||UTAH VEGETATION â€" FACTORS AND TYPES 7 Utah Vegetation Factors We can perhaps visualize the changes that have occurred in Utah vegetation during the past century if we consider briefly first the nature of Utah plant life and the factors responsible for its distribution. Our state is extremely rich both in the number of plant species and in the types of communities in which they are assembled. Good roads and trails today make it possible for one to pass through all of these vegetative types of Utah in a single day. In so doing the traveler traverses all of the principal plant communities to be found in North America, from the semi-tropical heat of Mexico to the eternal snows of the Arctic. Several factors contribute to this phenomenon. First, a favorable latitudinal position plus an alti-tudinal range from 2,500 feet to more than 13,000 feet provide here all of the temperature zones of North America exclusive of the tropics. Second, precipitation varies from an annual total of less than five inches west of the Great Salt Lake to nearly ten times this amount in the high Wasatch Mountains seventy miles to the east. Third, soil and topography are extremely variable. The hydrogen ion concentration of some alkali soils of the desert is as low as pH9 and as high as pH4 in some mountain soils, which means that the soils in the Sphagnum bogs at the head of the Provo River are a hundred thousand times more acid than the heavily alkaline soils around the Great Salt Lake. Hundreds of square miles of the Great American Desert lying west of the Great Salt Lake consist of colloidal clays so heavily impregnated with salt as to preclude even the most salt tolerant seed plant. And from this extreme and in remarkably close juxtaposition are gradations of soil leading to the rich black loam of verdant mountain slopes. All of these factors of climate an,d soil have operated throughout the ages in assembling plants of similar requirements into definite communities, usually dominated by one or a few species, and thus it is possible to recognize rather permanent and particular association types. Proceeding from the semi-tropical desert of Utah's Dixie to the perpetual snow of our mountain summits, the principal plant communities are the following. Types I. Desert Vegetation Types A. Warm Temperate Desert Shrub Utah's only claim to the semi-tropical climate is the Dixie area of Washington County. There, elevations are between 2,500 and 3,500 feet. The vegetation is similar to that of the warm deserts of southern Arizona, New Mexico and Texas. It consists of broad-leafed evergreen shrubs, such as the creosotebush, a rich variety of cacti, the Joshua tree and other yuccas. Mesquites flourish in the richer bottom lands.