Burning questions: sifting through the ashes for a verdict on Yosemite's 'prescribed burns.' - Cover Story

Science News, Oct 27, 1990 by Richard Monastersky

Burning Questions

Staffers at Yosemite National Park geared up for throngs of tourists this summer as they prepared to celebrate the park's centennial. For a week and a half in August, however, Yosemite Valley lay unnaturally empty. Severe forest fires, ignited by lightning, had forced officials to close the park for the first time in history.

Only two summers ago, an epidemic of wildfires plagued another precious wilderness area, Yellowstone National Park in northwestern Wyoming. Ultimately spreading through 793,000 acres inside the park, the Yellowstone conflagration sparked a national controversy over the Park Service's view of fire as a vital, primeval force in the cyclic rejuvenation of wilderness ecosystems (SN: 11/12/88, p.314).

While almost all national parks share the ultimate goal of restoring fire's natural role to the greatest extent possible, officials at Yosemite and Yellowstone have developed individualized strategies that reflect vastly different local conditions. At Yosemite, managers routinely burn parcels of the forest -- in part to reduce the risk of uncontrollable future fires, and in part to encourage a more natural mix of forest vegetations. Yellowstone does not conduct these so-called "prescribed burns."

Fire experts now believe that Yosemite's August infernos, fought from the beginning, could not have been avoided. But the wildfires alarmed many who have experienced the region's awesome beauty, once again drawing public attention to fire-management policies. Park officials, while recognizing the serious disruptions caused by the blazes, see the behavior of these wildfires as an unprecedented opportunity to assess the effectiveness of their fire strategy. Some are pondering the need to intensify prescribed-burning efforts. They are also questioning the feasibility of the program's long-term goal.

"In a way, I think this Yosemite fire is going to be the first real test of the extent to which the program has really made a difference," says David J. Parsons, a research biologist at the nearby Sequoia and Kings Canyon National Parks. This park pair sits along the backbone of the Sierra Nevada about 50 miles south of Yosemite, and all three parks have a vigorous program of lighting intentional fires, with the goal of eventually letting nature's own fire cycle resume as much control as possible.

Prescribed burns represent an attempt to reverse the efforts of previous generations of Californians, who eliminated most natural fires from the Sierra. Between World War I and 1970, government fire crews actively battled any blazes erupting in these forests. Prior to that, white settlers had smothered fire's natural role in the Sierran forests by moving in huge herds of sheep for grazing.

Before sheep tromped through the Sierra, low-intensity firest swept across much of the middle-elevation forest every decade or two, cleaning out dead wood. These frequent burns, usually sparked by lightning or Native Americans, kept the forest open and airy. But the sheep grazed away forest grasses and packed down pine needles on the ground, discouraging low-intensity fires by making the ground cover less flammable. Naturalist John Muir, who herded in this region during his youth, later decried the environmentally destructive impact of sheep, calling them "hoofed locusts."

With fires largely suppressed over the last century, dead wood and needles have accumulated on the ground while middleaged trees have risen unimpeded toward the forest canopy -- the ceiling of leaves and needles formed by older, taller trees. Ecologists fear that these changes have altered vegetation patterns and the mix of tree species, creating a type of forest that did not exist before white settlers arrived. The accumulation of "fuel" has also set the stage for unnaturally severe fires to sweep through.

In recent decadesM concern over such shifts -- which threaten many other public lands where fires have long been suppressed -- has forced a revolution in thinking within the U.S. Park and Forest Services. Those agencies, which once emphasized the destructive potential of forest fires with publicity campaigns featuring such symols as Smokey the Bear, today recognize that the forest can actually benefit from some burning. Within practical limits, parl officials now seek to restore fire to its "rightful" place in the ecosystem.

Most of the land in the Sierra parks is high-elevation forest, where fuel accumulates more slowly and granite outcrops break up the tree stands. In this area, the pre-1970 suppression efforts had little effect. Managers can thus allow lightning fires to burn under careful supervision there if the firest meet a long list of criteria designed to protect people and property.

But managers have less latitude in the middle-elevation forests, where unnaturally high fuel levels make the practice too risky during most times of the year. This is where prescribed burns come into play.

Concentrating on small patches at a time, workers light fires to burn the dead wood and needles, and to kill understory trees -- young and middle-aged trees that don't reach the canopy. Over the years, by igniting one or perhaps two prescribed fires in each targeted area, they hope to reduce the forest fuels to a safe level so that future lightning fires can be allowed to take their natural course in these middle-elevation regions, which make up about 10 percent of the Sierra parks. In short, the intentional fires in those areas represent a means, not an end.