YOSEMITE Refined Burning Prescriptions for Yosemite National Park NPS Occasional Paper No. 2

Early travelers and photographers in the mid-1800s recorded the forests of the Yosemite region as being parklike with little undergrowth and wide expanses of meadows. These forests were part of a mixed conifer zone dominated by ponderosa pine (Pinus ponderosa Laws)¹ with some incense-cedar (Libocedrus decurrens Endl.) and California black oak (Quercus kelloggii Newb.) at the lower elevations, and increasing numbers of sugar pine (Pinus lambertiana Dougl.) and white fir [Abies concolor (Gord. and Glend) Lindl.] at the upper end of the zone. Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] was more prevalent in the northern portion of the region.

Beneath the larger trees the forest floor was carpeted with needles, bear clover (Chamaebatia foliolosa Benth.), or various grasses and forbs.

The understory, where present, consisted of tree reproduction, ceanothus (Ceanothus spp. L.), and manzanita (Arctostaphylos spp. Adans.). These open conditions were attributed to low intensity surface fires set by Indians and lightning. If fires were frequent enough, fuel accumulations and undergrowth were kept at a minimum (Fig. 1).

FIG. 1. An open mixed conifer forest in Yosemite National Park. Ponderosa pine, sugar pine, and incense-cedar are in the overstory. The sparse understory consists of manzanita plants and coniferous reproduction. This area has been managed with periodic prescribed fires.

Subsequently, in 1890 an era of protection began when the area was set aside as Yosemite National Park. Cessation of Indian burning and subsequent park management under a philosophy of preservation caused many changes. Overprotection from natural surface fires permitted the forest floor to become a tangle of understory vegetation and accumulated debris. Shade-tolerant incense-cedar and white fir thickets increased and caused an unnatural ecological succession away from the less shade-tolerant ponderosa and sugar pines. Pines are better adapted to a regime of periodic surface fires and would be favored.

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¹Tree nomenclature is from Little (1953). All other taxonomic references are from Munz and Keck (1968).

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As undergrowth increases, fuel volumes expand and a continuous fuel layer extends from the ground to the forest canopy. Currently, the total fuel layer, including dead trees, branches, needles, and other debris, is so thick that a wildfire would soon reach catastrophic proportions.

The early management decision to extinguish all natural fires has led to conditions which not only threaten to destroy those park values which were intended to be preserved but also modify the forest to an unnatural state. Recent changes in National Park Service policy, in part due to recommendations of the Leopold Committee (1963), direct that each park be restored as nearly as possible to the conditions that existed when Europeans first visited the area. A management philosophy of perpetuating natural processes rather than preserving and protecting objects has evolved.

Prescribed fire is now being introduced into the mixed conifer forests of Yosemite National Park as the most natural method of manipulating the vegetation to recreate and perpetuate what are thought to be pristine conditions and also to reduce fire hazards. In order to use fire effectively and judiciously, burning prescriptions need to be developed and refined. A prescription specifies the weather and fuel conditions under which a fire could be ignited to meet specific management objectives.

Objective

The objective of this study was to develop refined burning prescriptions for environmental restoration and fire hazard reduction fires in the mixed conifer forests of Yosemite National Park. This was done by evaluating the effects of prescribed spring fires burning in different understory fuel types, within a specific range of fuel and weather conditions, on fire characteristics, fuel reduction, and understory plant composition.

Previous Work

That fire played an important role in Yosemite's forest can be surmised from historical accounts. John Muir (1894) not only described the open nature of the original forest in his book The Mountains of California but also wrote about periodic fires which occurred in the forest. Reynolds (1959) and Vankat (1970) have both attempted to determine the exact nature of the pristine Sierra Nevada forests. There is general agreement that the forests were quite open compared to present conditions and that fire was important in maintaining the openness.

Changes in Yosemite's vegetation subsequent to fire suppression activities have been chronicled by Ernst (1949, 1961). He noticed the forest encroaching on the meadows of Yosemite Valley and the increase of undergrowth throughout the forest. Using a sequence of historical and modern photographs, Gibbens and Heady (1964) showed the dynamic nature of the vegetation increases.

Fire ecology studies in California have included fuel reduction and modification investigations in giant sequoia-mixed conifer forests (Biswell 1967), fuel conditions in similar forests (Agee 1968), and the restoration of fire to various ecosystems of Sequoia and Kings Canyon National Parks (Kilgore 1970, 1971). No specific studies relating fire behavior to weather variables and fuel consumption have been made for this region. Data from a study of this type have been published for the southern coastal plains in Georgia (Hough 1968), although burning was restricted to one fuel type and no attempt was made to burn at specific moisture levels.

Prescriptions for burning to maintain fuel breaks in the central Sierra Nevada have been established by Schimke and Green (1970). To obtain safe, satisfactory fires, the prescriptions specify upper and lower limits for air temperature, relative humidity, wind speed, and fuel moisture content. These prescriptions must be further refined, however, if prescribed fires are to be used for environmental restoration and fuel reduction in a mosaic of understory fuel types.

Considerable research concerned with fire characteristics has been done by using fire models in laboratories. At the U.S. Forest Service Missoula Fire Laboratory, Beaufait (1965) characterized backfires and headfires in prepared fuel beds. Rothermel and Anderson (1966) developed equations to relate fire spread characteristics to fuel moisture and wind speed, and Anderson (1969) used mathematical models to describe heat transfers and fire spread. Project Fire Model conducted at the Macon Laboratory used model fires to characterize fire behavior (Byrum et al, 1966). Field studies under natural conditions, however, have been limited (Mount 1969). Van Wagner (1971) suggests that field research is essential if fire behavior knowledge is to receive practical application.