17 



seasonal extremes of 90° and —25° F. are rarely exceeded. The 

 growing season is shorter than in either the north coast region or 

 the Sierra region, but longer than in the country farther north. 

 Killing frosts sometimes occur in July and August. Snowfall is 

 not extremely heavy, but during the winter there are occasional pro- 

 longed periods of intense cold. 



CENTRAL ROCKY MOUNTAIN REGION. 



The central Rocky Mountain region is characterized by the pre- 

 dominance of the lodgepole pine type. This forest type is best devel- 

 oped in Wyoming, southern Idaho and Montana, and northern 

 Colorado and Utah. The eastern slope of the Cascades in Washing- 

 ton and the "mountain ranges of northeastern Oregon also belong to 

 this region by virtue of their very similar climates and the general 

 character of their forests. 



Here, as elsewhere, altitude divides the forest into distinct types. 

 The most generally distributed types in this region are as follows,, 

 beginning wath the highest: An alpine type of Engelmann spruce,, 

 alpine fir, and other less important alpine trees, becoming in the 

 low^er portions a nearly pure spruce forest of considerable commercial 

 importance; a lodgepole pine type, covering whole watersheds with 

 pure, unbroken stretches of lodgepole pine, interspersed with occa- 

 sional small pure and mixed stands of Douglas fir ; a Douglas fir type, 

 sometimes in extensive pure stands, but more often mixed with 

 lodgepole pine, Engelmann spruce, and alpine fir above, and with 

 yellow pine below ; and a lowland type of yellow' pine. Besides these 

 there are large, even-aged, pure stands of aspen, which comes in 

 densely on fresh soils after burns. The competition between lodge- 

 pole pine and Douglas fir is decided chiefly by soil moisture. Doug- 

 las fir mixes Avith yellow pine in the moister parts of the lowland 

 type, displaces lodgepole pine on drier soils of the intermediate type,, 

 and mixes in the alpine forest wherever, as on burned areas, it re- 

 ceives light enough to cope successfully for a generation or two with 

 the more tolerant spruce and balsam. In the mountains of north- 

 eastern Oregon and along the eastern slope of the Cascades other 

 species, notably lowdand fir and western larch, form a considerable 

 portion of the stand. 



In this region daily and seasonal ranges of temperature are great, 

 precipitation is small or moderate, and the vegetative season is often 

 less than three months long. The w^inters are long and severe, with 

 heavy snowfall and frequent periods of extreme cold, during which 

 the temperature sometimes falls to —30° or — iO° F. The summers, 

 are hot, and in some parts of the region very dry. The annual pre- 

 58398— Cir. 150—09 3 



