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