22 BULLETIN 211, U. S. DEPARTMENT OF AGRICULTURE. 
lower levels into which the arroyos debouch, with a few that have come 
in from the adjacent mesas or ridges. 
The physical factors that determine this distribution relate in some 
way to local water and air drainage. While the upper courses of 
such arroyos are likely to be deep and full of plants, the lower reaches 
are usually dry, broad, flat, gravelly channels, at most but a few feet > 
lower than the surrounding land and practically bare of vegetation 
or occasionally having a crop of range weeds. 
Woodlands.—As used here, the term woodlands refers to those areas 
that are covered with a more or less scattering growth of low trees, 
a plant formation occupying a zone between the grass-covered plains 
and the forest-covered areas of the higher mountains. Typical wood- 
lands occur on the lower parts of the mountains, ranging upward 
from 1,000 to 2,000 feet above the level of the surrounding plains. 
Where these plains are relatively low, as they are in the southern part 
of the State, the wooded areas begin at about 5,000 feet elevation, 
while farther north, where the plains are much higher, the lower limit. 
of woodland is sometimes as high as 7,000 feet. Throughout the 
State the area is characterized by the presence in greater or less_ 
abundance of low scrubby trees and numerous shrubs. Among these 
occur various bunch grasses and numerous herbs during the growing 
season. This plant formation often covers the lower drier ridges and 
mountains to the summits, especially on the southern exposures 
where the zone is nearly always broad, while on the higher mountains 
and especially on steep northern slopes the zone is apt to be narrow 
or sometimes almost absent. 
Forests —Above the woodland zone in the mountains come the 
forests. First, as one goes upward, are the open forests of western 
yellow pine with interspersed parks (the transition zone), then the 
denser forests of pine and Douglas spruce (mainly the Canadian zone), 
and last the dense spruce forests (Hudsonian zone), reaching the 
timber line. These forests exist only because of the rainfall that 
occurs at these levels, and the growing conditions thus produced 
result in an abundant and varied flora, most of which is readily eaten 
by stock. As the elevation increases, the forests become denser, 
darker, and wetter. The growing season also is shortened, as is the 
grazing season, the area above the transition zone bemg mostly 
summer pasture. 
Above the timber line there are some ridges and peaks which have 
a short-lived crop of grass, sedges, herbs, and a few low shrubs, but 
this area is very rarely reached by stock even in the warmest of the 
summer weather. 
Practically all of the lands held by the Federal Government lying 
in the woodland zone and those above it are now administered as 
national forest. 
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