Changes in fire hazard as a result of the Cerro Grande Fire

Fire Management Today, Winter 2002 by Greenlee, Dawn, Greenlee, Jason

Like similar fires elsewhere, the Cerro Grande Fire burned hotter than historical fires because of fuel buildups from years of fire suppression.

On May 4, 2000, a prescribed burn was ignited on the Upper Frijoles Burn Units 1 and 5 on New Mexico's Bandelier National Monument. The units were located at between 9,000 and 10,000 feet (2,700-3,000 m) elevation in the Jemez Mountains, 6 miles (10 km) west of Los Alamos, NM. The burn was part of the Valle Project, an interagency fuel reduction program designed to reduce the risk of catastrophic fire in the Los Alamos region. The burn's objectives were to reduce tree densities and fuel loads in overgrown meadows and stands of aspen, ponderosa pine, and mixed conifer (NPS 2000). Two large wildfires had threatened Los Alamos in preceding years (the 1977 La Mesa Fire and the 1996 Dome Fire), causing researchers at the Los Alamos National Laboratory to publish a prediction that proved to be an uncanny harbinger of the events that followed (LANL 2000).

On May 5 and 6, the burn escaped and suppression actions failed.**

The fire moved eastward through mixed-conifer vegetation into the lower elevation ponderosa pine vegetation on the Pajarito Plateau, where Los Alamos is located. It then skirted the northern and southern edges of town (fig. 1), burning about half of the town's perimeter. Before the fire was suppressed, it burned 42,858 acres (17,344 ha) and 235 residences. Like other recent wildland fires in the United States, this fire burned hotter than historical fires because of the buildup of fuels that had resulted from years of fire suppression.

Fire Hazard Study

Following the fire, the Federal Emergency Management Agency (FEMA) was asked to facilitate a management program that would ensure that the fire hazard did not become greater than it had been prior to the fire. FEMA commissioned a study by the authors to predict changes in fire hazard in the Cerro Grande Fire area by modeling fire behavior in postfire fuels over the next 23 years. The model could serve as a tool for fire managers elsewhere in determining the importance of timber salvage and other fuel reduction treatments following high-severity wildland fires.

Whereas low- and moderate-severity prescribed burns are conducted to reduce understory ladder fuels, high-severity wildland fires can have the opposite effect. Postfire fuel conditions can become more hazardous than before the fire, because fire-killed snags fall to create thick slash fuels. Trees are often removed after high-severity fires to prevent slash fuels from developing. In about 20 percent of the area of the 1994 Tyee Fire in Washington, fire-killed timber was salvage-logged or fuels were piled and burned, particularly in wildland-urban interface (W-UI) areas, both to recover timber value and to reduce future slash accumulation (Ellis 2000; Forest Service 1994). Following the 1998 Florida fires, trees in the community of Palm Coast were salvaged both for the safety of local residents and to reduce fire hazard and facilitate suppression efforts (Kuypers 2000). Similar treatments have been applied to high-severity fires elsewhere (Keeves and Douglas 1983). The authors examined changes in fire hazard resulting from the Cerro Grande Fire and addressed the need for salvage treatments or other fuel treatments, both in the areas burned and in unburned areas near the Los Alamos townsite.

Vegetation Types

The area of the Cerro Grande Fire is characterized by three primary vegetation types: ponderosa pine, mixed conifer, and pinyon/juniper.

Ponderosa Pine. Ponderosa pine (Pinus ponderosa) forests are the dominant vegetation in and around Los Alamos. This forest type extends from 6,500 feet to 8,800 feet (2,000-2,700 m) elevation and dominates south-facing aspects in the Jemez Mountains below 8,000 feet (2,400 m) (Balice and others 1997). Prior to 1900, open stands with grassy understories and only 50 to 100 trees per acre (125-250 trees/ha) were maintained by frequent low-intensity surface fires, which thinned stands and limited the buildup of dead fuels on the ground. Historical mean fire return intervals were between 5 and 15 years (Allen 1989), but most of the area burned by the Cerro Grande Fire had not burned since 1883 (Allen 1989; Foxx and Potter 1981).

Exclusion of fire produced tree densities of between 286 (Balice and others 1997) and 1,300 (Forest Service 2000) trees per acre (706-- 3,200 trees/ha) and an accumulation of between 8 and 40 tons of fuel per acre (18-89 t/ha) on the forest floor (Balice and others 1997; Miller 1999). Stand basal areas were 60 to 80 square feet per acre (14-18 m^sup 2^/ha) (Forest Service 2000). Crown bulk densities were estimated at between 0.02 to 0.03 pounds per cubic foot (0.3-0.5 kg/ m^sup 3^) (Armstrong 2000), much more dense than the 0.006 pounds per cubic foot (0.1 kg/m^sup 3^) needed to sustain crown fire activity (Agee 1996). Armstrong (1998) calculated that these pine stands would not, on any day in the fire season, develop 4-foot (1.2-m) flame lengths, even on the steepest (40-- percent) slopes.