THE FORESTS. 15 



are grassland; and somewhere between, the forest and the grassland 

 merge; also that the northern and eastern slopes are wooded to the 

 base, while the others are bare. These features are purely a response 

 to the distribution and storage of soil waters. Southern and western 

 slopes exposed to sun and wind are too dry for the growth of trees; 

 the northern and eastern exposures are more favorable, for there the 

 snow lies later in the spring, the heat is less intense in summer, and 

 at all times evaporation proceeds more slowly. The drier slopes, if 

 wooded at all, are usually clothed sparsely with yellow pine and 

 Douglas spruce ; the other slopes with larch and Douglas spruce and 

 lodgepole pine. Thus it appears that not only the features of local 

 forest distribution are under topographic control, but also the qpm- 

 position of the forests and the proportions of the various species. 



One is impressed by the fact that the forest species themselves 

 differ much in their requirements. Some are like the meek ; they are 

 modest in their demands and they inherit the earth ; others maintain 

 a sort of aloofness, circumscribed by rigid demands of their own 

 organization to a particular set of conditions. As an example of the 

 latter is the western white pine ; limited by requirements of moisture 

 in soil and air, by a scant production of seed and by peculiar habits 

 of growth from the seed, by a relatively narrow range of favorable 

 temperatures, by a marked susceptibility to disease and to injury by 

 fire, its distribution is restricted to the Cascade Mountains and 

 especially favorable situations in the northern Rockies. On the 

 other hand its relative, the yellow pine, easily satisfied with soil 

 and moisture conditions, enduring a temperature range of 160 de- 

 grees or more, early and prolific in bearing seeds of good vitality 

 and germinative power, resistant to fire and disease, is found from 

 the British Possessions to Mexico and from the Black Hills to the 

 shores of the Pacific, 



Thus might be summarized the nature and habits of any forest 

 species, and it would be found that such facts bear an intimate 

 relation to its geographic distribution ; and conversely the distribu- 

 tion of a species indicates the range of its requirements and adapta- 

 bility. 



Among the most significant and interesting facts of forest distri- 

 bution are what are known as vegetation cycles or succession of types. 

 A given forest may not be the permanent or final type of forest 

 which is to occupy the area. An example may be found in the 

 transition from grassland to heavy forest evident in various places 

 in the intermountain region. The prairie is gradually invaded by 

 the forest and ultimately yields to it fully. The pioneer in this 

 invasion is the common yellow pine, closely followed by the Douglas 

 spruce. Through centuries the forest increases, and the soil, enriched 

 by the deposits of vegetable matter, becomes more capable of retain- 

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