FOREST VEGETATION. 
the range of conditions peculiar to these forest units 
there may be found species which are also dissimilar in 
their climatic requirements. Thus, for instance, sugar 
pine ( Pinus larhbertiana) , which is such a prominent 
component species of the sugar pine-yellow pine-in¬ 
cense cedar forests of California, will not grow in the 
dry, pure forests of yellow pine in Arizona, but on all 
intermediate situations all the species may be found 
growing together and are interchangeable one with 
another. 
All these forests, although apparently different in 
character, in reality partake of the same biological 
characteristics and have more or less similar conditions 
of growth, as do the two principal species which give 
character to these different forests, namely, western 
yellow pine and Douglas fir. 
Pure yellow pine forest (fig. 8).—Of the three great 
types of forests which make up the western yellow pine- 
Douglas fir division, the pure forests of yellow pine 
are by far the most extensive and economically most 
important. They occur on dry, hot slopes or flats, 
ma inl y at low elevations from 3,500 to 4,500 feet, on 
the west slope of the Sierras, and on the east slope they 
occupy the entire timbered slopes, and consist largely 
of Jeffrey pine ( Pinus jeffreyi) , western yellow pine, 
and white fir. Pure yellow pine forests constitute by 
far the greater part of the forest's containing most of 
the merchantable timber in Arizona and New Mexico. 
In the southern part of each of these States the belt lies 
between about 6,000 and 7,500 feet. At its upper edge 
Douglas fir and Engelmann spruce come in, and at about 
9,000 feet dominate the stand. In the northern part of 
New Mexico these forests are nearly 1,000 feet higher. 
The curious fact that these forests occur at lower eleva¬ 
tions in the southern part of the State of New Mexico 
is in some way connected with the level of the adj acent 
plains. Pure western yellow pine stands also occur in 
northern Nebraska at an elevation of 3,000 feet, and in 
the Black Hills at from 3,000 to 5,000 feet; also on 
south slopes and at all lower altitudes in eastern Wash¬ 
ington, Oregon, and central Idaho between 2,500 and 
4,500 feet, and in northwestern Montana and northern 
Idaho at the lower altitudes on warm, sunny exposures, 
low ridges, and rather dry benches and gravelly flats. 
The pure yellow pine forest, except in Washington, 
Idaho, Montana, Nebraska, and the Black Hills, occu¬ 
pies a belt that lies between the pinon-juniper wood¬ 
land or chaparral below and Douglas fir or lodgepole 
pine above. It lies almost wholly within the “transi¬ 
tion zone ” as defined by biologists. It is not continu¬ 
ous, but often includes open grazing lands that give a 
parklike effect to the region. This effect is heightened 
by the comparative absence of underbrush and the 
presence of a more or less dense growth of grass and 
other herbaceous plants beneath the trees. The trees 
themselves are usually large, mature, growing in 
groups, or widely spaced, so that the sunlight reaches 
the ground in all parts of the forest with but little 
interference by the crowns. The typical forest litter 
of leaves, twigs, decaying stems, and branches is absent 
or found in small quantities near the base of the trees, 
and in many regions the only litter beneath the trees 
is a pile of dry cones, the accumulation of a number of 
seed years. These typical western yellow pine forests 
much resemble the open longleaf pine forests of the 
southeast and may, where the surface is not too rocky, 
be as easily traveled. 
A forest fire or logging, as a rule, will not change 
the character of the forest; in other words, its natural 
cycle is western yellow pine following western yellow 
pine. Occasionally, however, after very severe fires, 
particularly at the upper and lower extension of the 
forest, the type of vegetation above or below may 
temporarily encroach on the burned or logged-off yel¬ 
low pine forest. The alternation of aspen ( Populus 
tremuloides) and pine in the Black Hills of South 
Dakota and of chaparral and pine in the southwest are 
good examples of such sequences of growth. 
The climate of pure western yellow pine forests is 
that characteristic of hot and dry interior plateaus and 
mountains. This applies particularly to the Rocky 
Mountain form, which occurs in the central and south¬ 
ern Rockies, and occupies generally drier sites than 
the true Pinus ponderosa. Most of the area has a rain¬ 
fall of from 20 to 30 inches. The forests occur in sit¬ 
uations where the mean annual temperature ranges as 
low as 40° F., and the mean temperatures during the 
growing season from about 50° to 60° F. Maximum 
temperatures of 110° F. are not uncommon. The 
trees withstand winter temperatures as low as —30° F. 
The land occupied by this forest has not been exten¬ 
sively used for agricultural purposes, as it lies at a 
rather high elevation and is characterized for the 
greater part by a short growing season. It occupies 
also uneven and stony soils. It is suitable, however, 
for crops adapted to cool weather and short seasons. 
The open parks in the pure yellow pine forests within 
Arizona and New Mexico offer the best “dry farming” 
possibilites in these States, though but little of the land 
is in cultivation when one considers it in relation to 
the entire area of yellow pine forests. Oats, barley, 
wheat and rye are the principal cereals, while pota¬ 
toes and alfalfa are grown to a limited extent. Vege¬ 
tables also can be grown within this area, although the 
total crop production on land occupied by western yel¬ 
low pine is relatively small. Because of the open char¬ 
acter of the forests and the grassy cover, this land is 
especially adapted to grazing. 
Tlie yellow pine-sugar pine-dncense cedar forest 
(fig. 10).—Next in importance are the mixed stands, 
chiefly of sugar pine, western yellow pine, white fir, 
Douglas fir, and incense cedar ( Libocedrus decurrens ). 
This forest is confined largely to the west slopes of 
the Sierras and to the Coast' Range in California, and 
presents probably the most favorable climatic condi¬ 
tions of the western yellow pine-Douglas fir division. 
Still, the limits of precipitation and temperature are 
not unlike those of the pure yellow pine stands. The 
average temperature during the growing season for the 
region ranges between 44° and 60° F.; for pure yellow 
pine stands it 1 is between 50° and 60°. The annual pre¬ 
cipitation of 20 inches practically limits both the pure 
western yellow pine forests and the mixed yellow pine- 
sugar pine-incense cedar forest, although the best devel¬ 
opment of the latter takes place when the rainfall is 
considerably above 20 inches. This forest varies in 
composition according to altitude, soil, and exposure, 
so that there can be distinguished several fairly dis¬ 
tinct types. Thus, on moderately dry western slopes, 
Figure 11.—Mixed forest—western larch and western white pine. 
(Cedar-hemlock.) The stand is about 140 years old. Western red 
cedar and hemlock are shown in the photograph to come in as an 
understory. Northwestern Idaho. Photographed by Austin Cary. 
and especially on soils of serpentine formation on the 
western slope of the'Sierras, the most prevalent type 
of forest is that of western yellow pine-incense cedar. 
Under the same conditions of temperature and soil, 
but at an elevation of 3,500 to 5,500 feet on the west¬ 
ern slope of the Sierras, it is yellow pine-sugar pine. 
White fir plays a conspicuous part in all these types 
of forest, as well as in the pure yellow pine forests. 
Agricultural development within the area has been 
limited largely by the unfavorable mountainous con¬ 
ditions of the land. 
Rocky Mountain Douglas fir forest . 2 —Just above the 
yellow-pine belt there occur mixed stands of western 
yellow pine and Douglas fir. These are found in the 
central Rockies at elevations from 6,000 to 9,000 feet, 
chiefly on sandstone and granite soils; on the west 
slope of the Sierras on northwesterly slopes at eleva¬ 
tions of from 4,500 to 5,500 feet; and in Arizona at 
elevations between 8,500 to 10,000 feet on the south 
slopes and between 7,500 feet and 9,000 feet on the 
north exposures. On cool northerly and northeasterly 
slopes, at altitudes of about 5,000 to 6,000 feet on the 
west slope of the Sierras and between 8,000 and 10,000 
3 Rocky Mountain Douglas fir is used not as a geographic term but 
to designate a form distinct from the Pacific Coast Douglas fir of 
Washington and Oregon. 
feet in the central Rockies, the mixed stands of yellow 
pine and Douglas fir become practically pure Douglas 
fir stands. Along the lower altitudinal limit of the 
lodgepole pine zone in the central Rockies, on south 
slopes and on dry rocky knolls, the Douglas fir mixes 
freely with lodgepole pine, giving rise to a mixed 
lodgepole pine-Douglas fir stand. 
These several forest types, although distinct in com¬ 
position and even in their altitudinal range, still be¬ 
long to the western yellow pine-Douglas fir series. 
They are all characterized by the presence of Douglas 
fir as one of their principal species. Throughout the 
Rockies Douglas fir and western yellow pine alternate. 
This forest belt receives slightly more precipitation 
than the pure western yellow pine belt. The annual 
precipitation in the Douglas fir zone in the Rockies is 
between 25 and 35 inches, largely in the form of snow. 
The summer and winter temperatures are, however, 
about the same as in the pure western yellow pine belt, 
and in general it is characterized by the same dryness 
of soil. 
Lodgepole pine forest (fig. 9).—The lodgepole pine 
forests are chiefly characteristic of the Rocky Moun¬ 
tains, although they occur also in Idaho, Washington, 
Oregon, and California. In Colorado the best stands 
of lodgepole pine are usually found between 7,500 and 
9,500 feet, in Wyoming between 7,000 and 9,000 feet, 
and in southwestern and central Montana between 
6,000 and 8,500 feet. They occupy usually an altitu¬ 
dinal belt of from 2,000 to 2,500 feet in width. They 
often descend as low as 4,500 feet on northern expos¬ 
ures and go up as high as 11,500, as in Colorado, 
southern Wyoming, and California. This wide alti¬ 
tudinal range of lodgepole pine is due to the fact that 
it is a pioneer tree which invades ground left open by 
other species, chiefly as a result of fires. It occupies 
ground belonging both to Engelmann spruce and to 
Douglas fir. The present extensive lodgepole pine 
forests have undoubtedly been brought about to a 
large extent by burns which enabled the lodgepole 
pine to spread at the expense of Douglas fir and Engel¬ 
mann spruce ( Picea eng el/mannii ). Lodgepole pine 
stands, therefore, occupy any intermediate zone on the 
line between the Douglas fir and spruce. They rarely 
go to the lowest and driest sites occupied by Douglas 
fir and only occasionally become alpine, generally at 
8,000 to 11,000 feet in elevation. The lodgepole pine 
forests belong, therefore, to two divisions, the western 
yellow pine-Douglas fir and the spruce-fir. Those on 
the drier and southerly exposures and at altitudes be¬ 
low the natural altitudinal range of Engelmann spruce 
may be classed with the western yellow pine-Douglas 
fir forests. The bulk of the lodgepole pine forests 
must be placed in the western yellow pine-Douglas fir 
division, since they occur largely in the same climate 
as the western yellow pine and Douglas fir stands. 
Lodgepole pine forests occur where the annual aver¬ 
age precipitation is 18 inches or more. The best de¬ 
veloped stands occur where the precipitation exceeds 
21 inches. This species endures for short periods ex¬ 
tremes of temperature varying from approximately 
100° F. to —55° F. The growing season in lodgepole 
pine areas is short. Killing frosts are likely to occur 
until about the middle of June, and the first autumn 
frost comes in late August or early in September. In 
their requirement for moisture and temperature, the 
lodgepole pine forests stand between Douglas fir, on the 
one hand, and Engelmann spruce and alpine fir ( Abies 
lasiocarpa) , on the other. 
The lodgepole pine forests are economically the most 
important timber belts of that portion of the Rocky 
Mountains lying between northern Colorado and cen¬ 
tral Montana. They are the source of mine timbers, 
converter poles, railway ties for the transcontinental 
railroads, and of lumber to a small extent. 
Western larch-—Douglas fir forest. —In northwestern 
Montana and northern Idaho, on dry benches, as well 
as on southerly and southwesterly slopes, there occur 
mixed stands of western larch, Douglas fir, western 
yellow pine, and lodgepole pine. After burns, western 
larch occasionally forms practically pure stands, espe¬ 
cially on fairly dry ridges and benches. On the more 
favorable sites it is gradually crowded out by the 
western white pine, which in turn is later replaced to 
a large extent by western red cedar, western hemlock, 
and lowland white fir. On the warmer southern slopes 
or the more shallow rocky soils Douglas fir and yellow 
pine enter in mixture with larch. On very dry, steep 
southern and southwestern slopes at lower altitudes 
the larch gives way entirely and western yellow pine 
forms pure forests. 
Cedar-Hemlock Forest. (Figs. 11 and 12.) 
This forest may be divided into two large units: (a) 
The western white pine and larch region of western 
Montana and northern Idaho, and (5) the Pacific 
Douglas fir of the western slope of the Cascades. 
These forests and the large number of minor forest 
types, although they differ in composition, have many 
