NATURAL VEGETATION. 
Figure 5.—An open oak stand (Chaparrai) merging with California grassland. It is characteristic of most of 
the foothills and interior valleys of California and is found also in Arizona. Sequoia National Forest, Tulare 
County, Calif. Photographed by Alfred Gaskill. 
age, and poor soil drainage of the 
wetter portion are, in large part, re¬ 
sponsible for their treeless condition. 
Repeated fires undoubtedly pushed 
the forest back and extended its 
boundaries farther eastward. For¬ 
ests have begun to work westward 
since the settlement of the prairies 
has broken up the virgin sod and pre¬ 
vented to a great extent the occur¬ 
rence of prairie fires. West of the 
prairies lie the plains, which have 
always separated the two great forest 
regions, since the moisture is insuffi¬ 
cient to develop forest growth. 
The western coast forest is an ex¬ 
cellent example of the effect which 
rainfall, here almost entirely of the 
cyclonic type, combined with topog¬ 
raphy, altitude, and temperature, has 
upon the forest. Nearly the entire 
region is mountainous, with a steep 
and high coast range almost at the 
water’s edge. The prevailing winds 
are westerly and northwesterly, but 
the chief rain-bearing winds are 
southeasterly, southerly, and south¬ 
westerly, under the influence of pass¬ 
ing cyclones, which are most frequent 
in winter and cause the strongest 
winds and heaviest rainfall in that 
season. Moisture thus brought in 
from the relatively warm ocean far to the south and 
southwest yields in the north over both mountains and 
valleys, and in the south principally over the moun¬ 
tains, low-hanging clouds, and much rain. Precipita¬ 
tion is heaviest in the northern part of the area, where 
cyclonic centers pass more frequently than in the 
south, and heaviest on the western flanks of the ranges, 
where the air is forced to rise over the mountains and 
to drop its excess moisture because of the expansional 
cooling resulting from its ascent. The eastern slopes 
are comparatively dry because the air is there descend¬ 
ing. Such moisture as passes over one range unpre¬ 
cipitated helps to cause rain on the range next to the 
east if the ascent there is great enough. 
Summer conditions are radically different, especially 
in the southern part of the area. The prevailing wind 
on the coast is westerly and northwesterly, and, com¬ 
ing from the relatively cool waters of more northern 
latitudes, brings only moisture enough to cause a belt 
of frequent fog and cloud limited almost entirely to 
Figure 6.—Chaparral composed principally of small-sized oak, 
ceanothus, and manzanita, characteristic of the foothills of southern 
California. It ranges from an impenetrable thicket of low shrubs to 
open oak stands. A fire line in the center. Southern California. 
Photographed by E. A. Sterling. 
the ocean slopes of the coast ranges. Summer cyclones, 
comparatively weak and few in number, bring a little 
rain to the outer coast in the north. In the south, 
where such cyclones are rare and rain is rarer still, 
the prevailing onshore winds are thoroughly “ dried ” 
over the sunny land, especially in the valleys, while 
an occasional hot wind from the continental interior 
still further emphasizes the dryness of the summer 
season. Thus the region is one of winter rains and 
summer droughts, except along the northern coast. 
This combination of wet winters and dry summers 
determines the location and character of the forests. 
On the lowlands and valleys of the south the hot 
summers and the scant winter rainfall make forest 
growth impossible, leaving the land either to grasses 
or to desert shrubs which can adapt themselves to 
semiarid conditions. The mountains, having a precipi¬ 
tation that is greater and of longer duration, with much 
less evaporation, furnish better growing conditions, 
and usually are forested. Growth is in proportion to 
the altitude up to an elevation of from 8,000 to 9,000 
feet. Above this, the winds and the severity of the 
winter, and possibly the decrease of precipitation, make 
conditions adverse to dense forests, the timber line 
occurring usually at about 10,000 feet. 
It is a noticeable fact that nearly all the principal 
mountain ranges have their heavy timber on the slopes 
exposed to the prevailing wet winds, while the opposite 
sides are usually only scantily clothed with timber. 
This is most strikingly shown in the Sierra Nevada. 
The rain-bearing winds, striking the westerly slopes, 
precipitate a part of their moisture before they cross 
the summit. Thence as they travel downward and 
become heated they cease to precipitate and even absorb 
what little moisture there may be on these slopes. As 
a result, the eastern slopes are in general very different 
in character from the western, the latter receiving 
enough rain and snow to support a strip of magnificent 
forest. In western Washington and Oregon, on the 
other hand, and along the northern half of the Cali¬ 
fornia coast, the rainfall in winter and the fog and 
cloud in summer are unusually heavy, and as a result 
the forest growth is very dense, particularly in the 
redwood belt of California. 
The Rocky Mountain region, being nearer the in- 
interior, receives less rain and snow than the Coast 
mountains and the Sierra Nevada, and the forest has 
a severer struggle with the elements. In the northern 
portions the forest is dense and well developed, but in 
general, as a result of unfavorable condition of growth, 
it is broken up into smaller areas and is more open 
than the typical Pacific coast forest. The southern 
Rockies derive much of their scanty moisture from 
the Gulf of Mexico. 
Since hardwoods require for their development a 
high humidity and rainfall, especially in the grow¬ 
ing season, they are largely absent from the western 
forest. The long, dry summer seasons of the western 
part of the continent prevent their development and 
yield the land to more hardy conifers. In the wet 
coast country the hardwood growth is more thrifty, 
and many species, such as maple, elder, oak, ash, and 
chinquapin, reach merchantable size; but, in compari¬ 
son with the magnificent coniferous growth, they con¬ 
stitute an insignificant part of the forest. 
GRASSLAND. 
Lying between the western and eastern forest belts, 
and extending from Canada on the north to Mexico on 
the south, is the great grassland area, broken only by 
river courses and occasional buttes or low mountains. 
Grasslands characterize areas in which trees have 
failed to develop, either because of unfavorable soil 
conditions, poor drainage and aeration, intense cold 
and wind, deficient moisture supply, or repeated fires 
Grasses of one kind or another are admirably suited to 
withstand conditions of excess moisture, excess drought, 
and fires which would destroy tree growth. 
Grasslands usually are well supplied with water in 
the surface soil during the growth period and do not 
depend to any extent upon deeply stored soil moisture. 
In fact, water in the deeper soil usually enables taller- 
growing plants, such as shrubs or trees, to replace the 
grasses. Grasses are, therefore, characteristic of re¬ 
gions of summer rainfall. A great profusion of showy 
flowering plants are usually found within the grass¬ 
lands, and they often present a varied appearance 
during the early part of the season, due to plants which 
later play no very important part in 
characterizing the areas. 
DESERT SHRUB. 
Between the Rocky Mountains on 
the east and the Cascade-Sierra on the 
west, and extending from the Cana¬ 
dian to the Mexican boundary, lies 
the great inland desert, characterized 
largely by xerophytic shrubs. The 
deserts occupy the Great Basin, except 
for isolated mountain forests, most of 
the drainage basins of the Columbia, 
the Snake, the Colorado, and the Rio 
Grande, and small areas east of the 
Rockies drained by the Big Horn, 
Platte, and Arkansas rivers. In the 
south, near the Mexican boundary, the 
desert broadens out, reaching from 
the Pacific nearly to the Gulf of 
Mexico. It is interrupted here and 
there by intrusions of grasslands or at 
higher elevations by forests. This 
whole area, is characterized by a de¬ 
ficient rainfall and an excessive evapo¬ 
ration rate. The perennial vegeta¬ 
tion which gives character to the 
deserts consists principally of shrubs. 
In the south succulent plants become 
more important. The relatively small 
size of the plants, together with their 
small leaves well protected against 
excessive transpiration, the small 
amount of growth produced each year, and the wide 
spacing enable these plants to conserve the scanty 
moisture supply and continue their growth during the 
long rainless periods. Such dry periods may occur at 
any time of the year. The loss of moisture from desert 
or dry-land soils is almost entirely due to plant trans¬ 
piration. A relatively small quantity is lost from the 
surface of the bare ground between the plants, except 
surface water immediately following rains. Wide 
spacing is, therefore, one of the most effective means 
of conserving a scant moisture supply. 
DETAILED DESCRIPTION OF THE FOREST 
VEGETATION. 
THE WESTERN FOREST REGION. 
The western forests, with all their variety of spe¬ 
cies, their physiographic and climatic differences, can 
be classified into two units of woodland and three large 
Figure 7.—Pinonand one-seeded juniper forest. (PiRon-juniper). 
Characteristically open. The lowest belt of forest in most of the 
southwestern part of the United States. Santa Fe, N. Mex. All 
photographs not otherwise acknowledged were taken-by H. L. Shantz. 
natural units of timberland. The woodland divisions 
are: 
Chaparral, or southwestern broad-leaved woodland. 
Pinon-juniper (Pinus-Juniper us ), or southwest¬ 
ern coniferous woodland. 
The timberland divisions are: 
Western yellow pine-Douglas fir ( Pinus-Pseudo - 
tsuga ), or western pine forest. 
Cedar-Hemlock (Thuja-Tsuga ), or northwestern 
coniferous forest. 
Spruce-fir {Picea-Abies ), or northern coniferous 
forest. 
These divisions correspond mainly to distinct re¬ 
gions, yet different divisions, often several of them, 
may occur within the same region but on different 
slopes or at different altitudes. Each is practically 
always associated with a distinct climatic belt. The 
description of these main divisions of forest vegeta¬ 
tion begins with the belt which altitudinally is the 
lowest and therefore the driest. Each successive alti¬ 
tudinal belt corresponds to a moister and cooler cli¬ 
mate. This is true of all the five divisions, with the ex- 
