Ozone may be a big culprit in Hill tree deaths

The understanding that tourists and residents alike have of the dead trees in the San Jacinto Mountains is that they have been killed by the rampaging bark beetle infestation brought on by the drought of the past several years.

But that, says researchers, is only part of the story.

While drought plays a significant role in tree mortality, particularly by allowing bark beetles to successfully attack a weakened tree, there is evidence that another threat is reducing the area’s forests — ozone.

Ozone monitoring sites are primarily in the San Bernardino Mountains, says Mike Arbaugh, project leader of the Air Pollution and Global Change Impacts on Western Forest Ecosystems study at the U.S. Forest Service’s Pacific Southwest Research Station in Riverside. The sites were founded in the 1960s by Paul Miller, considered to be one of the “fathers of air pollution research,” Arbaugh said.

“He established a network of study sites in the San Bernardinos, 18 of which we currently maintain,” Arbaugh said. “It is one of the longest existing air pollution effects gradients in the world.”

Researchers first suspected ozone was a problem in the late 1950s, Arbaugh said.

“In the 1950s, forest managers noticed that pine needles were becoming mottled and sickly,” he explained. “It was called the X-disease, because no one could find out what was causing it. After several years, Forest Service researchers found out that it was caused by ozone produced from car exhaust.”

This summer, he said, there will be two stations added to the San Jacinto Mountains to find out if the project’s findings from the San Bernardino Mountains apply here — that ozone attacks not only the trees themselves, but also the soil.

“Air pollution, including ozone … has altered forest composition, growth rates and root biomass,” he said. “As a result, forests are now much more vulnerable to both drought and bark beetle attacks. The recent forest mortality, in part, is due to the long-term effects of air pollution.”

Ozone pollution damages a tree after it is absorbed through leaves or needles, and it can reduce the amount of sugars transported to the roots. Laura Merrill, an entomologist for the Forest Service’s Forest Pest Management office in Riverside, says that ozone’s damage to leaves and needles leaves a plant vulnerable to bark beetle attacks.

“Ozone enters the needles through the stomates and kills cells it contacts. Initially, the damage can be seen as yellow spots on the needles (although other, less harmful conditions can also cause yellow spots),” she said. The damage, she explained, can reduce a tree’s ability to photosynthesize, a process of turning sunlight into energy. That can reduce a tree’s ability to ward off beetles by sapping the tree of sugars needed to create resin, the substance a tree uses to kill attacking beetles.

A tree that suffers from both lack of sufficient water and exposure to ozone is in serious trouble, according to Michele Eatough Jones, an entomologist with the University of California at Riverside.

“By far, the biggest factor behind our current bark beetle problems was the drought,” she said. “Water is essential for resin production. Air pollution can also stress pine trees, so the combination of drought stress and air pollution can further increase pine susceptibility to beetle attack.”

The result of the combination of drought and ozone exposure is causing the composition of the forest to change. As ozone-sensitive trees die off — trees that the Forest Service calls bioindicators, such as ponderosa pine and Jeffrey pine — they are replaced with species either more tolerant to drought conditions, ozone concentration or overcrowding (a major problem that exacerbates a lack of water or soil nutrients as trees fight for limited resources).

“Trees that are weakened by air pollution become more susceptible to drought. Bark beetles attack trees already weakened by air pollution and drought and finish them off,” says Andrzej Bytnerowicz, an ecologist for the U.S. Forest Service’s Pacific Southwest Research Station in Riverside. “Dense forests are more susceptible to drought (too many trees and too much root biomass for the limited water resources).” Arbaugh says that the monitoring program is giving researchers an excellent opportunity to track the forest’s changes.

“One important result of our project was to document the amount of mortality at sites in the forest due to us maintaining our long-term monitoring sites,” Arbaugh said. “Few sites are available that have 30 years of measurements. We have been able to track both changes in species composition (more white fir and incense cedars, fewer ponderosa and Jeffrey pines), and mortality (100 percent at some sites).”

One last possible change to the forest may be a reduction in its role as a giant oxygen factory, sucking up carbon dioxide and releasing oxygen in exchange. A study by Michigan Technological University and the North Central Research Station of the U.S. Forest Service in 2003 suggests that forests exposed to carbon dioxide and ozone produce less oxygen and remove less carbon dioxide from the atmosphere as forests exposed solely to carbon dioxide. In other words, as scientists have long theorized that large forest tracts can help solve global warming by absorbing the greenhouse gas carbon dioxide, it appears forests near urban areas, where ozone levels are high enough to be toxic to humans and plants alike, are less equipped to perform that function.

The two monitoring stations coming to the San Jacinto Mountains will help researchers assess ozone levels in the area and make some determination about the damage it is causing. While the consensus is that ozone levels are lower here than in hot spots such as Crestline and Big Bear, these sites will give researchers some real data to make accurate comparisons and to begin the process of understanding the changes occurring in the area.