Fires and Forest Succession in the Bitterroot Mountains of Northern Idaho

Abstract

Foresters have recently begun to seek a more intimate knowledge of the natural, successional stages by which forests regain terrain lost by extensive fires or other pronounced denuding agencies. Studies in this field lead to a closer understanding of the factors which control the distribution, composition and density of the present forest, the silvical requirements of the various species which compose the existing forest and of the soil building or soil deteriorating influences which are operative after large fires. It is the author's privilege in this paper to record certain observations on forest succession in the Bitterroot Mountains in northern Idaho, gathered in the course of several years of forest research in that region. This is the territory lying north of the Salmon river between the crest of the Bitterroot divide and the Columbia river plateau. Perhaps no other region in the United States is visited by forest conflagrations of similar magnitude, frequency or degree of destruction. The contributing causes are mainly as follows: the moist winter and spring which give rise to a profuse and luxuriant forest vegetation of a highly inflammable nature; an invariably dry summer with afternoon temperatures often ranging between 95 and ioo degrees F. (350 to 380 C.); the low atmospheric humidity which frequently falls to I5 per cent and lower. These critical conditions are combined with strong, desiccating winds sweeping in from the arid region to the southwest. The fires, therefore, when once under way, travel with great speed and rapidly assume uncontrollable proportions. Not infrequently an entire township (approximately 95 sq. km.) of timber is wiped out in the course of one or two days. These large forest fires kill all of the trees and the seedlings. In from ten to twenty years most of this dead timber lies prone, and it then presents a fire hazard of greater magnitude than existed in the green, virgin forest. The second fire, on this account, is even less controllable and more destructive than the first. The return of the forest to the climax composition subsequent to these devastating fires proceeds along certain well defined steps or stages which may be considered parts of the entire cycle of succession, or the natural process of regeneration. The first steps in this succession begin with the appearance of species intolerant of shade, and capable of withstanding considerable drought and exposure, and concludes with the establishment of 67