April Sall stood in the charred remnants of a Joshua tree forest, bark peeling off melted black limbs. Above her, ridges once thick with 1,000-year-old piñon and juniper pines were scorched bedrock and stumps. More than 90% of the surrounding Pipes Canyon Preserve was consumed in last months Sawtooth blaze. It was one of half a dozen fast-moving fires this summer that burned 65,000 acres of the Mojave Desert, fueling debate over whether the desert is burning more frequently and explosively as a result of invasive weeds, smog, development and climate change.

"Its heartbreaking to see," said Sall, a biologist who manages the preserve and whose grandmother homesteaded the land a century ago. "Well never see those piñon or juniper trees again in our lifetimes, nor will our children, nor will their grandchildren. Its a bitter pill…. This land isnt meant to burn." Many scientists agree, saying the recent blazes offer fresh evidence that deserts across the Southwest are undergoing a profound shift, as ancient native pine, shrubs and cactuses give way to young, highly flammable weeds and grasses.

"Right now were losing very large pieces of landscape," said Todd Esque, an ecologist with the U.S. Geological Survey in Henderson, Nev., who studies the cause and effect of fires in the desert. "Its happening in Joshua Tree National Park, its happening in Mojave National Preserve … up in southwestern Utah … and in Arizona. We lost 750,000 acres of desert to fire in Nevada alone last summer."

This summer, five blazes have seared parts of Joshua Tree, where a fire only every few years was the norm for the last 50 years. Esque and other researchers say that unlike forests and chaparral, the sparsely vegetated desert is not meant to burn frequently. "The public has come to understand that fire is a necessary part of the life of forests," Esque said. "That is not the case with deserts. We have a major problem going on."

A vocal minority disagrees, contending there is no clear-cut evidence of far-reaching change. They blame this years fires on bumper crops of wildflowers nourished by heavy spring rains two years ago. According to the theory, dried remnants of the prolific blooms fueled a 50,000-acre fire in the Mojave National Preserve last summer and in this years conflagrations.

"The winter of 2004-05 was the wettest ever in 100 years of recorded data in the desert. We had a phenomenal crop of annual native wildflowers, and it was dry the next year and it stayed there," said Richard Minnich, a professor of Earth sciences at UC Riverside. "Its flash fuel of 1 to 2 tons per acre. Whats really scary is, theres still a lot of it out there."

Scientists do agree that it will take centuries, if not millenniums, for the desert to recover. "It wont be on a timeline we humans would like, but it will happen," said Tasha LaDoux, Joshua Tree National Parks botanist. Inside the park, new growth provides fodder for the debate over whether the fragile, arid landscape is undergoing dramatic change.

At the scene of a 1995 fire, not a single juniper or piñon pine seedling has come up after 11 years. But healthy, 3-foot "pups" have sprouted from the roots of once seemingly dead Joshua trees. The pups may or may not survive, scientists say, because in drought years they may be gnawed by thirsty rodents and ground squirrels. Meanwhile, native apricot mallow, bright-green cheesebush and golden California marigold are blooming even in August.

Along a sandy road in the western section, the scene of a 1999 blaze that scorched 14,000 acres, a beige sea of grasses spreads beneath burned Joshua trees bleached silver by sun and rain. The new growth consists of native bunch grasses and a pair of noxious, ankle-scratching weeds. These two nonnatives, known as red brome and cheatgrass, form highly flammable carpets between native shrubs and trees, and many scientists believe they are the main culprits behind increasing fires.

"These invasive grasses fill in the spaces between the desert plants. They carry the flame through at a very high rate, and much hotter. It spreads a lot faster," Sall said. The weeds are also bad for animals. "The ranchers call it cheatgrass because for the first few years its good grass, but after that it cheats the cattle of their nutrients," Esque said.

Native to Mediterranean Europe and Asia, the weeds were probably blown across the West by the wind, tracked in by hikers boots and construction equipment, and excreted by livestock. Researchers at UCLA and elsewhere say the weeds appear to capture nitrogen from smog-laden air more readily than native plants, eventually choking them out.
(By Janet Wilson, Los Angeles Times, 21/08/2006)
http://www.latimes.com/news/science/environment/la-me-desert21aug21,1,6168739.story?track=rss