Volcanic Gas Implicated in Tree Die-Off : Geology: High levels of carbon dioxide are found in the ground at the site near Mammoth Mountain. The area is heavily used in summer.

SAN FRANCISCO — Federal scientists are investigating whether the deaths of hundreds of trees near a popular recreation area at Mammoth Mountain are due to increased levels of carbon dioxide produced by volcanic activity.

The U.S. Geological Survey and the U.S. Forest Service said they have found no indication of a hazard to public health and are uncertain whether the high gas level is a sign of increasing volcanic activity within the mountain.

“There are a lot of things we still don’t know about this yet,” said David Hill, a USGS scientist who is in charge of monitoring seismic and volcanic activity in the Mammoth area. “We know that some carbon dioxide is in the ground and is coming out of the ground on the south side of Mammoth Mountain.”

In 1991, people in the area first noticed that trees were dying near a picnic area in the vicinity of Horseshoe Lake, above the town of Mammoth Lakes.

The area affected is about 200 yards in diameter with several acres of trees. It is not in the area used by downhill skiers in the winter but is heavily used in the summer because there is access by road and a parking lot nearby.

The Geological Survey is continuing to conduct tests in an attempt to pinpoint the source of the carbon dioxide but early investigation indicates volcanic activity is the most likely cause, agency scientists said.

Initial studies indicate there is more carbon dioxide in the ground than could be produced simply by the decay of vegetation. The emission of carbon dioxide from the ground is a phenomenon often associated with volcanic activity.

Since a series of earthquakes at Mammoth Mountain about 15 years ago, the USGS has been monitoring seismic and volcanic activity in the region. Since 1990, they have observed a bulge in the central part of the Long Valley Caldera that is growing an average of 2 centimeters a year.

Although plants absorb carbon dioxide, excessive quantities can kill plant life, scientists said. “The concentrations in the soil are quite high and it might indeed be the thing that is killing the trees,” Hill said.

It also is possible the trees have died from other causes. Vern McLean, a U.S. Forest Service geologist, said increasing ground temperatures associated with volcanic activity appear to have killed trees and vegetation elsewhere on the mountain.

Carbon dioxide is generally harmless to people unless breathed at high concentrations; humans and other animals convert oxygen to carbon dioxide when they breathe.

Hill said it is conceivable the carbon dioxide could affect people in the area if it is coming out of the ground fast enough. Because carbon dioxide is heavier than oxygen, it could displace oxygen and collect in low-lying areas on a still day, he said.

But the experts agreed that there is no evidence the gas poses a risk to the public. “I don’t think we have gotten any indication there is a reason to keep the public out of there yet,” McLean said.