July 13, 2009

Air pollution linked to increased fire threat

Fire-fueling plants thrive on nitrogen-enriched soil

Air pollution may indirectly contribute to the increase of wildfires in the desert. Fire-stoking weeds now thrive in the desert soil, enhanced by the nitrogen in the soil deposited by air pollution. (Jay Calderon, Desert Sun file photo)

Keith Matheny
The Desert Sun

Air pollution from engine exhaust — largely carried here from Los Angeles by the wind — isn't just harming people's health. It's also pushing native plant species toward extinction in and around the Coachella Valley, and indirectly contributing to increased wildfires in the desert.

Edith Allen, a University of California, Riverside botany professor, has spent years working with a team studying the impacts of engine exhaust pollution on native and invasive plant species in parts of Riverside and San Bernardino counties, including the Coachella Valley.

The research has found that car exhaust is inadvertently making plant fertilizer — nitrates, a form of the naturally occurring nitrogen that plants can readily use for food, Allen said.

The nitrates drop out of the atmosphere and onto the soil, where rain allows plants to absorb it through their roots, Allen said.

“Up to a point it's a good thing,” she said. “But we also know that weeds take up nitrogen. They're better at taking it up than crop plants or wildland plants.”

The nitrogen is naturally flushed away over time in areas where it rains frequently, Allen said. But in the desert, with only a handful of rainy days in a year, the nitrogen is able to accumulate, she said.

Invasive weeds were accidentally introduced to the region through movement of people, crop seeds and livestock over centuries, Allen said.

“In the desert, they are from the dry regions of Europe and North Africa,” Allen said.

Exhaust-related air pollution is helping the invasive weeds to flourish.

“Because they take up nitrogen more rapidly and grow more rapidly, they produce more seeds,” Allen said. “They are more competitive than the native plants and they replace the native plants.

“Areas that once were covered with native vegetation — including areas of the Coachella Valley — are covered with with invasive plants from the Mediterranean.”

Change in fire cycle

In the western valley and areas in and around Joshua Tree National Park, the plants thriving on the exhaust-provided nitrates are often invasive grasses.

“Those exotic grasses are the ones that are effecting the fire ecology in the park,” said Joe Zarki, a longtime ranger at Joshua Tree.

“They create carpets out in the desert where previously we had a lot of open, bare ground. When they die every year, they create fuel.”

It's leading to wildfires that are more intense and happen more often, experts said.

The desert burned “very infrequently” over hundreds of thousands of years, Allen said. But the cycle changed around the mid-1980s, she said, as air pollution worsened in the Los Angeles area and moved into the region.

“Now what we see is some areas of the desert burning whenever there is a wet year,” she said.

“There are areas that have burned three times since the 1980s. That's not part of our natural fire cycle. There's something different going on.”

The most severe of the more frequent desert fires was the Sawtooth-Millard fire in July 2006.

The lightning-started fires combined to burn more than 85,000 acres in the Morongo Basin near Yucca Valley and in the San Gorgonio Wilderness north of Cabazon. They led to a man's death, cost more than $21 million to fight and nearly $12 million in property damage.

In July 2005, the Mojave National Preserve, near the old mining town of Kelso north of Twentynine Palms, had its largest wildfire in modern history, a lightning-started blaze that burned more than 70,000 acres.

Cameron Barrows, a research ecologist at the UC Riverside Center for Conservation Biology, sees a clear connection between engine exhaust pollution and the more intense, more frequent wildfires.

“You end up with very high concentrations of nitrogen from the Banning Pass to Snow Creek and the Desert Hot Springs area, and more towards Joshua Tree” National Park, he said.

“Not surprisingly, that's where most of our wildfires have been.”

A vicious cycle

The pollution-created nitrogen in soils, the invasive grasses and wildfires all create a vicious cycle that works against native desert species — Joshua trees, junipers, pinyon pines, chaparral, creosote bush and more.

“Once the fires go through, the desert vegetation is not adapted to fire,” Allen said.

“It takes a long time to recover, and in fact it does not recover. What it turns into is a greater abundance of invasive grasses.”

Zarki notices the trend at Joshua Tree National Park.

“We're seeing a type conversion from older, more mature type plants to exotic grasses and annual wildflowers,” he said.

“There are unknown ecological consequences to that. If you're a plant or animal that's best adapted to junipers and pinyon pines, and they're getting burned up and not getting a chance to re-establish themselves, there is a significant impact.”

The phenomenon isn't isolated to the Coachella Valley and high desert.

“It's an international problem,” said Andrzej Bytnerowicz a senior scientist studying air pollution with the U.S. Forest Service's Southwest Research Station in Riverside.

“We have huge problems in China, for instance, related to air pollution acting as plant fertilizer.

“I think we have to be much better in publicizing what we know. Ozone has been discussed for three, four, five decades. But nitrogen deposition, it's been maybe talked about for 10 years.”

Though the long-term ramifications of the pollution's effect on desert plant life look dire, Allen said she remains hopeful that the situation will turn around.

“Now that we have a federal administration in Washington that is willing to allow California to take the lead, I think air pollution issues are going to get better,” she said. “I think the air is going to get cleaner and some of these issues about invasive grasses and fire are going to get better.”