Winter 2000

Wildland Fires for Resource Benefits in Wilderness Areas:

the Yosemite Park Experience

Roberta Van de Water

Fire scientists and others today see fires, especially large ones, as ecologically significant events that may drive vital ecosystem processes. Enabling fire to play its natural role, especially in National Parks and other wilderness areas, has challenged researchers and managers alike. Simplistically, this approach has been termed the 'let burn' policy, but prescribed natural fire has evolved to sophisticated strategies by the National Park Service, Forest Service,Nature Conservancy and other land managers over the years.Ý The scientific basis of such policies is due in large part to three decades of study in the Sierra Nevada national parks.Ý

The Illilouette Creek basin was Yosemite National Park's first watershed to undergo prescribed natural fire, beginning in 1974. Fire researcher Jan van Wagtendonk, formerly with the Park Service and now with USGS, and various collaborators have been watching a pattern emerge on the landscape which is like that of a jigsaw puzzle being filled in with each fire season.What is emerging is a picture wherein the size of fires is limited by nearby past burns. New fires run out of fuel and go out, or at least burn withÝ reduced intensity and rate of spread.Ý In the case ofthe Illilouette watershed, the first prescribed natural fire, the 1974 Starr Fire, burned 4000 acres.Ý Reburns into it occurred in 1980 and 1988. In 1981 a fire burned to the edge and wentout. In 1991 the Ill Fire burned with an intensity that van Wagtendonk feels would have been less, and produced less smoke in the park and surrounding communities, had the 1974 fire not been limited due to earlier smoke concerns.

Before a Wildland Fire for Resource Benefit zone is established, the historic role of fire must be considered. This includes fire frequencies of various fire size and intensity classes. (See Carl Skinner's article in this issue.)Ý The fuels and predicted fire behavior should be within their natural ranges in order to meet prescription. Then concerns such as risk of escape outside the zone, safety, tactical suppression needs and, increasingly,Ý smoke need to be addressed.Ý Risk of escape includes endangerment of critical resource values, as well as life and structures.

Gradually, as experience was gained, the Park Service increased its proportion in the Resource Benefit zones from 25% to 87% of Yosemite. A total of 528 fires have been allowed to burn over 24,000 hectares in the zones (Table 1) over the life of the project. Most of these fire were less than 0.1 hectare (0.25 acres) in size, usually only a single tree (Table 2). Most of these occurred in the red fir and lodgepole pine types where weather and fuel conditions were not conducive for burning.Ý Only 18 fires have grown larger than 400 hectares (1000 acres) primarily in the white fir mixed conifer type.Ý In 1999, however, the largest fire to have occurred during the 27-year program has burned over 8,000 acres in chaparral immediately above Hetch Hetchy Reservoir.Ý Van Wagtendonk feels that the success of the program can be measured by the closeness of the calculated fire rotation (the number of years needed to burn the entire vegetation type based on historical performance) to the predicted, or historical, rotations for each vegetation type. Table 3 summarizes total area burned and percent burned by major type, along with calculated fir rotation.Ý The red fir, Jeffrey pine, and lodgepole pine types all had calculated rotations within estimated ranges.Ý However, ponderosa pine mixed conifer was twice what would be expected; 33 vs. 10-12 years.Ý Ponderosa pine is the most extensively naturally-burned type by far, with over 90% of it burned over the last 27 years.Ý This is because lightening ignition sources are prevalent in this type, and weather and fuel conditions are conducive to larger fires.

Table 1. Presribed natural fires in Yosemite National Park 1972-1999

Table 2. Distribution of prescribed natural fires by size category; percent of total number of fires and of total area burned in Yosemite National Park, 1972-1999

Table 3. Area burned by all fires in conifer vegetation types and calculated fire rotations, Yosemite National Park, 1972-1999

The 1999 season has once again brought Yosemite Park and other wilderness area fires into the limelight.Ý As this goes to press, many are being allowed to burn while public opinion ranges from outrage over the smoke, to worry about the lost pristine nature of a favorite wilderness area, to a creeping realization that Nature is in command. Each fire can potentially offset the negative effects of the really big one which fire scientists predict is lurking just around the corner.Ý And maybe it will do good things for ecosystems we only dimly understand.-

References:

van Wagtendonk, J., 1995, Symposium on Fire in Wilderness and Park Management; 1993 March 30-April 1; Missoula, Montana. General Technical Rep. INT-GTR-320. Ogden, UT: USFS Intermountain Research Station.

van Wagtendonk, J., personal communication, and tables and figuires8/99.