Salix sitchensis Sanson, Sitka willow, includes S. coulteri 
Anderss., Coulter willow. 
These four species of willow, Salix, accepted in the 1953 
Check List are omitted as shrubs only rarely reported to attain 
tree size: 
Salix ligulifolia (Ball) Ball, strapleaf willow. 
Salix lutea Nutt., yellow willow. 
Salix melanopsis Nutt., dusky willow. 
Salix padophylla Rydb., serviceberry willow, now a synonym 
of S. monticola Bebb, park willow. 
Leucaena greggii S. Wats., Gregg leadtree, of Mexico, is 
omitted, having been reported also from Texas in error. 
Quercus undulata Torr., wavyleaf oak, has been omitted as a 
hybrid of Q. gambelii Nutt., Gambel oak. 
NOTES ON RANGES 
Some observations on tree distribution may be noted from the 
maps of the 210 species in this volume. However, analyses of 
the ranges are outside the scope of the Atlas. An attempt has 
been made to record where each species grows naturally now, 
not to explain or speculate how and why. 
Many western tree species are confined to the States along 
the Pacific Coast, California, Oregon, and Washington. Others 
are inland through the Rocky Mountains. A prominent element 
of the western hardwoods is found along the Mexican border 
from southeastern California through parts of Arizona and New 
Mexico to southwestern Texas. These mostly small trees are 
found at low altitudes in the southwestern desert and mountain 
regions. Additional subtropical species of northeastern Mexico 
spread northward into southwestern, southern, and sometimes 
central Texas. 
Many maps show distinctive or unusual ranges. Especially 
noteworthy are the species of farthest geographic extent and 
greatest variation in environmental conditions. Some species 
may be widely adapted to many types of climates. Others 
apparently are composed of geographic races. 
Relatively few species have broad east-west distribution 
nearly across the continent and can be classed as transcontinen- 
tal. At least 21 of this volume extend eastward beyond the maps 
of the 18 Western States. Eastern ranges of the same species 
will be shown in Volume 4. These minor hardwood species of 
broad east-west distribution are listed below. 
Amelanchier alnifolia, western serviceberry 
Bumelia lanuginosa, gum bumelia 
Cephalanthus occidentalis, common buttonbush 
Cercis canadensis, eastern redbud 
Cornus stolonifera, red-osier dogwood 
Corylus cornuta, beaked hazel 
Crataegus succulenta, fleshy hawthorn 
Prunus americana, American plum 
Prunus angustifolia, Chickasaw plum 
Prunus pensylvanica, pin cherry 
Prunus virginiana, common chokecherry 
Ptelea trifoliata, common hoptree 
Rhus glabra, smooth sumac 
Salix bebbiana, Bebb willow 
Salix discolor, pussy willow 
Salix exigua, coyote willow 
Salix nigra, black willow 
Salix petiolaris, meadow willow 
Sapindus drummondii, western soapberry 
Viburnum lentago, nannyberry 
Viburnum trilobum, American cranberrybush 
Thirteen species of minor western hardwoods in Volume 3, 
including 4 of the previous list, have a broad geographical 
occurrence in Western North America from Alaska southeast 
across western Canada to the lower 48 States. In Volume 1, 13 
species of conifers (also 1 shrubby) and 5 of important 
hardwoods have similar ranges. Many of these are found in the 
coastal forests from southern or southeastern Alaska south along 
the Pacific Coast of British Columbia to Washington or beyond 
to Oregon or California. A few are characteristic of the interior 
forests of Alaska and Rocky Mountains southward. The Alaska 
distribution is shown in detail in Volume 2. A list of these 
species follows: 
Acer glabrum, Rocky Mountain (Douglas) maple 
Alnus sinuata, Sitka alder 
Alnus tenuifolia, thinleaf alder 
Amelanchier alnifolia, western serviceberry 
Malus diversifolia, Oregon crab apple 
Salix bebbiana, Bebb willow 
Salix hookeriana, Hooker willow 
Salix lasiandra, Pacific willow 
Salix scouleriana, Scouler willow 
Salix sitchensis, Sitka willow 
Sambucus callicarpa, Pacific red elder 
Sorbus scopulina, Greene mountain-ash 
Scorbus sitchensis, Sitka mountain-ash 
Five shrubby species of Volume 3 are native from Canada 
southward across the United States to Mexico, as follows: 
Artemisia tridentata, big sagebrush 
Cephalanthus occidentalis, common buttonbush 
Cornus stolonifera, red-osier dogwood 
Rhus glabra, smooth sumac 
Sambucus glauca, blueberry elder 
One of the above, Cornus stolonifera, red-osier dogwood, 
reaches from central Alaska across Canada to Newfoundland 
and south to the Northeastern States as well as mountains of 
northern Mexico. Another, Rhus glabra, smooth sumac, is the 
only species classed as sometimes reaching tree size and native 
in all 48 lower States. However, it has been found in California 
only once and in Nevada at only two localities. 
Nearly all tree species of this volume are confined to North 
America. However, a few tree species, mostly northern or 
subtropical, extend beyond. Examples are willows (Salix), across 
interior Alaska into northeastern Asia. Cephalanthus occiden- 
talis, common buttonbush, ranges from southeastern Canada 
and New England to Canada and south through Mexico into 
Central America and into Cuba. The same or a closely related 
species is recorded from eastern Asia. 
Several subtropical species of small trees of the Mexican 
border region extend southward into South America. Koeberli- 
nia spinosa, allthorn, a desert species, reappears in deserts of 
Bolivia along with a few associated shrubs. Dodonaea viscosa, 
hopbush, including several varieties, is widespread also in the 
Old World tropics. 
The number of tree species native in an area obviously 
increases from North to South. For example, Alaska, the largest 
State, has only about 20 tree species of hardwoods, also 6 more 
rarely reaching tree size (also conifers totaling 12 tree species 
and 2 shrubby), as mapped in Volume 2. 
The 5 Northwestern States of Washington, Oregon, Idaho, 
Montana, and Wyoming together contain about 50 tree species 
