A new study confirms that salt used to keep roads clear of snow and ice appears largely responsible for the death of hundreds of trees along Lake Tahoe highways.
A fungus found in many sick Tahoe trees also could have contributed to the deaths, but the significance of the infection remains unclear and is being studied, said Shouhua Wang, a plant pathologist with the Nevada Department of Agriculture.
"There's a clear link with de-icing," Wang told the Reno Gazette-Journal. "I would say de-icing is the most important factor."
The study focused on wilting and dying trees along the Mount Rose Highway, U.S. 50 in Nevada and State Route 28 in 2002.
Preliminary results of Wang's research were released to the Nevada Department of Transportation last week.
"We know salt kills trees, we've known it all along," NDOT District Engineer Thor Dyson said.
Dyson added that concerns about road salt are nothing new and are not restricted to Lake Tahoe.
"The challenge is to keep the roads safe, balancing the environment and economic consequences," he said.
"We're always looking at anything new and we're going to be watching a lot closer. We're looking at other chemicals, other methods, new technologies," he told the newspaper.
In the study into the tree deaths, scientists selected 42 locations along the three affected Tahoe highways, sampling pine needles of both blighted and apparently healthy trees as well as the soil beneath them.
In the healthy trees, samples revealed no salts or very low concentrations. Salts were found in both the needles of blighted trees and in soil samples, with the severity of blight in a given tree linked to higher levels of salt concentrations, Wang said.
The problem seemed significantly more noticeable along Nevada highways than those on Tahoe's California side. One possible explanation is that the California side of the Tahoe Basin gets more snow and rain than the drier Nevada side, officials said.
Nevada's transportation department uses two types of salt in its wintertime effort to keep highways clear. A liquid brine solution is sprayed on roads as a preventive measure when storms approach. Once snow or ice is caked on roads, trucks drop a mixture of sand and salt granules to melt slick surfaces.
Dyson said NDOT has cut its overall use of road salts by half since it began using the liquid brine solution at Tahoe in 1996-1997.
He was also troubled by the die-off experienced in 2002.
"It was very much upsetting. A lot of trees went brown," Dyson said, adding that some trees later recovered.
He's unsure why the winter of 2001-2002 was so tough on Tahoe's roadside trees but he does have some theories.
Though the season was very dry, there was enough regular snowfall to require regular application of road salt, Dyson said.
But once salt was applied, it could be that post-storm runoff was too little to wash salts away and roadside trees may have suffered as a result, he said.
"Nothing was getting diluted. Concentrations were high because total moisture was so low," Dyson said.
Another possibility, he said, is that the liquid brine applied during warm storms might be sprayed into roadside trees by passing vehicles.
John Christopherson, resource management officer for the Nevada Division of Forestry, said the affected trees might have suffered from the double-whammy impacts of being first weakened by drought and then poisoned by road salts.
"Combined, that could pack a wallop," Christopherson said.