14,431?

Alpine

11,600?

Sub-Alpine

10,000?

Montane

8,000?

Foothills

5,500?

Plains

3,500?

Climate, which is a composite of prevailing temperature, length of season and average moisture, is the chief factor in deciding where plants of any given species can grow and propagate. Soil type also plays a part, and if extremely unfavorable may totally exclude some species of plants from a large and otherwise favorable area, but in general, soil is the minor factor. In Colorado, climate is largely determined by altitude, so here, as we pass from one elevation to another, we find plant life arranged in horizontal layers or zones of the sort illustrated in the above sketch. The thinness of air, in the sense of less oxygen per cubic foot of air, that goes with high elevation, seems in itself to have little effect on plant life, but the prevailing cold, the long period of snow cover, and the increase in annual precipitation, that go with elevation in our mountains, do have a profound influence on plant growth. High latitude has much the same effect as high altitude, so that the timberline conditions we find in Colorado at from 11,000 to 12,000-foot elevations are very similar to those existing at sea level near the Arctic Circle. Growing conditions, and prevailing plant species, at these widely separated places, are, for this reason, much alike.

These zones of life have no sharp boundaries, but tend to intergrade into each other. Many species of plants normally inhabit parts of two or more zones, and local conditions may so influence climate that particular species of plants will be found growing at lower elevations, or at higher, in one part of the state than in another. Generally, however, in Colorado like elevations result in plant populations of quite similar makeup, even though a whole range of mountains or a deep wide valley may lie between. The principal factor causing exceptions to this rule is the tendency of many areas in western Colorado, particularly those between about 6,000 and 10,000 feet in elevation, to receive greater average annual precipitation than is received by corresponding areas east of the Continental Divide. As a result of this, many species which in eastern Colorado occur only in moderately high elevations will be found clear down in the foothills in western sections.

The individual life zones of Colorado are illustrated and described on the next five pages.