SUMMER - DRY

Sequoia-Kings Canyon National Parks, Yosemite National Park and the Selway-Bitterroot Wilderness fall into the summer-dry category. These study areas have relatively cool moist winters and warm dry summers with most annual precipitation occurring outside the summer months. Therefore, during the growing season, plants are largely dependent on water that is stored in the form of snow or deep in the soil. As can be seen in the following figures, the pattern in Sequoia-Kings Canyon and Yosemite is very dramatic with virtually no precipitation at all during July and August. The Selway-Bitterroot has a similar, albeit less pronounced, pattern. Please click on a graph to view a larger image.

Sequoia-Kings Canyon National Parks are located on the western slope of the Sierra Nevada in central California. Combined acreage for these two parks is 349,500 ha (863,300 acres). Elevations range from (420 to 4410 m (1380 to 14,470 feet). Vegetation ranges from the foothill grassland and chaparral, through ponderosa pine, to the mixed conifer zone, to red fir and lodgepole pine and finally to high elevation pine near treeline. Generally, fire frequency increases toward drier environmental positions, and as fire frequency increases, fire severity tends to decrease (Caprio and Swetnam 1995). Lightning is most common on higher elevations, but can still be significant at lower elevations. In the past, lightning ignitions may have been supplemented by burning by native Americans and by sheepherders during the late 1800s. Fire suppression since the early 1900s has disrupted the fire regime allowing dead fuel to accumulate and understory tree density to increase. Currently, the fire and fuels management program at the Parks seeks to restore and maintain the natural fire regime in a manner consistent with firefighter and public safety. This program includes the use of prescribed fire, managing unplanned fires for the benefit of ecological values, and fire suppression. Because Sequoia-Kings Canyon has been actively involved with interagency fire management planning (Southern Sierra Geographic Information Cooperative; JFSP project # 99-1-3-04), we extended our analysis beyond Park boundaries and used the watershed boundaries that this planning effort has adopted. Our study area is the Kings and Kaweah watersheds, a very large landscape (782,700 ha; 1,932,600 acres) managed by multiple agencies including the US Forest Service, Bureau of Land Management, and state and local governments.

Yosemite National Park is approximately 100km north of Sequoia-Kings Canyon National Parks. It is 303,000 ha (750,000 acres) in size and ranges in elevation from 660-3950m (2100-13000 feet). Vegetation in Yosemite is similar to that in Sequoia-Kings Canyon, but Yosemite has less foothill and low elevation vegetation. Fire regimes and the history and management of fire in Yosemite is also similar to that of Sequoia-Kings Canyon NPs. Seventy-five percent of the park is managed to allow wildland fires to burn in a prescribed natural fire zone with another 8% in a conditional zone where fires have been allowed to burn under some conditions.

The Selway-Bitterroot Wilderness is located on the border of north-central Idaho and western Montana. We buffered the 0.5 million-hectare (1.2 million-acre) designated Wilderness to obtain a study area of 1.1 Mha (2.7 M acres). Elevations range from 430-3070 m (1410 to 10,060 feet). The climate ranges from inland-maritime in the northwestern part of the Wilderness to a continental rain shadow climate in the southern and eastern portions (Finklin 1983). The vegetation ranges from open stands of ponderosa pine at lower elevations, to mixed conifer forests at intermediate elevations, to whitebark pine, alpine larch, and Engelmann spruce at higher elevations (Habeck 1976). The area experiences a mixed severity fire regime: many fires are nonlethal surface fires but under suitable weather and fuel conditions, lethal surface fires and even stand replacing crown fires occur (Brown et al. 1994). The fire season typically runs from late June to mid-September; during this time, lightning-caused fires accompany frequent thunderstorms. Within the wilderness boundary, unplanned ignitions are often allowed to burn, although if a threat is perceived to the wildland-urban interface outside the wilderness, fires within the wilderness will be controlled (Law et al. 1997).