18 



cipitation in the Douglas fir zone is fi'oni 15 to 25 inches, largely in 

 the form of snow. In general, snow falls more heavily on the west- 

 ern than on the eastern sides of the mountains. 



SOITTHERX EOCKT MOI'XTAIX EEGI0>7. 



The southern Eocky Mountain region takes in that part of the 

 range of Douglas fir south of central Colorado and northern Utah. 

 It difl'ers from the northern Eocky Mountain region in the absence of 

 lodgepole pine as an important associate, in the gTeat predominance 

 of the yellow pine type, and in the presence of white fir. and, in the 

 extreme south, of Arizona cypress and Mexican white pine. Alti- 

 tudinal forest types, determined principally by moisture, include a 

 woodland type at the lowest elevations, composed of pinons, junipers, 

 scrub oaks, mountain mahogany, and other drought-enduring chapar- 

 ral species : a yellow i^ine type in somewhat moister soils above this ; 

 next a fir type, principally Douglas and white firs, mixed with yellow 

 pine, which these firs replace wherever they can obtain enough 

 moisture; and an alpine type, which gradually replaces the fir type 

 at high altitudes on northerly exposures and is made up of Engel- 

 niann sjDruce. alj^ine and Arizona cork firs, and limber pine. Aspen 

 is more abundant in this region than in the northern Eoclvy Moun- 

 tains, and is a useful associate of Douglas fir, since it is exceedingly 

 active in restocking burns, where its light foliage produces ideal light 

 and moisture conditions for an undergrowth of Douglas fir. 



In the northern paii of this region the climate is similar to that 

 of the central Eoclrr Mountain region. Farther south, however, it 

 becomes more moderate, with a longer growing season, smaller range 

 of tem^^erature. and hea^-ier precipitation: in the Gila Xational 

 Forest, Xew Mexico, the precipitation within the altitudinal range 

 of Douglas fir is from about 20 to SO inches, more than half of which 

 falls in the " rainy season,'' from July to September. The winters 

 are not severe, but there is heavy snowfall. The seasonal range of 

 temx^erature is usually between the extremes of —10° and 95"^ F. 

 The growing season is about six months long. Late frosts occasion- 

 ally do much damage in this region. 



According to Forest Service estimates. Douglas fir as a type forms 

 nearly one-fourth of the forests in the northern Eocky Mountains, 

 but decreases as it extends southward until in southern Arizona and 

 Xew ^lexico it averages less than 1 per cent of all the forest types. 

 On the other hand, Douglas fir stumpage, irrespective of type, is a 

 little more than a quarter of all stumpage in the north, while in the 

 south it aggregates about 14 per cent. 



[Cir. 150] 



