OCR Text |
Show replaced in the valleys by a warmer climate. Winter snows are moderate. Total annual precipitation is from 17 to 26 inches, occasionally as much as 32 inches, and sometimes more. 2 Annual mean temperatures usually are between 40° and 50° F., except in a few Transition areas of extreme cold and aridity such as the Upper Green River Valley at Pinedale and Kendall, Wyo. 3 Throughout a large part of the Colorado River Basin, this zone coincides very closely with the open, parklike ponderosa pine forests of the mountains and foothills, but north of the San Juan Mountains, Uncompahgre Plateau, and the Aquarius Plateau, the Transition Zone becomes a partially open region, characterized by sagebrush valleys and grassy slopes, often covered with Gambel oak and other chaparral shrubs. This reduction of forest growth may be due to the fact that precipitation in that region is relatively light even at the higher altitudes, while soil moisture and average relative humidity are further reduced by frequent 2 37.5 on summit of Mount Lemmon, in Arizona ( Schwalen, 1942, p. 442 and map). 3 Martin and Mattice, 1930, p. 12 high winds. 4 In this northern part of the basin the ponderosa pine gradually is replaced by Douglas- fir, but distribution of the latter within the zone often is restricted to steep slopes, shady areas, or the crests of plateaus above 7,000 feet, leaving the major part of the zone to the sagebrush and chaparral. Other plant indicators are the narrow- leaved Cottonwood, Utah oak, bur oak, bearberry, Rocky Mountain birch, Oregon or holly grape, elderberry, maple, locust, and alder. The sagebrush previously mentioned cannot be used as an exclusive indicator of the Transition Zone because it occurs even more extensively in the next warmer zone and sometimes enters the lower limits of the Boreal Zone. Since hardly any two kinds of plants, or animals, have precisely the same climatic requirements, the location of zone boundaries is based upon the presence of not one but a large number of the appropriate indicators. Obviously, therefore, the establishment of sharp lines of division between zones represents merely a compromise among various arbitrary decisions that it is necessary to make in order to draw up an intelligible small- scale map. 4 F. Martin, 1930, p. 2. ' « ta 111 s&* siillWJ' asiiB i$& m Figure 7.- Upper Sonoran Zone- Canyon Lands of southeastern Utah. 8 |