of hardwoods in Volumes 1 and 3 of which about 40 are mapped 
in this volume. In contrast, New Mexico has about 70 or more 
tree species of native hardwoods and Arizona, 95. Totals in 
California and western Texas are even larger. 
The great variation in altitude is one of the most significant 
factors in the distribution of tree species in Western United 
States. As a result, the ranges of many species are irregular, 
broken, and discontinuous, especially southward. Many species 
are characteristic of certain zones of climate and vegetation, 
which rise from near sea level in the North to mountain 
summits in the South. Overlay 3, Topographic Relief, in Volume 
1. has contour lines, which may indicate some relationships in 
distribution. 
RARE AND ENDANGERED WESTERN 
HARDWOODS 
The maps of this Atlas show clearly which tree species are 
confined to limited geographic areas. From these trees of local 
occurrence, lists of rare and endangered species may be 
compiled. The subject of rare and endangered trees is being 
treated in a series of separate publications. 
The first report, “Rare and Local Conifers in the United 
States” (Little 1975) is based upon Volume | and contains maps 
of 35 species from that source. Similarly, the species maps of 
Volume 3 reveal much information about rare and endangered 
hardwoods in western continental United States. A summary 
may be appropriate here. 
The Endangered Species Act of 1973 (U.S. Public Law 93- 
205, approved December 28, 1973) directed the Smithsonian 
Institution to prepare a list of endangered and threatened plant 
species, to review methods of adequately conserving these 
species, and to report the Institution’s recommendations to the 
Congress. Accordingly, the Secretary (Smithsonian Institution 
1975) presented to the Congress on December 15, 1974, its 200- 
page “Report on Endangered and Threatened Plant Species of 
the United States.” As explained in the foreword by Secretary 
S. Dillon Ripley, this report provides lists of endangered, 
threatened, recently extinct, and exploited species of native 
plants of the United States. 
The preliminary lists for continental United States comprise 
about 10 percent of the flora, 2,099 species (including some 
varieties and subspecies). Fortunately, the number of tree 
species and varieties is small, only about 32 (also 11 varieties), 
of which 14 are western hardwoods mapped in Volume 3. 
However, the number of species of rare trees is somewhat 
larger. 
Next, a few definitions. A rare species has small numbers of 
individuals throughout its range, which may be restricted or 
widespread. The term local (also endemic) species is used here 
for a species of relatively small range, such as a small part of a 
State or a few islands. Border or peripheral species reach the 
limit of their natural range a short distance into the United 
States, for example, from Mexico into the Southwest. An extinct 
species (recently or possibly extinct) is no longer known to exist 
after repeated search of the type locality and other known or 
likely places. 
As defined in the Act, “the term endangered species means 
any species which is in danger of extinction throughout all or a 
.. “The term threatened 
species means any species which is likely to become an 
significant portion of its range 
endangered species within the foreseeable future throughout all 
or a significant portion of its range.” 
About 60 species of minor western hardwoods of the 210 
mapped in Volume 3 have relatively small ranges in the United 
States and may be classed under one or more of the above 
terms. Also, several rare varieties are not mapped separately or 
mentioned here. Of these, 34 species are border or peripheral 
species mostly widespread south of the border in Mexico and 
thus not endangered at present. No tree species of Volume 3 is 
classed as extinct. However, a border species noted below 
apparently is extinct except in cultivation and in Mexico. 
The 26 species of hardwoods with local distribution in western 
continental United States, as mapped in Volume 3, may be 
grouped geographically in the five lists below. The 9 on the 
endangered list compiled by the Smithsonian Institution and the 
5 on the threatened list are designated. 
Trans—Pecos Texas 
Leucaena retusa, littleleaf leadtree 
Ostrya chisosensis, Chisos hophornbeam (threatened) 
Populus hinckleyana, Hinckley cottonwood (endangered) 
Quercus graciliformis, Chisos oak (endangered) 
Quercus tardifolia, lateleaf oak (endangered) 
Arizona—California 
Cereus giganteus, saguaro 
Condalia globosa, bitter condalia 
Fraxinus gooddingti, Goodding ash (endangered) 
Rhus kearneyi, Kearney sumac (endangered) 
V auquelinia pauciflora, vauquelinia (threatened) 
Washingtonia filifera, California washingtonia 
California 
Fremontodendron mexicanum, Mexican fremontia (endan- 
gered) 
Juglans californica, California walnut 
Juglans hindsii, Hinds walnut (endangered) 
Prunus fremontii, desert apricot 
Quercus engelmannii, Engelmann oak 
Rhus integrifolia, lemonade sumac 
Rhus laurina, laurel sumac 
Channel Islands of California 
Ceanothus arboreus, feltleaf ceanothus (threatened) 
Cercocarpus traskiae, Catalina cercocarpus (endangered) 
Lyonothamnus floribundus, lyontree (endangered) 
Prunus lyonii, Catalina cherry 
Quercus macdonaldii, McDonald oak 
Quercus tomentella, island live oak (threatened) 
Oregon—California 
Salix fluviatilis, river willow (threatened) 
Salix tracyi, Tracy willow 
The 34 border or peripheral species mostly widespread south 
of the border in Mexico may be arranged geographically in the 
three lists below. Though of limited distribution in the United 
States, these species, with one exception, are not endangered or 
threatened with extinction. Nevertheless, some may merit 
special protection in this country. Berlandier esenbeckia, Esen- 
beckia berlandieri, apparently is extinct in extreme southern 
Texas except in cultivation, though not endangered in Mexico. 
