J. A. Larsen

Fires and Forest Succession in the Bitterroot Mountains of Northern Idaho

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 authors 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

Relation of Leaf Structure of Conifers to Light and Moisture

Trees range from shade-enduring to light-demanding (in forestry designated tolerant and intolerant) according as they thrive in degrees of light from dense shade to full sunshine. Also, species which are most shadeenduring require more moisture and a location characterized by lesser extremes of temperature or moisture than the light-demanding species. Examination of form and development of trees of different tolerance and moisture requirement, such as may be observed in northern Idaho, reveals pronounced differentiation in form of root system, thickness of bark, and crown development. These differences are evidently natural adaptations which the tree has acquired in response to the demands of its environment. In the study of such adaptations and differentiation we naturally turn to the leaf first of all, since this is the most specialized organ in respect to environment, and because it is the seat of photosynthetic activity. Cross-sections of leaves were prepared for a microscopic study of the various tissues, which were expected to indicate structural or other differences in the leaves now classified as tolerant and intolerant, moisture-loving or less moisture-loving. All leaf sections used were apparently of normal representative leaves picked from the leader of the tree, and cut from the center of the leaf. The species of trees studied are as follows (placing the most shade-enduring and moisture-demanding first):

Effect of Removal of the Virgin White Pine Stand Upon the Physical Factors of Site

It has long been a matter of speculation to what extent the complete or partial removal of the dense virgin timber alters the air and soil temperature in relation to humidity, soil moisture, and evaporation. Information on this subject would be of value in explaining the survival and growth of seedlings under different conditions, and might also be of use in a study of invasion or succession of vegetation after cutting and in fire protection. In order to secure such data, therefore, the U. S. Forest Service on July 12, i919, installed three meteorological stations near the Priest River Forest Experiment Station in northern Idaho. Since it was possible to keep the records at these stations for only a limited period, July and August, the months which are the hottest and most critical part of the growing season, were selected. Much credit is due Mr. Herman Bauman, Forest Service Field Assistant in i919, for his conscientious care and painstaking accuracy in obtaining the records. The stations were installed on the Binkly sale area, near the Priest River Forest Experiment Station. Station A was located on land cut clear in i909I9io, about 250 feet south from the edge of the 300-year-old western white pine stand. On this site there are a few down logs and stumps, and practically no forest reproduction and no trees. Station B was placed about 400 feet north well within the tract logged in i9i8, where there were several large western hemlocks and western red cedars, which are usually associated with white pine and made about one third forest cover. There was considerable unburned slash on the ground, but none right at the station. Station C was placed well within the uncut virgin timber, at least 300 feet from the opening on the south side. Each station was equipped with maximum and minimum air thermometers of the Weather Bureau pattern, and Stations A and C had air thermographs and soil thermographs. These instruments were placed 4? feet above the ground within a box made of half-inch board, painted white on the outside, and open to the north. At each station there was one psychrometer and one Livingston porous cup cylindrical atmometer. The latter was placed 6 inches above the ground; the bottle containing the water was placed entirely below the surface. Daily soil temperature readings and semi-weekly soil moisture samples were obtained at a depth of 6 inches near each station on a bared plot of

Association of Trees, Shrubs, and Other Vegetation in the Northern Idaho Forests

It is the purpose of this paper to set forth the association of the most common herbs, shrubs, and trees in the forests of northern Idaho, in the hope that the information may afford a better insight into the quality and general characteristics of the habitats. The herbs and shrubs listed were collected at the Priest River Forest Experiment Station of the U. S. Forest Service. The writer is much indebted to Drs. C. H. Shattuck and E. L. Greene, Messrs. E. C. Rogers and H. P. Rigdon, and Mrs. Agnes Chase for their help in the work of collection and identification. The forests of northern Idaho occur in altitudinal belts often spoken of as forest types. At the lowest elevations, bordering on the Columbia River Plateau, the forest is usually of pure western yellow pine. Higher up and on more uneven ground there is western larch and Douglas fir with some western yellow pine in mixture. Still higher and on more broken and rough topography the forest is composed of western white pine, western hemlock, western red cedar, and lowland white fir. Above this again are the subalpine forests of mountain hemlock, alpine fir, Engelmann spruce, and lodgepole pine. These altitudinal differences in the composition of the forest are, of course, the result of differences in air temperature and precipitation. The annual precipitation in or near the yellow pine forest is from I8 to 20 inches, and that in the forest of western white pine and associates from 27 to 35 inches. The mean growing season, May to September, inclusive, in the yellow pine forest shows a temperature from 6o0 to 650 F., and from 54 to 6o degrees in the white pine forest. (Data from the U. S. Weather Bureau Cooperative Stations.) One very marked feature of the climate in this region is the occurrence of abundant precipitation during the colder months, and of a pronounced drought in summer, particularly in July and August. This dry period is further intensified by the warm, dry winds which blow from the Spake River Desert region. It must not be supposed that the forest belts show a clear-cut debarkation from one type to the other. The transition zones are sometimes as wide as the belts themselves. There is much overlapping according to variations in 63