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THE NATURAL VEGETATION OF THE UNITED STATES. 
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INTRODUCTION. 
The United States, which includes within its borders 
a wide range of climatic, physiographic, and soil dif¬ 
ferences, presents a very great diversity in natural vege¬ 
tation. The vegetation ranges from the deciduous for¬ 
ests of the East to the sparse grasslands of the Great 
Plains, from the alpine meadows among the snow¬ 
capped peaks of the Rockies, Sierra, Cascade, and 
Olympic ranges to the subtropical forests of southern 
Florida, and from the luxuriant forests of the North¬ 
west to the almost barren deserts of southeastern Cali¬ 
fornia. It is difficult to present a clear and coherent 
picture of such complex vegetation. Of the several 
proposed classifications some are based on purely geo¬ 
graphic lines, other groupings are based on physio¬ 
graphic regions, and still others on the requirements 
of temperature or water. The present description is 
based largely on the distinctive features of the vege¬ 
tation itself. 
In classifying the vegetation no attempt has been 
made to correlate it with geography, physiography, or 
with climatic or other physical factors. The natural 
vegetation is the expression of environment, it is the 
integration of all climatic and soil factors, past as 
well as present, and, therefore, if it can be distinctly 
and clearly indicated, provides often a better basis for a 
classification of environments than any one factor or set 
of factors. The forms of vegetation here described are 
not merely aggregations of species but are biological 
communities characterized by certain similarity in their 
biological aspect, in their environment, in their past 
history, and in their ultimate development. The 
biological unit is thus made the basis of classification 
and the environment is measured in terms of vegeta¬ 
tion, and not the vegetation in terms of temperature, 
moisture, evaporation, or any other factor. Since the 
attempt to correlate the vegetation with any one factor 
or set of factors has been avoided, the temptation to 
force the vegetation to correspond to the assumed con¬ 
trolling factors of its distribution has been done away 
with and the establishment of real differences in the 
vegetation itself made easier. 
The natural vegetation of a country, when properly 
analyzed and classified, may serve a very concrete and 
practical purpose. As a new country becomes settled 
the natural vegetation must be replaced gradually by 
agricultural crops, orchards, pastures, and man-made 
forests. The suitability of the virgin land for va¬ 
rious crops is usually indicated very clearly by the 
natural vegetation. After a correlation is established 
between the different forms of natural vegetation and 
various agricultural or forest crops, it provides a 
means of dividing the country into natural regions 
of plant growth, which can be used as indicators of 
the potential capabilities of the virgin land for agri¬ 
culture and forest production. 
In preparing the accompanying map, published 
vegetation studies and maps, local, floras, soil, geo¬ 
logical, land, military, and biological surveys, and 
Forest Service maps and reports have been consulted. 
The selected bibliography does not cover the sources 
from which this map is drawn. It is chosen to sup¬ 
plement the information here presented and lack of 
space alone has prevented the inclusion of many im¬ 
portant papers. A large number of persons inti¬ 
mately acquainted with different regions have freely 
given advice and criticism. 1 Although the aim has 
been to make the map as accurate as possible, it must 
still be regarded as preliminary. If it will stimulate 
further and more detailed studies in the classification 
of the natural vegetation by states or smaller units, 
the work of preparing it will be fully justified. Such 
studies will make possible the preparation at some fu¬ 
ture time of a more accurate and detailed map of our 
natural vegetation. On a map of small scale and with 
lack of abrupt vegetation changes, the plant cover of 
certain localities, especially on the border line between 
two types, might be classified differently by different 
persons. It is necessarily a generalized map, and 
many may find that the vegetation represented does 
not exactly tally with that with which they are inti¬ 
mately familiar in certain localities. An attempt is 
iThe authors are particularly under obligation to the following 
persons whose sympathetic criticism and suggestions greatly helped 
them in their work: A, B. Aldous, L. H. Bailey, O. E. Baker, C. It. 
Ball, C. G. Bates, H. H. Bennett, S. F. Blake, F. E. Clements, F. 33. 
Cobb, John S. Cole, E F. Frothingham, F. C. Gates, David Griffiths, 
Roland Harper, A. S. Hitchcock, E. R. Hodson, T. H. Kearney,. C. F. 
Korstian, W. E. McLendon, C. F. Marbut, F. J. Marschner, W. R. 
Mattoon, Edw. N. Munns, G. A. Pearson, R. L. Piemeisel, C. W. Piper, 
J. F. Preston, J. N. Rose, W. E. Safford, W. N. Sparhawk, J. W. Stokes, 
Geo. B. Sudworth, I. E. Tidestrom, J. J. Thornber, R. H. Weidman, and 
E. O. Wooton. 
made only to indicate such vegetation as gives charac¬ 
ter to the area, necessarily omitting smaller areas of 
type differing from that of the region. In discussing 
each unit of vegetation the more important variations 
have been considered, although they could not be indi¬ 
cated on the map itself. Not the least difficult task 
in preparing the map has been the classification of the 
vegetation and the determination how far the map 
should show subdivisions. Gradual changes in vege¬ 
tation, such as occur in passing from the humid prairie 
to the arid grassland of the high plains, are as im¬ 
portant as the more abrupt ones. Where such changes 
occur a wide transition zone is found. Transition 
zones have not been shown on the map, but the types 
have been separated by definite lines indicating as 
nearly as possible the division between the two areas. 
GENERAL OUTLINE OF THE VEGETATION. 
The vegetation of the United. States may foe broadly 
divided into forest, grassland, and desert shrub. 
The forest vegetation forms two broad belts, one 
extending inland from the Atlantic Ocean and the 
other inland from the Pacific Ocean. The eastern 
is relatively continuous, while the western is broken 
by many interspersed areas of nonforested land. In 
the region east of the Cascade-Sierra the forests are 
confined largely to mountain tops and high plateaus. 
The eastern portion of the western forest extends down 
NATURAL VEGETATION 
ORIGINAL AREAS OF THE ^AJGR DIVISIONS 
TOTAL AREA OF THE UNITED STATES 
1903 MILLION ACRES 
Figure 1.—The relative original areas of major divisions of vegeta¬ 
tion in the United States. 
over the Rocky Mountain range and divides the unfor¬ 
ested portion of the United States into two broad 
strips, one of which lies east of the Rocky Mountains 
and the other between the Rockies and the Cascade- 
Sierra. Of these two belts the eastern constitutes the 
great grassland area, while the western constitutes the 
desert-shrub area. The desert area broadens out to¬ 
ward the south, and along the Mexican boundary forms 
a continuous strip extending from the Pacific Ocean to 
the Gulf of Mexico, interrupted only here and there at 
higher elevations. 
FORESTS. 
The forests of the United States include a variety 
of economically useful timber trees hardly equaled 
anywhere in the world. In the Tropics the forests are 
richer in species; in Russia there are larger areas of 
contiguous forests; hut nowhere els© can on© find such 
a vast area of forests combined with such a large num¬ 
ber of species. Leaving out of consideration species 
of limited distribution, such as those of semitropical 
origin found along the coast in southern Florida and 
those which overlap from the Mexican flora, as well as 
all species which do not exceed 1 foot in diameter, 700 
or more arborescent species are known to occur within 
the limits of the United States. Of these, not less than 
100 species are of recognized economic value and about 
200 species may be considered in forest management. 
The forests of the United States are divided into the 
Eastern or Atlantic and the Western or Pacific, which 
are effectually separated in the central part of the con¬ 
tinent by grassland that acts as a barrier between 
the species of the two regions even more effectively 
than, a body of water of the same extent. In the Brit¬ 
ish possessions, north of the fiftieth degree of latitude, 
the two forest divisions come together in one great 
stretch of continuous subarctic forests, extending from 
the Atlantic to the Pacific. In the south the forests 
of the eastern and western regions are also united by 
a narrow strip of forest peculiar to the plateau of 
northern Mexico and possessing features common to 
both regions. 
The Eastern forest is essentially broad-leaved in 
composition and of unbroken distribution, covering both 
valleys and mountains, while the Western forest, which 
is distinctly coniferous in character, is characterized 
by abrupt changes in type, and is interrupted by tree¬ 
less valleys. Excluding tropical species, the Eastern 
forest has 200 species of broad-leaved trees, many of 
which cover large areas and are of immense value, 
while the Western forest has only about 100 broad¬ 
leaved species, few of which are of any considerable 
value. On the other hand, the Westerh forest has 65 
species of conifers and the Eastern but 29. It is inter¬ 
esting to note that only a few species {Abies dalsamea, 
Picca ccmadensisi Picea mariana , Larix laricina ) are 
common to both regions. 
The Eastern forest was formerly unbroken and com¬ 
prised more than 1,000,000 square miles. Although 
an estimate of the present area of the Eastern 
forest can only be approximate, it may be said with 
some certainty that out of the original seemingly 
boundless supply there is left now, after a little more 
than 100 years of settlement, not more than 260,000 
square miles of merchantable forest lands, about 500,000 
square miles having been cleared for farm lands and 
settlements, and the remainder culled of its valuable 
timber, devastated by fire, or turned into almost useless 
brush land. 
The further division of the 260,000 square miles of 
merchantable timber east of the one hundredth me¬ 
ridian falls into about 130,000 square miles predomi¬ 
nantly of conifers and 130,000 square miles predomi¬ 
nantly of broad-leaved trees. The Northeastern States 
have about 20,000 square miles, the Lake States 20,000 
square miles, and the Southern States about 90,000 
square miles of coniferous forests. 
The Western forest still corresponds largely with its 
original natural limits, although large areas have al¬ 
ready been made* unproductive by unrestricted lumber¬ 
ing and by destructive fires which have swept over 
enormous areas. It now includes about 130,000 square 
miles of merchantable forest, of which about 65,000 
square miles lie in the Cascade-Sierra and Coast 
Ranges in the States of Washington, Oregon, and Cali¬ 
fornia, and about the same amount in the Rocky Moun¬ 
tains. 
The forests of the United States, therefore, are very 
unevenly distributed over the continent. Two-thirds 
of the forest is concentrated in the eastern part of the 
continent, while the remainder is found on the western 
side and is mostly coextensive with the Rockies and 
the Cascade-Sierra and Coast ranges. 
The causes which have influenced the present position 
and density of the large bodies of forests must pri¬ 
marily be sought in the peculiar distribution of rain¬ 
fall in this country. The region occupied by the East¬ 
ern forest is unbroken by any great mountain ranges, 
except in the Appalachian system, and is mainly com¬ 
posed of rolling country with good soil conditions. The 
moisture-laden winds from the Gulf of Mexico sweep 
inland to a great distance, the precipitation being 
heaviest during the growing period of the summer. The 
favorable distribution of rainfall, together with favor¬ 
able temperature and good soil conditions, enables the 
hardwoods to reach their greatest development in this 
region and excludes the conifers almost entirely, or 
relegates them to sandy plains and benches, rocky 
slopes, inclement altitudes, and cold swamps. The low, 
rolling character of the region of Eastern forest and 
the fact that the prevailing winds in the growing season 
are from the south, southwest, and southeast, allows the 
rainfall and consequently the forest to be distributed 
much farther inland than on the western front of the 
continent. In the central portion of the continent, how¬ 
ever, far from the moist ocean winds, the moisture is in¬ 
sufficient to support a dense forest, and grassland vege¬ 
tation becomes dominant, the natural forest being found 
chiefly along watercourses. The fact, however, that the 
rainfall in the eastern portion of the prairie region is 
still sufficient to insure the growth of a heavy forest 
leads to the inference that it is not low precipitation 
which has prevented the growth of trees. The flatness 
of the prairies, the absence of a clearly developed drain- 
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