The Historical Role of Fire on the Bitterroot National Forest Research Paper INT-187

Effects of Fire Exclusion

Data from all study areas (tables A-1, A-2, A-3) show a marked decrease in fire since about 1920. Modern, airborne fire-suppression techniques theoretically can result in very effective control of potentially low-to-medium intensity fires. Thus a major ecological force has been largely excluded for half a century, and the years since the last burn on most of the forested land now exceed the longest fire-free intervals recorded between 1735 and 1900 (table 1). In some forest types, this has allowed fuels to accumulate. In many stands were Douglas-fir is the potential climax tree, dense understories of this species have developed, making a ladder of fuels that now endangers even the fire-resistant, old-growth overstory of ponderosa pine, Douglas-fir, and western larch. Throughout the subalpine forest on the Bitterroot, additional heavy fuels have been generated as a result of massive overstory kills of lodgepole and whitebark pines by the mountain pine beetle (Dendroctonus ponderosae) in the 1920's and 1930's.

Also, severe infestations of dwarf mistletoe (Arceuthobium spp.) in Douglas-fir, western larch, and lodgepole pine at various locations on the Bitterroot have contributed to stand stagnation and loss of vigor. Presumably, the effect of dwarf mistletoe adds to the fuel generation rates and thus increases vulnerability of the forest to severe fire damage. This situation is partially a result of fire exclusion. Alexander and Hawksworth (1975) have analyzed current knowledge of the interrelationships between natural fire and dwarf mistletoe infestations, and have commented on possible applications of prescribed fire for control of the parasite. Other possible relationships of continued fire exclusion to insect damage, such as to western budworm in Douglas-fir and mountain pine beetle in overly dense ponderosa pine stands on the Bitterroot, also warrant concern.

Continued fire suppression without fuel management apparently promotes accumulation of fuels as well as greater continuity of fuels over the landscape; this probably favors the development of high-intensity fires.

Means for Reducing Fuels

High fuel loadings eventually will be reduced by decay, fire (wildfire or prescribed fire), or removal. Decay is relatively slow in these cool, dry forests and thus seems inadequate unless augmented by various types of prescribed fire or logging, or a combination. Where resource managers do not establish a fuel reduction program, nature may eventually do so by means of a large, uncontrollable wildfire. Mankind does not generally find this alternative safe or otherwise acceptable. Logging and slash disposal can reduce fuels in many areas. However, in some areas, this may be either uneconomical or unacceptable because of other management goals.

In the latter situation, the prescribed use of fire should be seriously considered for fuel reduction and stand management. Prescribed burning in dense stands on Pseudotsuga habitat types has recently been demonstrated on the Lubrecht Experimental Forest in western Montana (Norum 1976). Also, information from the following sources can supply much of the framework necessary for using fire in such stands:

1. Findings of natural fire studies in national parks (Kilgore 1975) and wilderness (Habeck and Mutch 1973; Mutch and Habeck 1975);

2. Techniques developed for managing fire in wilderness (Aldrich 1973);

3. Other current research of the Fire in Multiple Use Management Research, Development, and Application Program (USDA Forest Service 1975).

Fire could be applied when burning conditions are moderate, and are forecast to remain stable, especially in early autumn.

A burning program would require carefully developed fire prescriptions designed to accomplish management objectives, while minimizing unwanted fires (Barrows 1974). After the initial fuel reduction, maintenance of desirable fuel and stand conditions should become less difficult. A fuel management plan might incorporate both prescribed fire and naturally caused fires burning under certain conditions in specified areas. Natural fires not burning as desired would be suppressed.

Extrapolating Fire History

Resource managers should be able to approximate pre-1900 fire frequencies in various parts of the Montana portion of the Bitterroot National Forest by extrapolating from table 1. Most areas of the Forest probably had frequencies comparable to those found at Tolan and West Fork, except that slopes of the Bitterroot and perhaps the Sapphire Range immediately adjacent to the main Bitterroot Valley may have had rates more like those at Onehorse.

The Forest habitat-type map and recent topographic maps will provide the basic information for assigning the appropriate habitat types in table 1 to the land in question. Analysis of the latest timber-inventory data and timber-type maps will be important for obtaining a knowledge of fuels and stand conditions necessary for developing fire management programs.