Horseshoe Lake Trees - FAQ

Many people think the dead trees at Horseshoe Lake are the result of a fire, bark beetles, or a disease. None of these are the cause. Carbon dioxide gas venting up through the soil is the cause. Measurements of gas in the soils in the tree kill areas indicate high levels of carbon dioxide gas. The normal level of carbon dioxide in the atmosphere is .03% and the level in the soil within the tree kill areas can be as high as 60%-80%. Plants use carbon dioxide in the photosynthesis process in their leaves or needles and have adapted to the .03% level in the atmosphere. The high levels of carbon dioxide in the soil actually interfere with the roots' ability to absorb nutrients.

What is the Source of the Carbon Dioxide?
There may be a link between elevated levels of carbon dioxide and seismic activity that occurred at Mammoth Mountain during the fall of 1989. Carbon dioxide, a naturally occurring gas in magma, appears to be making its way to the surface, possibly from an intrusion of magma at great depths. There is no seismic data to support the theory that this venting of gas indicates increasing chances of an eruption. Researchers believe that there would be more seismic activity in the vicinity of Mammoth Mountain if an eruption were imminent.

How Long has this Been Occurring?
The first indication of carbon dioxide venting to the surface was in March of 1990 when a Forest Service employee, on ski patrol, sought refuge from a heavy snowstorm inside a snow survey cabin near Horseshoe Lake. After entering the cabin he experienced symptoms of rapid pulse and breathing, followed by headaches and dizziness. The following summer, two other Forest Service employees reported similar symptoms when entering utility vaults in the Horseshoe area. During the remainder of 1990, some trees appeared to be dying and a larger area of tree death was observed in 1991. By 1992 the area of tree death had reached a size of 15 acres. In 1994 the area had expanded to 28 acres. As of 1995, the area had expanded again by 2-3 acres.

Is There Any Threat to our Health from the Carbon Dioxide?
Carbon-dioxide is heavier than air and tends to collect in low and enclosed spaces. All of the reports of breathing difficulty have occurred in poorly ventilated spaces not open to the public. The U.S. Geological Survey and the Forest Service have been collecting air samples in the area in both tents and restrooms and have found the levels of carbon dioxide to be well within public health standards. There are plans to intensify the monitoring of the gas. Since carbon dioxide gas dilutes quite quickly once it enters the soil, persons spending time in the open air at Horseshoe Lake should experience normal levels of carbon dioxide.

Why Were Some of the Trees Cut Down?
Trees with potential of striking a campsite, picnic site, car, or restroom are considered hazard trees and must be felled. Hazard trees include only 7-9 acres of the total 30 acres of dead trees. There are no plans at present to cut down anything but hazard trees.

How is the Wood Being Used?
The wood has been sold to a commercial firewood company in Mammoth Lakes. Most of the permanent residents in town depend on firewood for heat during the area's long winters.

For more information, see US Geological Survey information about monitoring carbon dioxide gas at Mammoth Mountain.