those numbers, there were still a lot of living trees,” Brown said.

At that time, insects that infested trees weakened by a lack of water took out both young and older trees, Brown said.

Bark beetles are considered a normal part of the ecosystem, and a few dead trees here and there is nothing to worry about, Brown said.

Much of the forests of Nevada County were logged during the Gold Rush to supply lumber to area mills.

Even so, groves of large trees do exist.

“We do have areas on the forest with big trees. It depends on how you define old growth,” said spokeswoman Ann Westling.