Introduction xv 



northern Canada, where it merges with the Northern Forest 

 region. It is bordered on the east by the Great Plains and on 

 the west by the Pacific Coast forest. The forests are not con- 

 tinuous in this region, but are Hmited to the more humid higher 

 areas which are interspersed between large treeless stretches 

 occupying the arid lowlands. 



The forests characteristically are grouped in conspicuous 

 life zones or belts determined by the moisture, temperature, 

 and topography. These zones tend to increase in altitude as 

 they become more remote from polar regions, so that trees 

 typical of the Northern Forest in northern Canada at eleva- 

 tions of a few hundred feet above sea level might be found at- 

 elevations of 8,000 feet in Montana, 10,500 feet in Colorado, 

 and 12,500 feet in Arizona. In the higher, colder, and more 

 humid belt, often called the Canadian zone, are dense forests 

 of Engelmann spruce, alpine fir, western white and lodgepole 

 pine, aspen and balsam poplar, and many other species closely 

 resembling those found in the Northern Forest. Below this 

 belt, in the transition zone, the characteristic tree is ponderosa 

 pine, together with less important species such as western larch, 

 Douglas-fir, various oaks, narrowleaf cottonwood, and water 

 birch. This zone is warmer and drier, being intermediate be- 

 tween the humid Canadian zone and the arid Sonoran zone. 

 The Sonoran zone is semi-tropical in nature and ranges from 

 the high plains and foothills in the south to low plains and 

 valleys further north. It is typified by pinyon-juniper forests, 

 broad-leaved cottonwoods, mesquite, and numerous other 

 shrubby tree species. 



PACIFIC COAST FOREST REGION 



Many highly important trees of very large size and growing 

 in dense stands characterize this region, which contains the 

 major portion of our remaining saw timber. This region, bounded 

 on the east by the Rocky Mountain Forest, includes the Pacific 

 Coast states plus the coastal portions of British Columbia and 

 Alaska. Numerous forest types are recognized which include 

 one or more of such species as the coast redwood, Douglas- 

 fir, ponderosa pine, sugar pine, Jeffrey pine, western hemlock, 

 true firs, cedars, spruces, cypresses, red alder, maples, myrtle, 

 and oaks. While some of the southern portions of this region are 

 dry, the great part of the coastal forests lies in a zone of heavy 

 rainfall. The largest trees in the world (giant sequoia), the 

 tallest trees in the world (redwood with a maximum height 

 of 364 feet), and the most important timber species in the world 

 (Douglas-fir) are found in this region. 



