[Illustration: Life zones]
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.