trees which extend northward from Mexico into southern Texas 
but no farther eastward. 
Most tree species native in the six Midwestern States from 
‘North Dakota south to Texas are eastern in range and reach 
their western limits in one or more of these States. These 
eastern species not in Volume 1 will be in “Volume 4, Minor 
Eastern Hardwoods.” Thus, the native trees of these six 
Midwestern States are to be found among Volumes 1, 3, and 4. 
About 20 species of Volume 3 are widespread across the 
United States or else range both east and west of one or more of 
the six Midwestern States from North Dakota to Texas. The 
entire range of several is shown on a map of North America. 
Detailed maps of these species showing occurrence by counties 
in Eastern United States are in Volume 4. 
For Alaska trees, Volumes 1 and 3 contain maps of North 
America showing the distribution beyond the 49th State into 
Canada and the lower 48. The detailed maps in “Volume 2, 
Alaska Trees and Common Shrubs” do not indicate occurrence 
outside. For completeness, North American maps are inserted 
here for these 2 species of Alaskan tree willows ranging into 
Canada but not southward: Salix alaxensis (Anderss.) Cov., 
feltleaf willow, and Salix arbusculoides Anderss., littletree 
willow. Volume 1 maps the general ranges of Alaska trees in 
North America for 17 species (12 conifers, 5 hardwoods) and 
Volume 3, 15 hardwood species, total 32. However, North 
American maps have not been compiled for 6 shrubby species 
(5 of Salix, willow, and 1 of Alnus, alder) rarely attaining tree 
size in Alaska. 
The 210 species of minor western hardwoods (including 12 of 
monocotyledons) mapped in this volume are grouped in 83 
genera and 36 plant families. A botanical index of genera and 
families appeared in the Check List (Little 1953, p. 445-450). 
Totals are incomplete, because important hardwoods are in 
Volume 1. Largest genera in Volume 3 are: willow (Salix), 21 
species; oak (Quercus), 19; ash (Fraxinus), cherry—plum (Pru- 
nus), and yucca (Yucca), 9 each; sumac (Rhus), 8; and acacia 
(Acacia) and hawthorn (Crataegus), 7 each. Combined with 
Volume 1, the largest genera of western hardwoods are: oak 
(Quercus), 29 species; willow (Salix), 22; and ash (Fraxinus) and 
cherry—plum (Prunus), 10 each. 
PREPARATION OF THE MAPS 
The maps in this volume have been compiled from various 
sources, following the procedure explained in the first volume. 
Principal records on tree distribution include publications, 
herbarium specimens, field work, and review by botanists, 
foresters, and others. The more detailed publications consulted 
are listed under Selected References (p. 11). Credit is due many 
persons for their valuable assistance. The list of publications 
and persons consulted is too long for citation here. Naturally the 
compiler is responsible for all errors. 
Publications with information about tree distribution have 
been examined for each State. These included tree guides, 
State floras or manuals, and catalogs. References for several 
States summarize tree species distribution accurately and in 
detail almost as precise as maps, such as by counties or 
counties along the border or, for rare species, by localities. In 
other States, scattered published floras with lists by counties or 
similar geographic units have been helpful in filling the gaps. 
The classic Manual by Sargent (1926) contains important locality 
records. Notes on range extensions are found in various 
scientific periodicals. A few unpublished theses with local lists 
have been examined. 
Published maps have been valuable sources in this compila- 
tion. Altogether, more than one-fourth of the States now have 
publications with distribution maps of all or most native tree 
species, as noted in the first volume (Little 1971, p. 5). These, 
with author and year, for 7 of the 18 Western States of this 
volume, are: Alaska (Hultén 1941-49, 1968; Viereck and Little 
1972, 1974, 1975): Idaho (Johnson 1961); Montana (Booth and 
Wright 1962); Utah (Erdman 1970; C M. Johnson 1970); 
Nebraska (Pool 1951); Kansas (Hitchcock 1899; Gates 1938: 
Stephens 1969); and Oklahoma (Williams 1973). 
Other references treat larger areas. Maps of trees of south- 
western deserts covered parts of California, Arizona, and New 
Mexico (Benson and Darrow 1945, 1954). Trees of the Sonoran 
Desert extending north into Arizona and southeastern California 
from Sonora and Baja California, Mexico, have been plotted in 
detail (Hastings, Turner, and Warren 1972). One recent publica- 
tion shows dots for counties in four States, North Dakota, South 
Dakota, Nebraska, and Kansas (Stephens 1973). 
“Index Holmensis, a World Index of Plant Distribution Maps” 
(Tralau 1969-74) cites published maps by species and will be 
valuable in future compilation. The first three volumes on 
gymnosperms and monocotyledons have several species charted 
here. Hardwoods will be covered in the remaining volumes on 
dicotyledons. 
The ranges of many native tree species of Western United 
States continue northward into Canada and have been charted 
there. “Native Trees of Canada” (Hosie 1969) contains small 
maps of the forest tree species. Plants of Yukon Territory were 
included with Alaska by Hultén (1941-49, 1968). Two parts of 
the flora of British Columbia with maps of the rose family 
(Rosaceae) and the heather family (Ericaceae) have appeared 
(Szezawinski 1962; Taylor 1973). Also records of plant speci- 
mens of Queen Charlotte Islands were plotted (Calder, Taylor, 
and Mulligan 1968). 
Among the most detailed published maps are those of 
taxonomic monographs and those of a single species based upon 
extensive field work, such as a doctoral thesis. Unfortunately, 
few tree species of Western United States have been so 
thoroughly studied. Examples of generic monographs with maps 
among the minor western hardwoods are: Alnus (Johnson 1969); 
Amelanchier (Jones 1946); Artemisia (Beetle 196C); Bumelia 
(Clark 1942); Cercidium (Carter 1974); Cercis (Hopkins 1942): 
Fraxinus (Miller 1955); Prosopis (Benson 1941; Johnston 1962): 
Ptelea (Bailey 1962); Rhamnus (Wolf 1938); Rhus (Barkley 1937); 
Salix in Alaska and Yukon (Argus 1973); Yucca (McKelvey 
1938-47). 
Also, a few publications about plant families have species 
maps. Two in the legume family, Leguminosae are: “The 
legumes of Texas” (B. L. Turner 1959); Subfamily Mimosoideae 
(Isely 1973). Similar maps are available in the cactus family, 
Cactaceae, for Arizona (Benson 1969). Also, Aesculus (Hardin 
1957). 
Examples of generic monographs with distribution data but 
without maps are: Bumelia (Cronquist 1945); Bursera (Porter 
1974); Citharexylum (Moldenke 1958); Condalia (Johnston 1962); 
Erythrina (Krukoff 1939); Sorbus (Jones 1939); Viburnum (Mc- 
Atee 1956); Yucca (Webber 1953). Special studies of one or few 
species may contain maps, for example, in Quercus (Tucker 
1952; Tucker and Haskell 1960). Also, some studies of plant 
distribution contain maps, for example, Mc Vaugh (1952). 
