Forest Types and Life Zones

Plants and animals tend to group themselves into definite associations, based primarily on climate, physiography, and soil conditions. Some biologists refer to these natural biotic provinces as life zones. Ecologists, however, prefer a terminology of biomes for the major categories in the classification of communities of interdependent living things—plant and animal—in and with their environment. Many foresters prefer a descriptive name of the vegetative type, possibly supplemented by the name of the corresponding life zone. This latter method is used in this treatise so far as possible.

In the use of the phrase "life zones," it is necessary to remember that the boundaries of these zones vary with conditions of temperature, moisture, exposure, and soil, so that they are quite variable as related to lines of elevation or latitude. There is a merging of the species and types of one zone with those of the adjacent zone along their common boundaries. Thus one author may differ from another in the designation of the life zone where the line of separation is indistinct.

Zones of altitude on a mountain may be compared to the zones of latitude on the surface of the earth which have comparable conditions of temperature, moisture, exposure, and soil. Thus a mountain of great height at the equator might offer at its different elevations the general variations in environment that would be encountered during a journey from the equator to the Arctic Circle. In general, the altitude of timberline, where tree growth stops, decreases as the latitude north increases.

Because of the marked differences between vegetative types of the Western and Eastern United States, these two sections of the country are discussed separately.