THE FORESTS. 13 



indigenous, supposed to be relicts of a previous age when, owing to 

 a more humid climate, they were more widely distributed than now. 

 Among such are the Engelmann spruce, alpine fir, yellow and lodge- 

 pole pines, all very common and widely distributed species. 



Thus it will be seen that in the northern Rocky Mountain country, 

 the eastern and western floras overlap, and there, mingled with both, 

 are the remnants of an ancient indigenous flora and the traces of an 

 Arctic migration. 



Among the several climatic factors which influence vegetation 

 undoubtedly the most important are rainfall and evaporation. Tem- 

 perature is likewise important, but within the area under discussion 

 its influence is seen more in the distribution of species within certain 

 limits of altitude than of latitude. Important as the annual rain- 

 fall is in determining the character and extent of the forest, more 

 important still is the seasonal distribution of rain. The forests of 

 the Ohio Valley are deciduous, broad-leaved woods of oak, hickory, 

 maple, and other species demanding ample supplies of water, which 

 are provided in an annual precipitation of 40 to 50 inches, much of 

 which falls in the summer. Here also is a relatively low rate of 

 evaporation. On the other hand, the forests of western Oregon are 

 mainly coniferous and evergreen; the total rainfall is 50 to 100 

 inches, but it occurs mostly outside the growing season. The sum- 

 mers are usually too dry for anything but hard-leaved trees, except 

 where soil waters are near the surface. The Rocky Mountain region 

 is, in these respects, similar to the west coast, especially in its most 

 heavily timbered sections. 



Contrary to a somewhat prevalent theory, the forest follows the 

 rain; not the rain the forest. There is no evidence at present that 

 the forests have any influence on precipitation. The relation of 

 forest crops to climate has long been an object of thorough research 

 in Germany, and many facts are now well established. 



The southeastern States of the Union have a rainfall of 50 to 70 

 inches a year. East of an irregular line from eastern Texas to Maine 

 is a region having 40 to 50 inches a year. Westward the annual 

 rainfall decreases to 10 to 20 inches, including the plains country, 

 roughly from Canada to Mexico and from the eastern line of the 

 Dakotas to the Cascade Mountains. The higher altitudes of the 

 Rockies are favored with somewhat more, and are marked by the 

 heavier forest formations. Fifteen inches a year is about as little 

 as will allow forest growth, except by the mitigation of certain fac- 

 tors, as lower temperatures, less wind, higher relative humidity, and 

 lower rate of evaporation. The areas of lowest precipitation are 

 either prairie or desert. 



Throughout the mountain country there is more or less definite 

 relation between topography and local forest distribution. Tho 



