DEPAR TMENT OF AGRICULT URE

Pacific Southwest 

Research Station

Research Paper

PSW-RP-265

September 2013

United States Department of Agriculture 

Forest Service

Pinus ponderosa: 

Geographic Races and 

Subspecies Based on 

Morphological Variation

Robert Z. Callaham
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Author

Robert Z. Callaham (retired) was a botanist, geneticist with the Pacific Southwest 

Research Station, 800 Buchanan Street, West Annex Building, Albany, CA 94710-

0011. He served as station director from 1976 to 1983 and as de facto director (1983 

to 1990), Wildland Resources Center, University of California. He can be reached 

at: rzcallaham@comcast.net.

Cover photograph: Chris Evans, Illinois Wildlife Action Plan Bugwood org.
Abstract

Callaham, Robert Z. 2013. Pinus ponderosa: geographic races and subspecies 

based on morphological variation. Res. Pap. PSW-RP-265. Albany, CA: U.S. 

Department of Agriculture, Forest Service, Pacific Southwest Research Station. 

53 p.

Morphological variation of ponderosa pine (Pinus ponderosa Dougl. ex Laws.), 

growing north of Mexico, is described. A map shows distributions of five putative races that are analyzed and discussed. Characteristics of branches, shoots, 

and needles were measured for 10 or fewer trees growing on 147 plots located at 

1,500-ft elevational intervals along latitudinal transects. Characteristics of cones 

were measured from 120 of these plots. Cones from 78 plots yielded seeds for study. 

Subspecific boundaries were based on both visual interpretations for many traits 

and statistical cluster analyses of plot means. Boundaries were tested for appropriateness by multiple discriminant analyses. Univariate analyses of variance and stepwise discriminant analyses selected characteristics most capable of differentiating 

among hypothesized races. On average, 98 percent of plots were correctly classified 

as to race. Characteristics most capable of differentiating among the hypothesized 

races were selected. Characteristics of trees in three common-garden plantings 

were analyzed to determine the genetic basis of foliar characteristics measured on 

trees in situ. Results of analyses and review of literature confirm existence of five 

morphologically distinct, geographic taxa and four transition zones.

Keywords: Ponderosa pine, geographic races, morphological variation.
ii

Contents

1 Introduction

1 Literature Review

5 Collections

12 Foliage, Cones, and Seed

13 Estimation of Climate

13 Data Analyses (by Diebel)

16 Results

28 Pacific Race

35 Discussion

37 Pacific vs. Columbia Ponderosa Pine

41 Rocky Mountains Ponderosa Pine

43 Central High Plains Ponderosa Pine

44 Southwestern Ponderosa Pine

48 Conclusion

49 Acknowledgments

49 Metric Equivalents

49 Literature Cited
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Introduction

Ponderosa pine (Pinus ponderosa Dougl. ex Laws.) is the most prevalent and valuable conifer in the western United States. It is estimated to cover 33.7 million ac in 

16 Western States plus additional areas in adjacent British Columbia. Perry (1991) 

affirmed that it does not occur in Mexico, but this contention needs to be further 

validated by studies of pines growing in northern Mexico. Ponderosa pine occurs 

in eight of 23 major forest cover types in the western United States (Burns 1983). 

Across its vast geographic range, the species varies considerably in morphology, 

ecology, and habits of growth. Its patterns of inherent variation have been studied 

since 1925 at the U.S. Depatment of Agriculture Forest Service’s Institute of Forest 

Genetics (IFG), Pacific Southwest Research Station, Placerville, California.

In 1954, the author, then a geneticist at IFG, began comprehensive investigations of ponderosa pine from its range north of Mexico. The objective was to define 

the nature and extent of intraspecific variability. Several studies were conducted to 

establish inherent similarities and differences of trees growing in the United States 

and Canada (Callaham 1959, 1961, 1962). Reported here is the study of morphological variability of trees growing in situ.

Few investigators had scrutinized in situ variation in morphology of a widespread tree species to the extent done in this study. Critchfield’s (1957) comprehensive investigation of lodgepole pine (P. contorta Dougl. ex Loud.) throughout its 

range inspired this study and served as its model.

Literature Review

Taxonomically, ponderosa pine has long been recognized as being composed of 

three varieties. Little (1979) recognized:

(1) Pinus ponderosa Dougl. ex C. Lawson var. ponderosa as typical ponderosa 

pine because the species was originally described from a collection near Spokane, 

Washington, extending southward from British Columbia, west of the Continental 

Divide, into Idaho, Washington, Oregon, and northeastern California.

(2) Rocky Mountain ponderosa pine, P. ponderosa var. scopulorum Engelm. (1880), 

elevated to a subspecies by E. Murray (1982), occurring in Montana and Wyoming, 

east of the Continental Divide, and extending southward through Utah, Colorado, 

and Nebraska into Arizona, New Mexico, the Oklahoma panhandle (upper Tessequite Canyon in western Cimarron County),1

 western Texas, and Mexico.

1

 Oklahoma Biological Survey, http://www.biosurvey.ou.edu/shrub/pipo.htm. (2013).
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research paper psw-rp-265

(3) Arizona pine, P. ponderosa var. arizonica (Engelm.) Shaw, a five-needled form 

having elongated, curving, semipersistent cones, growing in extreme southwestern 

New Mexico, southeastern Arizona, and adjacent northern Mexico.

The first two taxa were expected to be refined as a result of the investigation 

undertaken in this study.

Most investigators have studied variation within pines by growing progenies of 

diverse provenances in one or more common gardens (a uniform plantation of trees 

at a geographic location) (Callaham 1964). So-called provenance studies of ponderosa pine began in 1913 and have been reported by many authors (see Conkle and 

Critchfield 1988). Wells (1964a) and Read (1980, 1983) comprehensively reviewed 

results of provenance studies of ponderosa pine. Both also presented their views on 

geographic races based on seedling progenies grown in nurseries.

Wells used seed from 60 of the provenance collections reported here, and 

Read’s seed collections were from 80 provenances mostly east of the Continental 

Divide. Wells (1964b) determined correlations between characteristics of seedlings, 

grown in a provenance study in Michigan, and Callaham’s range-wide climatic 

data.

Rehfeldt (1986a, 1986b, 1990, 1991, 1993) conducted a series of provenance 

studies to determine patterns of genetic variation in a broad north-south band 

extending from the border with Canada almost to the border with Mexico. 

Sorenson (1994) used Rehfeldt’s approach in a provenance study in Oregon east 

of the Cascade Range.

Only two authors have studied in situ variation across the range of ponderosa 

pine. Haller (1965) studied occurrence of two-needled fascicles and proposed racial 

boundaries for ponderosa pine based on geographic distribution of two-needled 

fascicles, mean annual precipitation, and mean minimum temperatures in January. 

Haller’s boundaries coincided closely with those of races drawn earlier by Weidman 

(1939) (fig. 1).

Smith (1977), after studying monoterpenes from xylem oleoresin at 68 locations, divided ponderosa pine into five regional types with four transitional zones. 

Except for combining regions in the north and designating some new regions in 

southern California and Arizona, his boundaries are synonymous with Haller’s. 

Sturgeon (1979) refined Smith’s boundary between races in northern California and 

southern Oregon.

Critchfield (1984) established both close identity of Washoe pine (Pinus washoensis H. Mason and Stockwell) (1945) with Weidman’s “North Plateau ponderosa 

pine” and extent of significant barriers to crossing among three races of ponderosa 
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Figure 1—Distribution of ponderosa pine and plots established for this study with tentative boundaries between races used for 

analyses of data.
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research paper psw-rp-265

pine. He concluded: “There are partial reproductive barriers between the Pacific 

and Rocky Mountains races and likely barriers between Pacific and North Plateau 

races.”

Critchfield’s scholarly review of this situation and his personal knowledge of 

genetics and geography of ponderosa pine resulted in his boundaries and transition 

regions among geographic races being drawn much as is done in this report.

Linhart et al. (1989) were first to publish data on differences in allozymes—

allelic variants of genes that determine enzymes detected using electrophoresis—

associated with races of ponderosa pine. Their work involved only 15 individuals of 

the Pacific race (from five plots closely clustered at low elevation on the west slope 

of the Sierra Nevada) and five individuals of the Rocky Mountains race (two each 

from Nebraska and Wyoming and one from South Dakota). Their findings “suggest 

... there are significant differences between ponderosa and scopulorum varieties.” 

Niebling and Conkle (1990) studied variation in allozymes and reported: “Major 

differences existed between the Rocky Mountain(s) race of ponderosa pine and ... 

the Pacific race of ponderosa pine (actually trees growing at two locations in the 

transitional zone between Pacific and North Plateau races), and North Plateau race 

ponderosa pine.” Differences in allozymes among all races of ponderosa pine have 

not yet been assessed.

Grant et al. (1989) studied 14 morphological characters of 3-year-old seedlings 

grown in a greenhouse in Colorado from intravarietal and intervarietal crosses. 

Female parents grew near Placerville, California. Male parents grew near Placerville and in Nebraska, South Dakota, and Wyoming. They concluded: “Intravarietal 

progeny were distinctly differentiated genetically from intervarietal sibships in 

seven of fourteen characters examined.” They also estimated narrow-sense heritabilities for the 14 characters.

Monson and Grant (1989), using two of the seedling families studied by Grant 

and coworkers (1989), established heritable differences in physiological traits. 

Particularly relevant to this study of morphological variation was their discovery 

that ponderosa pines from California differed significantly from ponderosa pines 

growing in Niobrara County, Wyoming, in stomata per square millimeter, stomata 

per millimeter along one row, and stomata per millimeter of needle width. When 

compared with an interracial (California x Wyoming) hybrid family, the intraracial 

(California x California) family had “higher stomatal densities ... expressed in a 

greater number of stomatal rows per unit of needle width and a greater number 

of stomata per unit of needle length ... sizes of the stomatal openings ... were not 

significantly different.”
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Rehfeldt demonstrated the existence of clinal patterns of genetic variability 

in races of ponderosa pine. His studies covered provenances growing in a broad 

band (the land measured 217 mi across northeastern Washington to the Continental 

Divide and 416 mi across Utah, Colorado, Arizona, and New Mexico) extending 

from Canada to Mexico. In a series of parallel studies, he examined growth, shoot 

elongation, and damage during winter of seedlings in common gardens and in a 

greenhouse. For these studies, Rehfeldt collected seed-bearing cones from 353 

populations: 201 in Columbia basins (1986a, 1986b, 1991), 56 + 16 in Utah and 

Colorado, respectively (1990, 1993), and 78 in Arizona and New Mexico (1993). 

His purposes were to explain patterns of genetic variation and to provide guidance 

for transfer of seeds to reduce maladaptations in reforestation. Through multiple 

regression analyses, he demonstrated existence of complex clinal patterns of 

variation and their parallels to patterns of environmental variation—particularly 

topography, precipitation, and length of frost-free season. He did not study characteristics of parent trees from which he collected cones and seeds, so a great potential for understanding in situ variability in ponderosa pine from a broad north-south 

transect was not realized.

Wells (1964a, 1964b) reported characteristics of his seedling progenies grown 

in a nursery in Michigan from seed collected for the study reported here. His study 

of seedlings essentially was a common garden planting. The study reported here 

describes in situ attributes of some of his parent trees.

Little’s (1979) third taxon, Arizona pine, was specifically excluded from this 

study. Investigations at the IFG, Placerville, California, had already established that 

P. arizonica Engelm. was a species. Later, Peloquin (1984) and Rehfeldt (1993) 

confirmed the distinctness of Arizona pine.

This report purposely refers either to the two, long-recognized varieties of the 

species or to five races of ponderosa pine (fig. 1). These races are morphologically 

distinct geographic variants within the species.

Collections

In late summer 1955, nearly 200 foresters working on national forests, for state 

agencies, and for private industry were asked to establish tagged-tree plots and 

to collect specified materials. Because instructions and materials were posted to 

designated collectors, these immediately were named “mail order” collections. Collectors were asked to provide data describing sampled trees and collection localities 

(plots) and samples of cones, shoots, needles, bark, and wood. By return mail, 

they provided data sheets and specimens from 147 plots, cones from 120 plots, and 

viable seeds from 78 plots (table 1).
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research paper psw-rp-265

Table 1—Locations of plots from which data were available

Plot North latitude West longitude Elevation State or province Weather station

Feet

1 43°57’ 103°36’ 5,400 South Dakota Deerfield Dam/Custer

2 44°03’ 107°18’ 6,300 Wyoming Tensleep

3 38°58’ 108°06’ 7,300 Colorado Cedaredge

4 38°15’ 106°42’ 9,000 Colorado Cotchetopa/Cathedral

5 42°27’ 105°28’ 6,400 Wyoming Double-4 Ranch

6 39°00’ 104°46’ 7,100 Colorado Husted

7 37°04’ 106°15’ 8,600 Colorado San Antone R.S./Chama, New Mexico

8 40°58’ 105°27’ 7,700 Colorado St. Cloud/Fry’s Ranch

9 39°02’ 106°15’ 9,000 Colorado Twin Lakes/Buena Vista

10 37°28’ 108°29’ 7,500 Colorado Fort Lewis

11 37°10’ 107°00’ 7,500 Colorado Pagosa Springs

12 37°31’ 108°29’ 7,500 Colorado Fort Lewis

13 44°03’ 103°37’ 5,500 South Dakota Deerfield Dam/Custer

14 43°54’ 104°03’ 6,200 South Dakota Boyd/Newcastle, Wyoming

15 43°53’ 104°02’ 6,200 South Dakota Boyd/Newcastle, Wyoming

18 38°57’ 105°59’ 9,200 Colorado Hartsell/Cripple Creek

19 38°54’ 104°57’ 7,200 Colorado Monument

20 41°00’ 105°29’ 6,900 Colorado Box Elder/Fry’s Ranch

21 39°00’ 105°00’ 9,100 Colorado Monument

22 41°28’ 100°01’ 2,800 Nebraska Stapleton

23 42°45’ 99°32’ 2,100 Nebraska Newport

24 42°44’ 103°48’ 4,500 Nebraska Harrison

25 41°26’ 103°05’ 4,200 Nebraska Dalton/Sidney

26 43°04’ 102°38’ 3,000 South Dakota Pine Ridge

27 45°41’ 121°21’ 200 Oregon Hood River

30 44°55’ 119°41’ 3,200 Oregon Loverock and Ukiah1NE

31 44°59’ 119°45’ 4,500 Oregon Loverock and Ukiah1NE

32 45°00’ 118°06’ 4,500 Oregon Austin

33 45°00’ 117°00’ 4,600 Oregon Gumboot/Sparta

34 47°58’ 120°18’ 2,200 Washington Lakeside

36 46°56’ 120°32’ 1,600 Washington Ellensberg

37 46°58’ 120°51’ 4,600 Washington Ellensberg

38 48°58’ 119°23’ 1,600 Washington Oroville

39 48°55’ 119°17’ 3,000 Washington Chesaw

40 44°16’ 120°26’ 5,000 Oregon Ochoco R.S.

41 43°49’ 119°18’ 4,700 Oregon Bear Creek/Seneca

44 38°48’ 120°09’ 6,000 California Woodfords

45 39°14’ 119°56’ 6,500 Nevada Glenbrook

46 36°13’ 121°43’ 3,000 California Big Sur State Park/Priest Valley
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

49 39°09’ 122°47’ 4,400 California Clearlake Park

50 39°07’ 122°45’ 2,800 California Clearlake Park

51 41°57’ 120°46’ 5,400 California Lakeview, Oregon

52 41°05’ 120°55’ 4,700 California Adin R.S./Madeline

53 41°54’ 120°19’ 6,000 California Fort Bidwell

54 41°00’ 120°55’ 5,800 California Blacks Mtn Branch/ Madeline

55 34°10’ 116°56’ 6,000 California Seven Oaks

57 34°14’ 117°14’ 6,000 California Lake Arrowhead

60 41°04’ 121°35’ 4,600 California Fall River Mills Intake

62 40°59’ 122°26’ 1,600 California Vollmers/Weaverville R.S.

64 40°59’ 122°32’ 4,400 California Weaverville R.S.

66 40°59’ 121°35’ 3,100 California Hat Creek P.H. No.1

68 39°01’ 120°49’ 3,200 California Foresthill R.S./Gold Run

69 38°56’ 121°00’ 1,500 California Auburn

70 39°05’ 120°45’ 4,500 California Blue Canyon

71 37°05’ 119°17’ 4,500 California Big Creek P.H. No.1

72 37°04’ 119°15’ 3,200 California Auberry

73 37°02’ 119°07’ 6,100 California Huntington Lake

77 45°29’ 122°52’ 200 Oregon Portland

78 44°27’ 123°18’ 300 Oregon Corvallis

79 47°02’ 122°32’ 300 Washington McMillin Reservoir

80 42°59’ 122°31’ 4,300 Oregon Prospect

81 42°59’ 122°52’ 1,400 Oregon Riddle

82 43°00’ 121°48’ 4,500 Oregon Chemult

83 42°59’ 121°57’ 5,900 Oregon Crater Lake

84 43°00’ 122°45’ 3,150 Oregon Prospect

85 42°58’ 123°35’ 1,450 Oregon Glendale

88 38°50’ 122°42’ 3,100 California Hobergs/Lakeport

89 38°23’ 122°31’ 500 California Santa Rosa

90 38°45’ 122°46’ 1,400 California Kellogg/Clear Lake Park

93 44°55’ 117°05’ 3,000 Oregon Halfway

94 45°00’ 118°05’ 5,900 Oregon Ibex Mine/Baker

95 49°07’ 118°25’ 3,200 British Columbia Chesaw, Washington

96 49°05’ 115°08’ 2,600 British Columbia Fortine, Montana

97 50°09’ 115°53’ 3,300 British Columbia Kimberly

98 49°27’ 120°26’ 2,000 British Columbia Princeton

99 49°34’ 120°29’ 3,300 British Columbia Manning Park

100 50°12’ 119°32’ 1,700 British Columbia V. Coldstream Ranch

101 50°53’ 120°13’ 2,400 British Columbia Kamloops

Table 1—Locations of plots from which data were available (continued)

Plot North latitude West longitude Elevation State or province Weather station

Feet
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research paper psw-rp-265

102 48°56’ 116°25’ 1,800 Idaho Porthill

105 46°59’ 116°14’ 3,800 Idaho St. Maries

106 46°59’ 116°49’ 3,000 Idaho Potlach

107 47°01’ 116°15’ 3,000 Idaho Deary/St. Maries

108 47°00’ 114°41’ 2,900 Montana Superior

109 46°59’ 114°43’ 4,600 Montana Superior

110 46°57’ 112°38’ 4,400 Montana Ovando

111 47°02’ 112°10’ 4,500 Montana Ovando

113 47°06’ 112°15’ 6,000 Montana Adel

114 46°59’ 109°29’ 4,500 Montana Lewiston

115 45°39’ 104°09’ 4,100 Montana Rocky Point, Wyoming

116 46°15’ 108°28’ 4,500 Montana Rapelje 45

117 45°13’ 106°16’ 4,300 Montana Ashland

118 48°46’ 118°07’ 1,400 Washington Northport

119 46°09’ 115°45’ 1,500 Idaho Kooskia

120 46°00’ 115°55’ 3,100 Idaho Grangeville

121 45°53’ 115°58’ 4,400 Idaho Grangeville

123 46°05’ 114°11’ 4,200 Montana Darby

124 46°05’ 114°14’ 4,500 Montana Darby

125 46°06’ 114°17’ 5,800 Montana Darby

126 32°53’ 107°55’ 7,500 New Mexico Mimbres R.S./Fort Bayard

127 32°57’ 107°51’ 9,000 New Mexico Mimbres R.S./Fort Bayard

128 32°51’ 107°57’ 6,100 New Mexico Mimbres R.S./Fort Bayard

129 33°10’ 107°45’ 7,300 New Mexico Hermosa

130 32°58’ 105°50’ 7,200 New Mexico Mountain Park

131 33°00’ 105°47’ 8,000 New Mexico Cloudcroft

132 33°00’ 105°42’ 9,000 New Mexico Cloudcroft

133 32°54’ 105°29’ 6,000 New Mexico Mayhill R.S.

134 32°56’ 105°35’ 7,500 New Mexico Mayhill R.S.

135 34°55’ 112°59’ 7,200 Arizona Walnut Creek/Prescott

138 35°13’ 107°43’ 7,500 New Mexico Marquez/Grants

139 35°16’ 107°38’ 9,000 New Mexico Marquez/Grants

140 35°15’ 107°34’ 10,300 New Mexico Marquez/Grants

142 35°08’ 111°49’ 7,300 Arizona Flagstaff

145 36°23’ 113°10’ 7,300 Arizona Tuweep/Fredonia

Table 1—Locations of plots from which data were available (continued)

Plot North latitude West longitude Elevation State or province Weather station

Feet
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

146 36°58’ 106°09’ 9,000 New Mexico San Antone R.S./Chama

151 33°18’ 110°50’ 7,400 Arizona Globe

154 32°40’ 109°52’ 9,100 Arizona Fort Grant/Wilcox

156 31°51’ 109°14’ 8,525 Arizona Portal

158 44°52’ 116°30’ 4,900 Idaho McCall

159 44°14’ 116°09’ 4,200 Idaho Alpha 1NE

160 44°51’ 116°30’ 3,900 Idaho Mew Meadows R.S.

161 43°52’ 115°50’ 4,500 Idaho Idaho City

164 38°59’ 111°22’ 8,200 Utah Ephraim Hq., GBRC/Emery

165 39°00’ 111°19’ 8,200 Utah Ephraim Hq., GBRC/Emery

166 38°25’ 109°08’ 8,900 Utah LaSal

167 43°43’ 115°52’ 4,500 Idaho Idaho City

168 43°54’ 116°00’ 6,000 Idaho Idaho City

170 44°54’ 115°43’ 4,500 Idaho Big Creek 1S

171 44°53’ 115°40’ 6,000 Idaho Big Creek 1S

173 44°57’ 114°37’ 6,000 Idaho Big Creek 1S

174 40°53’ 109°38’ 7,600 Utah Manila

175 37°23’ 113°30’ 6,800 Utah Pine Valley/Cedar City

176 37°45’ 112°39’ 8,700 Utah Panguich

177 37°41’ 109°49’ 8,400 Utah Monticello

179 40°38’ 111°10’ 7,500 Utah Park City

180 38°15’ 112°28’ 7,500 Utah Beaver Canyon P.H./Beaver

182 45°33’ 113°57’ 4,700 Idaho Salmon

183 45°32’ 114°04’ 5,800 Idaho Salmon

184 36°59’ 106°59’ 6,200 New Mexico Dulce

190 33°58’ 109°48’ 9,000 Arizona McNary

191 34°03’ 109°42’ 7,500 Arizona McNary

193 36°47’ 113°45’ 6,900 Arizona MountTrumbull/Grand Canyon

194 36°07’ 113°48’ 6,100 Arizona Mount Trumbull/Grand Canyon

195 37°20’ 121°39’ 4,000 California Mount Hamilton

196 29°17’ 103°15’ 6,000 Texas Chisos Basin

197 40°58’ 123°41’ 1,700 California China Flat

198 37°03’ 122°08’ 1,600 California Ben Lomand

199 38°44’ 120°44’ 2,700 California Placerville

Table 1—Locations of plots from which data were available (continued)

Plot North latitude West longitude Elevation State or province Weather station

Feet

R.S. = ranger station, P.H. = power house, INE and IS = weather service designations, GBRC = Great Basin Research Center.
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Insofar as possible, plots were located within 5 mi north and south of the 

odd degrees of latitude from 29° to 51° north latitude. This provided a spacing 

of approximately 130 mi north and south between collections. Thus, each of five 

races identified by study of literature were sampled along three to five latitudinal 

transects (fig. 1). Where transects missed large portions of the range, subsidiary 

collections were made.

Along latitudinal transects, plots were established at elevational intervals of 

approximately 1,500 ft. Plots were located so as to provide two or three collections 

on each side of major north-south mountain ranges. Elevations of established plots 

ranged from 200 ft in Oregon to 10,300 ft in New Mexico. Insofar as possible, plots 

were located within 200 vertical ft of prescribed elevations: 1,500, 3,000, 4,500 ft, 

et seq.

The collector establishing a plot recorded exact location and elevation and 

described topography, soil, and stand conditions and identified each tree with a 

numbered tag. Plots were located in stands of natural origin containing at least 10 

cone-bearing trees, each separated from the others by a minimum of 100 ft.

The 10 trees selected for collection on each plot were to be of average form, 

growth rate, and branch habit. Conspicuously crooked, slow-growing, or thickbranched trees were to be avoided. Preference was to be given to thrifty young 

trees. Cones on selected trees were to be ripe, as evidenced by a browning in color 

and opening of basal scales, but many less-than-fully-ripe cones were collected.

Four characteristics of each selected tree were recorded in the field: 

(1) Average number of years that needles remained green (C1)2

 was estimated from 

an examination of four thrifty branches that could be reached from the ground. 

(2) Average distance (inches) from tip of branch to oldest green needles (C2) was 

estimated from measurements of the four branches. 

(3) After counting the branches in the top five whorls, the collector estimated the 

average number of branches per whorl (C3). (Ponderosa pine usually produces only 

one whorl each year.) 

(4) The angle branches at the tree’s apex made with the trunk of the tree (C4) was 

sketched by the collector and subsequently was measured in the laboratory with a 

protractor to the nearest 10 degrees.

Materials sampled from each selected tree included fascicles of needles, two 

vigorous shoots, five cones, a standard increment core, and a piece of bark. A 

2

 Characteristics used in this study are described here and henceforth are designated C1 

et seq. See table 2 for complete list.
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

fascicle of needles was picked from the center of each annual cluster of fascicles 

on each of the four thrifty branches measured for traits C1 and C2. Thus, if green 

needles were retained 3 years, then 12 fascicles were collected. These materials—

with the plot description and a map showing the location—were mailed to IFG. All 

materials collected, after processing, were deposited in IFG’s herbarium. The bark 

samples were mounted for study, but differences in color or other characteristics 

were not apparent. Increment cores were not used but are archived at IFG.

Note that during 1955, ripening cones were found primarily on plots in the 

eastern part of the range of ponderosa pine. During 1956, a second attempt was 

made to collect ripe cones from remaining plots. Collectors were asked, in the event 

of failure of the cone crop, to take five cones from the ground beneath sample trees 

or nearby trees. Ultimately, cones were collected from 120 plots, but seeds were 

available from only 78 plots. 

Trees growing in three common garden plantations in Oregon and Washington 

were studied to estimate heritability of characteristics. Observations were made in 

these plantations early in 1956, and materials were collected for measurements in 

the laboratory. Each plantation contained one 16-tree block planting of progenies 

from 10 widely separated provenances of ponderosa pine from the:

• Pacific race: Steilacoom (Washington), Willamette (Oregon), and Eldorado 

(California)

• Transition between the Pacific and North Plateau races: Rogue River 

(Oregon) and Lassen (California)

• North Plateau race: Bitterroot (Montana) and Deschutes (Oregon)

• Rocky Mountains race: Harney (Black Hills, South Dakota)

• Southwestern race: Coconino (Arizona) and Carson (New Mexico).

These plantations were established in 1928 and 1929 with 2-0 stock grown from 

seed collected in seven states (Squillace and Silen 1962). For this study one 16-tree, 

square plot was established for each of 10 provenance at each of three planting sites:

(1) Oregon State University’s McDonald Forest near Corvallis at 950 ft elevation 

(near native Pacific-race ponderosa pines)

(2) U.S. Forest Service’s Wind River Experimental Forest near Carson, 

Washington, at 1,300 ft elevation

(3) Deschutes National Forest near Bend, Oregon, at 4,200 ft elevation (among 

native North Plateau-race ponderosa pines)

None of these trees provided cones or seeds.
12

research paper psw-rp-265

Foliage, Cones, and Seed

Materials were received at Placerville and processed a few days after collection.

Shoot specimens were placed in a conventional plant press after hue (C5), value 

(C6), and chroma (C7) of foliage color had been estimated using Munsell Color 

Charts.3 Ten fascicles of needles were randomly selected and measured (C8). 

Lengths of 10 randomly selected needles produced in 1955 were measured. 

That length was expressed as multiples of length of primary bracts comprising 

the still-papery sheath surrounding the base of each fascicle. An average for this 

ratio—needle length/sheath length (C9)—was calculated. The number of needles 

per fascicle (C10) was determined.

From the middle of one needle in each of the 10 randomly chosen fascicles, a 

short section was removed and prepared for microscopic examination. Thickness 

of needles (actually radius of the fascicle, C11) was measured. Semicircles from 

two-needled fascicles and pie-shaped sections from three-needled fascicles were 

the subjects measured (in units of an optical micrometer). Each needle of ponderosa 

pine has two large resin ducts, one in each outside corner. Other smaller resin ducts 

were counted on adaxial and abaxial sides of each needle. Average numbers of 

adaxial (C12) and abaxial (C13) resin ducts were calculated. The average number of 

thick-walled hypodermal cell layers (immediately inside the epidermis) in corners 

of needles (C14) was determined.

Length and breadth of each of up to five dried cones from a tree were measured. Averages of length of cones (C15), width of cones (C16), and cone form 

(width divided by length) (C17) were calculated for each parent tree.

Calipers were used to measure length of seed plus wing, wing alone, and length 

and width of each of 10 seeds from each tree. Averages were calculated for seed 

length (C18), seed width (C19), length of seed plus wing (C20), seed form (length/

width) (C23), and absolute wing length (C24).

Wings attached to seeds varied in shape. Some were widest at midpoint 

between tip of the seed and tip of the wing. Other wings had their widest point 

closer to tip of the wing. For each of 10 seeds, the widest point was estimated as 

proportion of distance from tip of the seed to tip of the wing (i.e., 0.4, widest point 

closer to seed; 0.7, widest point closer to tip of the wing). The average proportional 

distance for widest point of wing (C21) was calculated.

3

 The use of trade or firm names in this publication is for reader information and does not 

imply endorsement by the U.S. Department of Agriculture of any product or service.
13

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Estimation of Climate

General data on climate for weather stations within the range of ponderosa pine 

were used to estimate climatic conditions where trees grew. Climatic summaries 

published by U.S. Weather Bureau (1954) for each state and provided by the British 

Columbia Forest Service for that province were used to develop climatic data for the 

station closest to each plot:

• Average precipitation in inches—

September through June, total and percentage of annual

May plus June, total and percentage of annual

• Average annual temperature in degrees Fahrenheit—

April through June = spring

July mean

January mean

July mean minus January mean

• Growing season—

Length in days

Date of start

Day length at start

Climatic data for each weather station were adjusted—using graphical methods 

developed by Baker (1944)—for differences in elevation between the weather station and the nearby plot.

Data Analyses (by Diebel)

Diebel (1984; see “Acknowledgments”) used a stepwise approach to analyze data. 

Visual interpretation of data for plots arranged geographically was followed by 

analyses of variance. Results of these analyses were used in cluster analyses to 

delineate racial boundaries. Tentative races were analyzed by multivariate and 

univariate statistical techniques to determine validity of boundaries. Data from the 

three plantations containing 10 provenances were used to estimate genetic control 

of characteristics used for discriminating races. Finally, simple correlations among 

climatic and elevational data and morphological characteristics were investigated.

Step 1: visual data interpretation— 

Plot mean and variance for each characteristic were calculated and plotted to determine ranges of variability and to identify where discontinuities occurred across the 

range of ponderosa pine (table 2). 
14

research paper psw-rp-265

Step 2: analyses of variance— 

An elementary one-way analysis of variance used plot means (usually for 10 trees, 

rarely as few as one tree) for each morphological characteristic (as the dependent 

variable). These analyses pointed to characteristics most likely to be useful as racial 

discriminators (table 2). Because of unequal numbers of trees within plots, the general linear model (GLM) program in SAS (Ray 1982) was used.

Step 3: cluster analyses—

Branch and foliage characteristics that differed significantly among plots were subjected to cluster analyses. Plot means were analyzed to determine whether racial 

Table 2—Significance of variation among characteristics of ponderosa pines from varying 

number of plots throughout the range of the species 

Characteristics Code F-valuea Plots

Years needles remain green C1 22.4 145

Foliaged length: terminal bud to oldest green needles (cm) C2 20.5 145

Branches per whorl C3 27.5 144

Angle of branching from stem (10°) C4 7.3 145

Hue of needle color C5 2.1 138

Value of needle color C6 9.5 138

Chroma of needle color C7 8.8 138

Needle length (mm) C8 35.6 143

Needle length/fascicle length C9 21.3 142

Needles per fascicle C10 20.9 143

Needle thickness (= fascicle radius) C11 6.2 143

Adaxial resin canals in needles C12 4.7 145

Abaxial resin canals in needles C13 3.3 143

Hypodermal cell layers in corners of needles C14 10.5 143

Seed cones and seeds:

Cone length (mm) C15 16.8 120

Cone width (mm) C16 11.8 120

Cone form (width/length) C17 1.2NS 120

Seed length (mm) C18 16.5 78

Seed width (mm) C19 12.6 78

Seed wing length (mm) C20 22.1 77

Position from tip of seed to where seed wing widest C21 14.2 77

Total smaller resin canals (= C12 + C13) C22 4.6 143

Seed form (length/width) C23 3.4 78

Absolute length of seed wing (mm) (= C20 - C18) C24 10.4 77

Proportional length of seed wing (= C20 / C18) C25 13.1 78

a F-values from a one-way analysis of variance on range-wide data; NS = not significant, p > 0.05.
15

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

boundaries could be delineated. Because cone and seed data were not available 

for many plots, only characteristics of branching, shoots, and foliage were used. 

Characteristics C1, C2, C3, C8, C9, C10, and C14 (table 2) were included in cluster analyses. The statistical procedure used was Ward’s method (Milligan 1980) as 

emulated by SAS (Ray 1982).

Step 4: analysis of east-west varieties—

Diebel used stepwise selection procedures (Klecka 1980, as emulated in SAS [Ray 

1982] with level for inclusion at 0.05) to isolate those foliar characteristics most 

capable of discriminating between the Rocky Mountains race and ponderosa pines 

growing to the west. Following isolation of likely discriminators, data from all plots 

were subjected to discriminant function analysis (Rao 1973) to classify plots into 

one race or the other.

Because discriminant function procedures require a priori decisions on boundaries, the boundary line was based on analyses of plot means and cluster analyses 

and on considerations of data and hypotheses described in the literature. For this 

analysis, meridian 112.5° West was used to separate eastern and western varieties of 

ponderosa pine in Montana.

This decision was based on a number of criteria. First, while “natural breaks” 

for needles per fascicle (C10) and hypodermal cells (C14) occurred at 110° meridian, all other morphological characteristics showed a pattern of continuous variation over that portion of the range. Second, using all plots with foliar data, cluster 

analysis placed plots 110, 111, and 113 in the Rocky Mountains region, despite the 

“natural breaks.” Third, Read (1980) identified the area between the 110° and 112.5° 

meridians as a transition zone, and he agreed with Weidman’s (1939) identification 

of the Continental Divide as a boundary between races. Therefore, the Continental 

Divide in Montana was accepted as the boundary between two varieties and races.

Step 5: analysis of races—

Eastern and western varieties of ponderosa pine, as defined by step 4 above, were 

further subdivided. Racial boundaries were drawn based on consideration of literature and interpretation of results of cluster analyses in step 3.

Five races of ponderosa pine were defined (fig. 1). Using multivariate techniques described in step 4, Diebel first compared all five races simultaneously (step 

5A). Then characteristics of foliage, seeds, and cones were compared between adjacent races (step 5B). Comparisons using 140 plots having foliar data were followed 

by comparisons using fewer plots that had foliar, seed, and cone data.

Multivariate analyses comparing races were supported by Hoetelling’s multivariate t-test (Brown 1977) to detect significant differences among races. A simple 
16

research paper psw-rp-265

univariate Student’s t-test was also used to test for differences between races for 

each discriminant characteristic. To characterize each race, mean and standard 

error of the mean were calculated for each characteristic (table 3).

Step 6: provenance study of foliar discriminators—

Three provenance studies were analyzed for foliar characteristics. A few of the 

characteristics measured on trees in situ were not measured in the provenance 

study. Of those used as discriminators of races in step 5, only distance from terminal bud to oldest green needles (C2) was not recorded. Data were analyzed using a 

linear model:

Yijk = P/i + SSj + Pi x SSj + errorijk

where

Y = trait measured,

P = plantation effect where i = 1, 2, 3, and

SS = seed source (or provenance) effect where j = 1, 2...J (usually 10).

Of particular interest in the analysis were relative strengths of seed source 

and plantation X seed source interactions for those characteristics used as racial 

discriminators. Diebel hypothesized that seed source effects would be significant, 

and genotype-environment interaction would be small. Acceptance of this hypothesis would indicate that races defined in step 5 might indeed be based on genetic 

affinities.

Variances resulting from plantation, provenance, and interaction were calculated by SAS (Ray 1982) according to Hartley, Rao, and LaMotte (1978). This 

provided data necessary for evaluating genetic significance of races defined by 

multivariate analyses.

Step 7: correlation between characteristics, climate, and geography—

Coefficients were calculated to determine significance of correlations between 

mean values of plots for each of 14 foliage, cone, or seed characteristics, with seven 

climatic variables and then again with three geographic variables. The 140 correlation coefficients were calculated first using means for all plots for the species, as a 

whole, and again for plots in each of five races.

Results

Step 1: visual data interpretation—

Visual examination of plot means and variances (tables 2 and 3) led to a conclusion 

that only two “natural breaks” in values of characteristics occurred at the 110° meridian. All fascicles had three needles (C10) on plots 110, 111, and 113 lying between 
17

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Table 3—Means with standard errors of means plus range of plot means [by Diebel] derived from multiple measurements on each of 10 trees growing at a varying number of plot locations for in situ morphological characteristics of five races of Pinus ponderosa

Races

Pacific North Plateau Rocky Mountains Southwest Central High Plains

Characteristics Means SE Ranges Plots Means SE Ranges Plots Means SE Ranges Plots Means SE Ranges Plots Means SE Ranges Plots

Foliage

Years needles remain green 3.9 (0.25, 1.4–6.5) 30 4.7 (0.14, 2.2–7.6) 50 5Y.7 (0.28, 3.6–7.9) 23 4.3 (0.18, 2.5–5.9) 24 4.7 (0.18, 4.1–5.0) 5

Foliage length on branch (cm) 25.1 (2.4, 7.4–52.1) 30 26.2 (2.2, 5.1–84.6) 50 21.1 (1.7, 11.4–42.9) 23 21.8 (2.7, 6.6–53.8) 24 42.2 (6.7, 25.9–65.0) 5

Needle color hue 34.8 (0.25, 32.3–40.4) 27 34.3 (0.10, 31.5–35.2) 47 34.3 (0.11, 32.9–35.0) 22 34.4 (0.16, 32.8–36.2) 24 34.6 (0.27, 33.5–35.0) 5

Needle color value 4.4 (0.09, 3.0–5.0) 27 4.4 (0.07, 3.6–5.1) 47 4.5 (0.09, 3.9–5.4) 22 4.8 (0.13, 3.7–6.1) 24 4.6 (0.19, 4.1–5.0) 5

Needle color chroma 4.6 (0.08, 3.5–5.0) 27 4.5 (0.06, 3.0–5.0) 47 4.2 (0.14, 2.5–5.0) 22 4.0 (0.16, 2.4–5.0) 24 4.8 (0.13, 4.3–5.0) 5

Needle length (cm) 19.8 (0.44, 14.4–24.3) 30 16.8 (0.29, 13.0–20.5) 48 11.2 (0.27, 9.2–14.4) 23 14.7 (0.45, 11.2–19.8) 24 15.6 (0.57, 14.8–17.9) 5

Needle length/fascicle length 8.1 (0.28, 5.8–12.8) 30 7.4 (0.15, 5.8–10.0) 48 6.1 (0.22, 4.2–7.8) 23 6.4 (0.26, 4.5–9.9) 23 9.3 (0.36, 8.2–10.4) 5

Needles per fascicle 3.0 (0.002, 2.99–3.04) 30 3.0 (0.002, 2.97–3.07) 48 2.6 (0.06, 2.2–3.0) 23 3.0 (0.03, 2.7–3.5) 24 2.4 (0.11, 2.2–2.8) 5

Needle thick. (arbitrary units) 45.9 (0.49, 41.9–54.6) 30 47.8 (0.51, 37.6–58.8) 48 46.4 (0.68, 39.4–53.4) 23 44.8 (0.87, 34.4–51.1) 24 49.7 (0.61, 47.4–51.0) 5

Adaxial resin ducts/needle 0.8 (0.04, 0.25–1.22) 30 0.8 (0.04, 0.1–1.5) 48 0.9 (0.10, 0.3–2.5) 23 1.2 (0.09, 0.7–2.4) 24 1.5 (0.35, 0.6–2.6) 5

Abaxial resin ducts/needle 2.6 (0.15, 1.2–3.9) 30 2.3 (0.13, 0.1–4.1) 48 2.4 (0.15, 1.5–3.9) 23 2.4 (0.18, 0.8–4.4) 24 3.9 (0.60, 2.1–5.6) 5

Small resin ducts/needle 3.2 (0.19, 0.9–4.9) 30 2.8 (0.17, 0.4–5.2) 48 3.1 (0.25, 1.8–6.4) 23 3.4 (0.27, 1.1–6.4) 24 5.4 (0.98, 2.5–8.2) 5

Hypodermal cell layers 3.2 (0.04, 2.7–3.7) 30 3.4 (0.03, 2.7–3.9) 48 4.1 (0.05, 3.6–4.4) 23 4.1 (0.06, 3.4–4.5) 24 4.7 (0.06, 4.5–4.9) 5

Branches

Branches per whorl 4.4 (0.13, 2.7–6.0) 30 3.7 (0.11 1.9–5.6), 50 3.0 (0.17 2.0–4.6), 23 3.4 (0.25, 1.6–6.6) 23 23 (0.11, 1.1–1.6) 5

Branch angle (degrees from vert.) 56 (1.8, 30–75) 30 51 (1.7, 30–79) 50 50 (2.3, 31–76) 23 48 (3.1, 30–79) 24 36 (1.9, 31–40) 5

Seed cones 

Cone length (mm) 101.4 (2.48, 63.6–121.7) 25 88.7 (1.24, 72.5–103.0) 36 70.7 (2.20, 52.4–92.6) 22 74.9 (2.51, 60.2–99.2) 20 71.1 (2.46, 64.8–77.1) 5

Cone width (mm) 77.1 (1.35, 56.3–88.6) 25 71.6 (0.73, 63.6–81.4) 36 61.5 (1.08, 53.1–69.1) 22 62.6 (1.77, 47.6–75.6) 20 63.3 (2.18, 58.6–68.9) 5

Cone form (width/length) 0.80 (0.03, 0.70–1.50) 25 0.84 (0.03, 0.80–1.80) 36 0.90 (0.02 0.70–1.00), 22 0.86 (0.02, 0.80–1.00) 20 0.90 (0.03 0.80–1.00) 5

Seeds

Seed length (mm) 7.5 (0.08, 6.9–8.1) 23 7.6 (0.16, 6.5–8.4) 14 6.3 (0.09, 5.4–6.9) 17 6.4 (0.18, 5.1–7.4) 16 7.0 (0.12, 6.5–7.2) 5

Seed width (mm) 4.9 (0.05, 4.3–5.3) 23 4.9 (0.08, 4.5–5.5) 14 4.1 (0.05, 3.6–4.6) 17 4.3 (0.09, 3.8–4.9) 16 4.5 (0.10, 4.2–4.7) 5

Seed + wing length (mm) 32.3 (0.58, 24.2–37.0) 23 24.8 (0.62, 20.2–29.5) 14 22.9 (0.63, 18.0–27.6) 17 23.3 (0.68, 18.2–26.2) 15 23.1 (0.78, 20.8–24.9) 5

Seed wing widest (base to tip) 0.36 (0.02, 0.30–0.50) 23 0.46 (0.01, 0.40–0.50) 14 0.44 (0.02, 0.30–0.50 17 0.44 (0.01, 0.40–0.50) 15 0.48 (0.02, 0.40–0.50) 5

Seed form (length/width) 1.52 (0.01, 1.40–1.60) 23 1.57 (0.02, 1.40–1.70) 14 1.52 (0.01, 1.40–1.60) 17 1.48 (0.02, 1.30–1.60) 16 1.56 (0.02, 1.50–1.60) 5

Wing length, absolute (mm) 24.8 (0.57, 16.1–28.9) 23 16.6 (0.70, 13.0–21.8) 14 16.4 (0.67, 10.0–21.1) 17 15.5 (1.47, 15.1–19.3) 16 16.1 (0.68, 14.3–17.7) 5

Wing length, proportional 4.34 (0.08, 3.00–4.90) 23 3.26 (0.10, 2.90–4.10) 14 3.62 (0.09, 3.10–4.30) 17 3.59 (0.07, 3.30–4.00) 15 3.30 (0.06, 3.20–3.50) 5
18

research paper psw-rp-265

110° and 112.5°. To the east, plots 114, 115, 116, and 117 were characterized by 2.35, 

2.25, 2.44, and 2.52 needles per fascicle, respectively. Trees in plots 110, 111, and 

113 and proximal plots west of 112.5° possessed fewer than 3.5 hypodermal cell 

layers in needle corners (C14). Eastward from 110° meridian, plots 114, 115, 116, 

and 117 had more than 4.0 hypodermal cell layers. These data confirmed earlier 

interpretations of this compact transition zone, using data collected both in situ and 

from provenance trials (Critchfield 1984, Korstian 1924, Read 1980, Weidman 1939, 

Wells 1964a).

Step 2: analyses of variance—

One-way analyses of variance of characteristics yielded highly significant plot differences for all characteristics except cone shape (C17) (table 2). Relatively low 

F-values were recorded for branch angle (C4), hue, value, and chroma of needle 

color (C5, C6, and C7), needle thickness (C11), needle resin ducts (C12, C13, and 

C22), and seed form (C23).

One possible explanation for relatively insignificant differences among plots for 

hue, value, and chroma of needle color may be the difficulty of subjectively evaluating these characteristics on samples received over a period of months and therefore 

varying in maturity and freshness. Relatively low F-values were calculated for some 

quantitative characteristics, i.e., branch angle (C4), needle thickness (C11), and 

needle resin ducts (C12, C13, and C22). Within-plot variance for these characteristics is relatively large on a range-wide basis.

Step 3: cluster analyses—

Cluster analyses using selected foliar characteristics did not yield clear racial 

boundaries. The first cluster delineated two general races separated at 112.5° meridian. Plots 110, 111, and 113 clustered with Rocky Mountains plots. Plots 96, 108, 

109, 124, 125, 182, and 183 clustered with western plots. Many anomalies were 

present, particularly in the western grouping. Plots 38, 41, 52, 79, 97, 98, 105, 107, 

123, 159, 158, 167, and 44 all clustered with eastern races while plots 23, 133, and 

135 clustered with western races. Plot 44, as will be discussed later, showed evidences of natural hybridization with Jeffrey pine (P. jeffreyi Grev. & Balf. in A. 

Murray). This may explain why it grouped with eastern races.

Cluster analyses suggested presence of several transition zones. These zones 

separated five geographic races:

• Pacific: including California west of the summits of the Cascade Range and 

the Sierra Nevada and moist western valleys of Oregon and Washington

• North Plateau: including the Inland Empire and eastern slopes of both the 

Cascade Range and Sierra Nevada
19

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

• Rocky Mountains: including all areas east of the Continental Divide in 

Montana and Wyoming and central Rocky Mountains north of latitude 

41°N except for the central high plains and the southwest described next

• Central High Plains: including drainages of the White, Niobrara, and Platte 

Rivers in South Dakota and Nebraska

• Southwest: including an area generally in Arizona and associated areas of 

New Mexico subject to bimodal rainfall patterns with monsoonal summer 

rains generated in the Gulf of California

Cluster analyses were useful for delineating five racial boundaries. They were 

too ambiguous to draw boundaries within the five racial boundaries comparable to 

those drawn by Read (1980, 1983).

Step 4: analysis of east-west races—

Stepwise discriminant analysis, using both forward and backward procedures, 

chose needle length (C8), needle thickness (C11), adaxial resin ducts (C12), and 

hypodermal cell layers (C14) as four discriminators between western and eastern 

races. However, a discriminant analysis using all foliar characteristics provided 

the best reclassification percentage (95 percent correct for eastern races and 100 

percent correct for western races). The high percentage of correct classification 

confirms the long-recognized distinction between taxonomic varieties of ponderosa 

and scopulorum.

Only plots 110, 111, and 113 were misclassified as var. ponderosa. These plots 

are adjacent to the boundary between varieties in Montana. Cluster analysis had 

grouped these with other eastern plots. Contradictions between the two analyses 

and geographic locations indicate that these plots are part of a transition zone centered in Helena, Montana. This finding agrees with Read’s (1980) narrow transition 

zone in Montana.

Step 5a: simultaneous analyses of hypothesized races—

Forward and backward stepwise discriminant procedures selected number of 

branches per whorl (C3), branch angle (C4), needle length (C8), needle length/ 

fascicle sheath length (C9), needles per fascicle (C10), needle thickness (C11), adaxial resin ducts (C12), and hypodermal cell layers (C14) as best discriminators among 

five races considered simultaneously. Canonical discriminant analysis, using these 

variables, indicated that five races are distinctly different (table 4 and fig. 2). Three 

evidences of this difference are:

• Very highly significant correlation coefficient (0.92, p < 0.001, table 4)

• Relatively large Eigen value (6.2, table 4)

• Large separation among group centroids (fig. 2).
20

research paper psw-rp-265

Table 4—Results of a canonical discriminant analysis evaluating 

five ponderosa pine races simultaneously using only foliar 

characteristics 

Standardized discriminant 

function coefficients

Branch or foliage character Code 1 2

Branches per whorl C3 0.2878 -0.1052

Branch angle C4 0.2753 -0.1497

Needle length C8 0.9154 1.2687

Needle fascicle ratio C9 -0.1594 -0.1626

Needles per fascicle C10 0.0763 0.8619

Needle thickness C11 0.4107 -0.6150

Adaxial resin ducts C12 -0.5238 0.1064

Hypodermal cell layers C14 -1.6595 1.3356

Eigen value 6.24 0.91

P-value 0.0001 0.0001

Adj. canonical correction 0.923 0.647

Source: Diebel.

2.0

1.5

1.0

0.5

-0.5

-1.0

-1.5

-2.0

-6 -5 -4 -3 -2 -1 1 2 3 4 5 6

Southwest

Central 

High Plains Pacific

North 

Plateau

Rocky 

Mountains

Discriminant Function 1

Discriminant Function 2

Figure 2—Scatter plot of group centroid values for each hypothesized race of ponderosa pine on the 

axes of the first two canonical discriminant functions (source: Kenneth Diebel).
21

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Discriminant analysis correctly reclassified 94 percent of plots from the Rocky 

Mountains race. Only two plots (10 and 11) were misclassified. Plot 10 was classified as belonging to the North Plateau race, and plot 11 to the Southwestern race. 

Plot 11 is located near the hypothesized boundary between the Southwestern and 

Rocky Mountains races. Its misclassification may be an indication of an introgressive transition zone between these races. Classification of plot 10 into the North 

Plateau race is an anomaly. No climatic, elevational, or geographic affinity exists 

between plot 10 and the North Plateau race.

All plots from the Central High Plains race were correctly reclassified. This 

was also the case for both the Pacific and Southwestern races.

Ninety-four percent of plots from the North Plateau race were correctly reclassified. Plots 34, 53, and 119 were misclassified as belonging to the Pacific race. Plot 

53 is located near the hypothesized boundary between these races. This misclassification may be related to a transition zone that will be discussed in the next section. 

There is no obvious explanation for misclassification of plots 34 and 119.

The high percentage of correctly reclassified plots is strong evidence of appropriateness of hypothesized racial designations. Potential transition zones among five 

geographically adjacent races were compared separately. Results of these paired 

comparisons are summarized in the following section.

Step 5b: analyses of adjacent races— 

Characteristics selected for discriminating between geographically adjacent races 

allowed correct classification of 86 percent of plots for all comparisons. Needle 

length (C8) was the best overall discriminant foliar characteristic among races 

(table 2). Hue (C5), value (C6), and chroma (C7) of foliage color were characteristics 

providing poorest discrimination. Other foliar characteristics were of intermediate 

value; that is, they were useful in discriminating between some, but not all, races. 

No cone or seed variable was singularly best or poorest discriminator between adjacent races (table 3).

Paired comparisons of selected characteristics (using student’s t-test) and a 

multivariate comparison based upon those selected traits (Hoetelling’s t-test) 

(table 5) generally reinforced appropriateness of five racial boundaries.

Pacific vs. North Plateau—Foliar characteristics useful for discriminating between 

these two races were number of branches per whorl (C3), needle length (C8), and 

needle thickness (C11). Using these three characteristics, 89.5 percent of Pacific 

plots could be correctly classified, and 86.2 percent of North Plateau plots could be 

correctly classified.
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research paper psw-rp-265

Table 5—Results of multivariate (Hoetelling) comparisons between adjacent 

races and of univariate (student’s t-test) comparisons between discriminating 

characteristics of adjacent races (by Diebel)

Multivariate Comparisons

Races compared Branch and foliage Seed cone and seed 

- - - - - - - - - - - P > F - - - - - - - - - - - 

Pacific vs. North Plateau 0.001 N.A.*

North Plateau vs. Rocky Mountains 0.001 0.001

Rocky Mountains vs. Southwest 0.001 0.008

Rocky Mountains vs. Central High Plains 0.001 0.001

Southwest vs. Pacific 0.001 0.001

Univariate Comparisons

Branch/foliage Means Code P > F Cone or seed Means Code P > F 

Pacific vs. North Plateau

Branches per whorl 4.4–3.7 C3 0.001 Seed wing length 4.3–3.3 C25 0.001

Needle length 198–168 C8 0.001 proportional (=C20 / C18)

Needle thickness 45.9–47.8 C11 0.001

North Plateau vs. Rocky Mountains

Needle length 168–112 C8 0.001 Seed+wing length 24.8–22.9 C20 0.052

Hypodermal cell layers 3.4–4.1 C14 0.001 Cone form 0.84–0.90 C17 0.004 

Seed form 1.57–1.52 C23 0.312

Seed wing length 16.6–16.4 C24 0.753

absolute (=C20–C18)

Rocky Mountains vs. Southwest

Needle length 112–147 C8 0.001 Seed form 1.52–1.48 C23 0.492

Needles per fascicle 2.6–3.0 C10 0.001 Seed wing length 16.4–15.5 C24 0.375

Needle thickness 46.4–44.8 C11 0.244 absolute (=C20–C18)

Hypodermal cell layers 4.08–4.07 C14 0.352 Seed wing length 3.62–3.59 C25 0.433

proportional (=C20 / C18)

Rocky Mountains vs. Central High Plains

Foliage length 21.1–42.2 C2 0.001 Seed+wing length 22.9–23.1 C20 0.935

Needle length 112–156 C8 0.001 Seed wing length 16.4–16.1 C24 0.500 

Adaxial resin ducts 0.9–1.5 C12 0.011 absolute (=C20–C18)

Hypodermal cell layers 4.1–4.7 C14 0.001

Southwest vs. Pacific

Years needles green 4.3–3.9 C1 0.194 Cone form 0.86–0.80 C17 0.001

Foliaged length 21.8–25.1 C2 0.483 Seed+wing length 23.3–32.3 C20 0.001

Branch angle 48–56 C4 0.023

Needle length 147–198 C8 0.001

Adaxial resin ducts 1.2–0.8 C12 0.001

Hypodermal cell layers 4.1–3.2 C14 0.001

*N.A. = not available.
23

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Misclassified plots included plots 33, 34, 38, 107, and 119 in North Plateau and 

plots 44, 52, 80, and 81 in Pacific. Misclassified plots were examined and judged 

neither to be associated with any particular climatic or elevational pattern nor the 

border between these races. Plot 44, located at 6,000 ft elevation east of Placerville, 

California, in a known region of introgression between ponderosa pine and Jeffrey 

pine, seemed to be intermediate between these two species. Other misclassified 

Pacific plots were on the border delineating Pacific from North Plateau. This suggests either a gradual transition between these races along that boundary or a meandering boundary where a straight line has been proposed (fig. 1). Sturgeon’s (1979) 

data supported location of this boundary and a major turning point at approximately 

41°40’N, 122°10’W.

In a univariate test, three discriminant foliar characteristics differed significantly between Pacific and North Plateau plots (table 5). Pacific trees had longer, 

thinner needles (table 3). Pacific trees also had more branches per whorl. These 

features make crowns of Pacific trees particularly distinguishable. Long needles, 

generally a mesic characteristic, may be a result of higher available moisture in the 

maritime climate west of the crests of both the Sierra Nevada and Cascade Range.

Only one cone or seed character—proportional length of seed wing (C25)—

was found to be useful in discriminating North Plateau race from the Pacific race. 

On the basis of this characteristic, 92 percent of North Plateau plots and 100 percent 

of the Pacific plots were correctly classified. Only plot 79, at the south end of Puget 

Sound, was misclassified, suggesting it should be included in the Pacific race.4 5

Haller found:

... North Plateau cones are usually heavier for their size and have more 

phyllotactic spirals than those from the Pacific race. They look denser. For 

me, this is the most easily observable difference between the two races. 

Cones from the North Plateau also are often smaller than those from the 

Pacific race, especially those from the western Sierra Nevada.

Statistics from this study confirm Haller’s points, but Diebel did not find 

statistically significant differences.

Pacific race includes western mesic portions of Oregon and Washington. 

Haller’s (1965) coastal ecotype, based on his in situ observations of adult trees, 

supports boundaries for the Pacific race. Haller and Vivrette (2011) named his 

coastal ecotype Pinus ponderosa var. pacifica.

4

 Haller, J.R. (retired professor, Univ. of California at Santa Barbara) 1992. Telephonic 

conversation with this author: “My numbers for the seed/wing proportion at this site support your conclusion.” 

5

 Haller, J.R. 1992. Personal letter to this author.
24

research paper psw-rp-265

North Plateau vs. Rocky Mountains—Foliar morphological characteristics to 

discriminate between North Plateau and Rocky Mountains races were needle length 

(C8) and number of hypodermal cell layers in needle corners (C14). Just these 

two characteristics yielded 97.7 and 94.5 percent correct reclassification of North 

Plateau and Rocky Mountains plots, respectively.

Only plots 98, 110, and 111 were misclassified. Plots 110 and 111 are east of 

the Continental Divide at the border between the two races (fig. 1). These plots 

generally coincide with Read’s (1980) narrow transition zone surrounding Helena, 

Montana. Plot 98 commonly was an anomaly in cluster analysis. Plot 98 is located 

approximately 27 mi south and 11 mi east of a known site of P. washoensis on 

Promontory Mountain, British Columbia (Critchfield 1984). Thus, trees from plot 

98 may be introgressants or, more likely, Washoe pine.

On the basis of univariate analyses, North Plateau and Rocky Mountains races 

differed significantly in needle length (C8) and number of hypodermal cell layers in 

the corners of needles (C14) (table 5).

The Rocky Mountains race had the shortest needles of all races (5 in) and more 

hypodermal cell layers than the North Plateau race (table 2). Short needles and a 

thick hypodermis may enable ponderosa pine to survive harsher conditions during 

both summer and winter in truly continental climates.

Four cone and seed characteristics—cone form (C17), length of seed plus wing 

(C20), seed form (C23), and absolute length of seed wing (C24)—were useful 

discriminators between North Plateau and Rocky Mountains races (table 5). Using 

these four characteristics, 100 percent of the North Plateau plots and 95 percent of 

Rocky Mountains plots were correctly classified. Only plot 95 was misclassified in 

Rocky Mountains. Of four cone discriminators, only cone form (C17) was found to 

differ significantly at the 5 percent level between the Rocky Mountains and North 

Plateau races.

On the basis of multivariate comparisons of both cone and foliar characteristics, the North Plateau race differed very significantly (p < 0.001) from the Rocky 

Mountains race.

Rocky Mountains vs. Southwest—Rocky Mountains and Southwestern races 

were discriminated on the basis of needle length (C8), number of needles per 

fascicle (C10), needle thickness (C11), and number of hypodermal cell layers in 

needle corners (C14) (table 5). Classification of Rocky Mountains plots was 86.1 

percent correct. Southwestern plots were 95.6 percent correctly reclassified; only 

plot 140, at high elevation near the northern boundary of the Southwestern race, 

was misclassified.
25

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Rocky Mountains plots 1, 3, 7, 12, 166, and 175 were misclassified as Southwestern. All of these plots are located near the southern boundary of the Rocky 

Mountains race. This suggests presence of a relatively broad transition zone 

between the Rocky Mountains and Southwestern races, centered on “Four Corners” 

(the meeting point of Utah, Colorado, New Mexico, and Arizona). As Read (1980) 

illustrated, this boundary has been difficult to define. Very intensive sampling will 

be needed to define trends within the zone.

On the basis of univariate analyses, the Rocky Mountains and Southwestern 

races differed significantly in number of needles per fascicle (C10) and needle 

length (C8). Rocky Mountains trees had shorter needles and fewer needles per 

fascicle than Southwestern trees (table 2). Differences in needle thickness (C11) and 

number of hypodermal cell layers (C14) were not significant at the 5 percent level. 

Frequent and heavy summer rains and milder winters in the southwest probably 

have contributed to evolution of these racial differences.

On the basis of three selected seed discriminators—seed form (C23), absolute 

length of seed wing (C24), and proportional length of seed wing (C25)—91 percent 

of Rocky Mountains plots and 82 percent of Southwestern plots were correctly classified. In the Rocky Mountains, plots 175, 117, and 115 were misclassified, and in 

Southwestern, plots 129 and 190 were misclassified. The relatively high percentage 

of misclassified plots may be due to small differences between races for the selected 

discriminators.

The Rocky Mountains race differed statistically from the Southwestern race in 

multivariate comparisons using data on branches, foliage, cones, and seeds (table 5).

However, in univariate comparisons no cone characteristic (C23, C24, or C25) differed significantly between the Rocky Mountains and Southwestern races (table 5).

Rocky Mountains vs. Central High Plains—The Rocky Mountains and Central 

High Plains races were 100 percent correctly reclassified by comparing foliage 

length (C2), needle length (C8), number of adaxial resin ducts (C12), and number 

of hypodermal cell layers (C14). The Rocky Mountains and Central High Plains 

races also differed significantly from one another in multivariate foliar comparisons 

(table 5).

All four traits used in a univariate test of differences between the Rocky 

Mountains and Central High Plains races differed significantly (table 5). Trees of 

the Central High Plains retained their needles 1 less year (4.7 vs. 5.7). Distance from 

branch tip to oldest green needles for the Central High Plains was twice that for the 

Rocky Mountains (1.3 vs. 0.7 ft). Needles from the Central High Plains were considerably longer than those from the Rocky Mountains (6.1 vs. 4.6 in). The Central 

High Plains also had more adaxial resin ducts (1.6 vs. 0.9) and more hypodermal 

cell layers (4.7 vs. 4.0) than did the Rocky Mountains.
26

research paper psw-rp-265

Foliar characteristics of Central High Plains’ trees appear to be contradictory 

with respect to drought hardiness. For example, more hypodermal layers may 

yield greater drought avoidance, whereas longer needles and much more foliage 

on the branch would appear to be counterproductive from the standpoint of water 

conservation. Trees of the Central High Plains have more resin ducts (both adaxial 

and abaxial) than all other races, which may impart a higher resistance to injury or 

damage from inclement weather, insects, and diseases.

Analyses of cone and seed characteristics between the Rocky Mountains and 

Central High Plains races were less definitive. Discriminant analysis based on 

length of seed plus wing (C20) and absolute length of seed wing (C24) yielded 85.7 

percent correct classification of the Rocky Mountains plots (misclassified were 

plots 19, 114, and 117), and 80 percent correct classification of the Central High 

Plains plots (plot 26 was misclassified). Because misclassified plots show little 

geographic affinity, it would appear that these two taxa are not solidly differentiated on the basis of cone and seed characters. Although results of the univariate 

t-test showed non-significant differences, Hoetelling’s multivariate t- test confirmed 

a statistically significant difference of the Rocky Mountains race from the Central 

High Plains race based on length of seed plus wing (C20) and absolute length of 

seed wing (C24) (table 5).

Southwestern vs. Pacific—Discriminant analysis of Southwestern and Pacific races 

yielded 100 percent correct classification of all plots based on number of years needles remain green (C1), foliage length (C2), branch angle (C4), needle length (C8), 

number of adaxial resin ducts (C12), and number of hypodermal cells (C14).

On the basis of univariate tests, the number of years needles remain green (C1) 

and foliage length (C2) did not differ significantly between these two races (table 5), 

but four other selected characteristics did differ significantly between the two races. 

Needles were one-third longer for Pacific than for Southwestern races (table 2). 

Adaxial resin ducts and hypodermal cell layers were more numerous in Southwestern than in Pacific races. These characteristics may help provide increased drought 

hardiness or pest resistance for the Southwestern race.

Discriminant analysis based on cone shape (C17) and length of seed plus wing 

(C20) also yielded 100 percent correct classification of Southwestern and Pacific 

races. Univariate analyses indicated significant differences between the Southwestern and Pacific races for both C17 and C20 (table 5).

On the basis of multivariate comparisons of foliar, cone, and seed data, the 

Southwestern race differed significantly from the Pacific race (table 5). These 

results support Critchfield’s (1984) finding of no phylogenetic affinity between the 

Southwestern and Pacific races.
27

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Step 6: provenance study of foliar discriminators—

Genetic control of foliar characteristics used in discriminant analyses was examined. Levels of significance were determined for effects on each characteristic of 

plantation, provenance, and plantation x provenance interaction (table 6). Of all 

characteristics used as discriminators of races, only branch angle (C4), used to 

discriminant the Pacific race from the Southwestern race, did not exhibit significant 

variation owing to provenance.

Relative contributions of plantation, seed source, and provenance x plantation 

interaction to total variance indicated that needle length (C8), needle thickness 

(C11), and number of hypodermal cell layers (C14) were under strongest genetic 

control (table 6). Needle length was picked as a discriminator of all races. Number 

of cell layers in corners was picked in four of five comparisons. Needle thickness 

was used in two of five comparisons.

Correlation between needle length of trees growing in plantations and in situ 

(r = 0.64), reported by Squillace and Silen (1962), supports this finding. Additionally, Grant et al. (1989) estimated a heritability value greater than 0.5 for length of 

primary needles on 3-year-old seedlings resulting from a nested mating design.

Wells (1964b) established that length of secondary needles, as measured in 

this study, was significantly correlated with four characteristics of climate at seed 

source. His data were from 2-year-old seedling progenies, grown in Michigan, from 

33 eastern sources of variety scopulorum. He established coefficients of correlation 

(r) between needle length and four characteristics of climate:

Table 6—Partitioning of variance for each effect in the provenance experiment 

(by Diebel)

Percent of total variance due to:

Branch or foliage character 

in plantations Code

Seed source 

(P)

Interaction 

(SS)

Error 

(P x SS)

Years needles remain green C1 39.1** 4.5** 29.5** 26.9**

Branches per whorl C3 43.7** 1.9** 4.2* 50.2*

Branch angle in tree top C4 0.7** 1.5 2.9 87.9

Needle length C8 68.8** 16.9** 5.1 9.2

Needle length/fascicle length C9 30.2 9.7** 9.3** 50.8

Needles per fascicle C10 0.3 4.0* 0.0 95.7

Needle thickness C11 4.9** 12.2** 8.2* 74.7

Adaxial resin ducts in needles C12 18.6** 2.0* 11.7* 67.7

Abaxial resin ducts in needles C13 94.7** 1.4** 0.1 3.8

Hypodermal cell layers in needles C14 7.0** 29.2** 7.3** 56.5

* Effect significant at p 0.05.

** Effect significant at p 0.01.
28

research paper psw-rp-265

• Average spring temperature, r = 0.54

• Annual temperature range, r = -0.52

• Length of growing season, r = 0.38

• Day length at start of growing season, r = -0.51.

The above evidences demonstrate that characteristics measured in situ reflect, 

at least in part, real genetic differences among populations.

Step 7: correlations between characteristics, climate, and geography—

For the species as a whole, coefficients of correlation (r) between foliage, cone,

and seed characteristics and climatic or geographic measures of site variability 

were usually significant (p < 0.05) or highly significant (p < 0.01). Within races,

182 correlations between characteristics of trees and either climatic or geographic 

variables were statistically significant (tables 7 and 8). To discuss each of these 

relationships is beyond the scope of this paper. Only some of the most interesting 

correlations for each race are considered.

Pacific Race

Only two foliar characteristics had multiple significant correlations with climatic 

variables. Years that needles remain green (C1) were significantly related to all 

but one climatic variable. Most of these correlations were negative indicating that 

years of needle retention increased as (1) precipitation decreased, (2) temperatures 

decreased, and (3) growing season became shorter. These relationships help to 

explain why years of needle retention (C1) increased significantly as elevation of 

plot increased. All of the foregoing would indicate that years of needle retention 

increase as conditions for growth become less favorable or more stressful.

Only one other foliar characteristic—needle length (C8)—had multiple significant correlations with climatic variables in the Pacific race. It was positively related 

to most of the climatic variables. Individual needles grew longer with increases 

in (1) summer precipitation, (2) growing season temperature, (3) growing season 

duration, and (4) seasonal temperature range. These relationships undoubtedly 

contributed to the negative correlation between needle length and elevation.

Surprising in the Pacific race was the significant, but contradictory, relationship of length of green foliage (C2) to two climatic variables: latitude and summer 

precipitation. Length of green foliage increased as latitude increased (northward 

through California and Oregon’s coastal valleys to Puget Sound). However, as 

summer precipitation increased (toward the Pacific Northwest and upward into the 

mountains), length of green foliage decreased.

Effects of longitude and latitude varied greatly within this long, narrow race. 

Some of the geographic changes in characteristics were interesting. Branch angle 
29

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Table 7—Significant coefficients of correlation between characteristics of branches and foliage and characteristics of climate and plot locations (by Diebel)

Precipitation Temperature Growing

July and Jan. Growing July Range season Location

Races and characteristics Code August Annual mean season mean July–Jan. length Elevation Longitude Latitude

Pacific (27 or 30 plots)Years needles stay green C 1 -0.52** -0.48* — -0.70** -0.47* 0.56** -0.67** 0.65** — —

Foliage length C 2 -0.43* — — — — — — — — 0.60**

Branch angle C 4 — — — — — — — — 0.35* 0.46*

Foliage color value C 6 — — -0.43* — — — — — — —

Foliage chroma C 7 — — 0.47* — — — — — — —

Needle length C 8 0.77** — — 0.64** 0.56* -0.47* 0.78** -0.59** — —

Needle length/sheath length C 9 — — — — — -0.60** 0.44* -0.38* — —

Needle thickness C11 — — 0.62** — -0.53** — — — — —

Adaxial resin ducts C12 — — — — — — — — — 0.40*

Abaxial resin ducts C13 — — — — — — — — — 0.38*

Hypodermal cell layers C14 — — — — — 0.42* — 0.54** -0.59** -0.57**

Total small resin ducts C22 — — — — — — — — — 0.39*

North Plateau (47–50 plots)Years needles stay green C 1 — — -0.29* -0.41** -0.33* — -0.51** 0.54** — -0.28*

Branch angle C 4 0.41** 0.46** — — — — — 0.29* — -0.39**

Foliage color value C 6 — — — — — — -0.31* — — —

Foliage chroma C 7 — — — — 0.35* — — — — —

Needle length C 8 — — 0.29* 0.49** 0.59** — 0.51** -0.33* — —

Needle fascicle ratio C 9 — — 0.32* 0.36* 0.55** — 0.38** — — —

Needles per fascicle C10 — — — — -0.33* — — — — —

Needle thickness C11 — — — — — — — — 0.29* —

Rocky Mountains (22 or 23 plots)Foliage length C 2 — — — — — — — -0.33* — 0.39*

Foliage hue C 5 — — — 0.35* 0.35* — — — — —

Foliage chroma C 7 — — — — 0.37* — — -0.39* — 0.39*

Needle length C 8 — — — — 0.43* — 0.36* -0.39* — —

Needle length/sheath length C 9 — — — 0.41* 0.54** — 0.34* -0.56** — —

Needles per fascicle C10 — — — -0.39* -0.48** — — 0.43** 0.63** -0.48**
30

research paper psw-rp-265 Needle thickness C11 — 0.35* — — 0.41* — 0.38* — — — Adaxial resin ducts C12 — 0.41* — — — — — 0.42* 0.37* -0.39* Hypodermal cell layers C14 — — — 0.36* 0.37* — — — -0.59** — Total small resin ducts C22 — 0.44* — — — — — — — —

Central High Plains (5 plots)Needles per fascicle C10 — — — — — — — — -0.94* —

Needle thickness C11 -0.90* — — — 0.88* — — — — —

Adaxial resin ducts C12 — — — 0.93* — — — — -0.88* —

Abaxial resin ducts C13 — — — — — — 0.97** — -0.90* —

Total small resin ducts C22 — — — 0.96** — — — — -0.91* —

Southwest (23 or 24 plots)Years needles stay green C 1 — -0.52* — — — -0.60** — — — 0.53**

Foliage length C 2 — — — — — — — — 0.44* 0.45*

Branches per whorl C 3 — — — — — — — -0.50* 0.44* —

Foliage hue C 5 — — — — — — — — 0.42* —

Foliage color value C 6 — — — — — — — — — 0.45*

Needle length C 8 — — — — — — — -0.43* — —

Needle length/sheath length C 9 — — — 0.52* 0.49* — — -0.71** — —

Needles per fascicle C10 — — — — — -0.57** — — — -0.52*

Needle thickness C11 — — — — — — — — — 0.47*

Abaxial resin ducts C13 — — -0.42* — — 0.58** — — 0.45* 0.57**

Total small resin ducts C22 — — — — — 0.56** — — 0.46* 0.52*

* Coefficient significant at p < 0.05.** Coefficient significant at p < 0.01.

Precipitation Temperature Growing

July and Jan. Growing July Range season Location

Races and characteristics Code August Annual mean season mean July–Jan. length Elevation Longitude Latitude

Table 7—Significant coefficients of correlation between characteristics of branches and foliage and characteristics of climate and plot locations (by Diebel) (continued)
31

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Table 8—Significant coefficients of correlation between characteristics of cones and seeds and characteristics of both climate and plot locations (by Diebel)

Precipitation Temperature Growing

July and Jan Growing July Range season Location

Races and characteristics Code August Annual mean season mean July–Jan. length Elevation Longitude Latitude

Pacific (23 or 25 plots)Cone length C15 0.57** — — — 0.58** — — — — —Cone width C16 0.50* — — — 0.58** — — — — —Cone form C17 -0.49* — — — -0.46* — — — — —Seed length C18 — — — — — — — — -0.50* —Seed width C19 — — — — — — — — -0.44* —Seed+wing length C20 0.58** — 0.47* 0.53* 0.69** — — — — —Seed wing widest C21 — — — — — -0.54* — — — —Wing length, absolute C24 0.62** — -0.51* 0.54* 0.74** — — — — —Wing length, proportional C25 0.65** — — 0.49* 0.77* — — — — —North Plateau (14 or 36 plots)Cone width C16 — — — — — — — — — -0.34*Wing length, proportional C25 — -0.65* — — — -0.59* — — — —Rocky Mountains (17 or 22 plots)Cone length C15 — — — — — 0.37* — — 0.66** —Cone width C16 — — — — — 0.33* — — 0.42* —Seed length C18 — — — — — — — -0.45* — 0.45*Seed width C19 — — — — 0.47* — — -0.55* — 0.52*Seed+wing length C20 0.61** 0.49* — — — — — — 0.65** —Seed wing widest C21 — — — — — — — -0.68** -0.59** 0.66**Wing length, absolute C24 0.61** 0.57** — — — — — — 0.67** —Wing length, proportional C25 0.57** 0.52* — — — — — — 0.61** —Central High Plains (5 plots) Cone form C17 — — — -0.90* -0.90* — 0.89* — — —Seed form C23 — — — — — — — — — 0.99**Southwest (16 or 20 plots)Cone length C15 0.57* 0.55* — — — — — — 0.57* —Cone width C16 — 0.61** — — 0.46* — — — 0.50* —Seed length C18 — — 0.73** 0.57* — — 0.74** -0.56* — —Seed width C19 — — 0.76** — — 0.66** 0.72** — — -0.59*Seed wing length C20 — — — — — — — -0.56* — —Seed form C23 — — — 0.55* — — — -0.55* — —Wing length, proportional C25 0.64* — — — — — — — 0.66** —

* Coefficient significant at p < 0.05.** Coefficient significant at p < 0.01.
32

research paper psw-rp-265

(C4) increased in northern and western plots. Hypodermal cell layers (C14) were 

more numerous at higher elevations; this also was the situation in the Rocky 

Mountains race. This significant relationship between number of hypodermal cell 

layers and elevation probably contributed to a negative relationship between number 

of hypodermal cell layers and longitude. In this race, the most eastern plots usually occurred at highest elevations. Both seed length (C18) and seed width (C19) 

decreased in the southern plots.

Haller (1962, p. 131) wrote: “Altitude also affects needle thickness. The needles 

of both P. ponderosa and P. jeffreyi are relatively thick at high elevations, and 

relatively thin at low elevations.” However, in contradiction he later wrote:

I found no increase at all in needle thickness with elevation in either of two 

elevational transects in the Sierra Nevada. In the San Bernardino Mountains, the only significant difference was between the lower elevation and 

mid elevation samples. This is probably anomalous, and may be related to 

the location of some of the low elevation trees in a moist, shady ravine.6

There was no significant correlation between needle thickness and elevation for 

this or any other race.

Haller (1962) reported for this race that proportional length of seeds to seed 

wings changed with elevation, but on subjecting his data to statistical analysis7 he 

found differences were not significant. The only significant difference was between 

his low-elevation and mid-elevation sites in the San Bernardino Mountains. Longer 

wings at his mid-elevation site appeared to be related to larger cones there. However, correlations between proportional seed wing length (C25) and elevation were 

not significant for this or any other race.

Several interrelated cone and seed characteristics showed greater values with 

increased precipitation during July and August and with higher temperature during 

July. These characteristics were cone length (C15), cone width (C16), length of seed 

plus wing (C20), absolute wing length (C24), and proportional wing length (C25). 

Both growing season precipitation and July temperature (but not growing season 

temperature) positively contributed to bigger cones and seeds and to longer seed 

wings. These highly significant relationships did not exist for any other race. Only 

in the Rocky Mountains race did length of seed plus wing (C20), absolute wing 

length (C24), and proportional wing length (C25) show similar highly significant 

correlations with summer precipitation (and annual precipitation), but these characteristics were not correlated with climatic temperatures.

6

 Personal communication from J.R. Haller, March 1992.

7

 Personal communication from J.R. Haller, March 1992.
33

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Ponderosa pine grows intermixed with Jeffrey pine at the upper limits of its 

elevational distribution from the southern Sierra Nevada north into the Cascade 

Range. In southern California areas of overlap, and possibly opportunities for 

hybridization, are somewhat greater.8 That natural hybridization occurs between 

these species has been known for many years9 (Haller 1962). Some ponderosa pines 

growing at highest elevations—both east of Placerville, California (plot 44), at 

6,000 ft elevation on U.S. Highway 50, and west of La Moine (plot 64), near Mount 

Shasta in northern California—exhibited unusually long cones and other features 

that would indicate presence of natural hybrids with the longer-coned Jeffrey pine.

North Plateau race—Two foliar characteristics of the North Plateau race, like 

the Pacific race, had multiple significant correlations with climatic variables. 

Apparently for ponderosa pine’s two western races, years of needle retention (C1) 

and length of needles (C8) differ strongly in association with climatic factors favoring growth. The negative correlation between needles per fascicle (C10) and 

July temperature would indicate a propensity for two-needled trees where summer 

temperatures are hotter. This same negative correlation was found for the Rocky 

Mountains race.

Within the North Plateau race, branch angle (C4) increased as:

• Annual precipitation increased

• Summer precipitation increased

• Elevation increased

• Latitude decreased

Branches of northern trees angled upward toward vertical, presumably as severity of winter and winter’s snow loading would increase. Interestingly, branch angle 

was not significantly correlated with the severity of winter (January) temperatures.

One can only guess at biological significance of correlations between proportional length of seed wing (C25) and climatic variables. Wings that are longer in 

relation to seed length should fall more slowly, for extra wing length should not 

add significantly to weight of seed plus wing. Why relatively longer seed wings 

were found on plots having greater annual precipitation and greater range in mean 

temperature (July minus January means) is puzzling.

8

 Personal communication from J.R. Haller, March 1992.

9

 Unpublished data on file at USDA Forest Service’s Institute of Forest Genetics, 

Placerville, CA.
34

research paper psw-rp-265

Rocky Mountains race—No correlation between needle retention (C1) and 

either climatic or geographic variables was significant for this race. This contrasts 

sharply with situations for Pacific and North Plateau races and less sharply for the 

Southwestern race.

Needle length (C8) increased as the mean temperature in July increased and as 

the growing season became longer. In this race, as for races to the south and west, 

needles became shorter (C8) as elevation increased.

The number of needles per fascicle (C10) increased as the temperature during 

the growing season decreased and as July mean temperature decreased. Needles per 

fascicle increased at higher elevations as Haller (1965) had found, but this was true 

only for this race. Needles per fascicle increased in frequency toward the west and 

south within this race.

The number of adaxial resin ducts in needles (C12) increased toward the west, 

south, and higher elevations. As in the Central High Plains race, number of adaxial 

resin ducts was higher where climate was wetter.

Hypodermal cell layers (C14) were more numerous eastward and where temperatures were warmer during the growing season.

Both cone length and width (C15 and C16) increased westward. Seed length and 

width (C18 and C19) both decreased at higher elevations and northward. Length of 

seed plus wing (C20), absolute length of seed wing (C24), and proportional length 

of seed wing (C25) all increased westward. Longer seed wings also were found 

where precipitation was more abundant.

Central High Plains race—Although only five plots from this race were studied, 

analyses showed eight significant correlations between characteristics of foliage and 

climatic and geographic variables. Needle thickness (C11) increased significantly as 

precipitation during July and August growing season decreased. As July mean temperature increased, so did width of needles.

Number of needles per fascicle (C10) and numbers of both adaxial and abaxial 

resin ducts (C12 and C13) decreased westward. The finding about the number of 

adaxial resin ducts is of special interest, for it is opposite to the relationship for the 

Rocky Mountains race where adaxial resin ducts (C12) were significantly fewer 

but also decreased toward the east. Number of adaxial resin ducts also increased 

as growing season temperature increased, but the number of abaxial resin ducts 

increased as growing season lengthened.

Cone form was positively related to length of growing season but negatively 

related to both July mean temperature and temperature during the growing season. 

That is, cones were less elongate and more ovoid where growing season was longer 
35

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

and temperatures were cooler. Such high correlations for this characteristic were 

not found in any other race.

The highest correlation in this study (r = 0.99) was between seed form and 

latitude in the Central High Plains race. Range in latitude for five plots in this race 

was less than two degrees. Yet from south to north, a distance of about 93 mi, seeds 

became steadily and consistently longer in relation to width.

Southwestern race—The Southwestern race displayed patterns of internal variation 

similar to those of other races as well as some unique patterns. Years that needles 

remain green (C1) increased toward north and as annual precipitation decreased. 

Green foliage length (C2) increased toward both north and west. Number of branches per whorl (C3) decreased at higher elevations and eastward. Needles per fascicle 

(C10) decreased significantly northward, which one might expect considering that 

north is predominantly toward the two-needled Rocky Mountains race. Number of 

abaxial resin ducts (C13) increased toward both west and north, as was the case for 

the Rocky Mountains race.

Both length and width of cones (C15 and C16) increased westward parallel to 

the situation in the Rocky Mountains race. Proportional length of seed wing (C25) 

increased westward, as it did in the Rocky Mountains race. Both seed form (C23) 

and seed length (C18) decreased as elevation rose, but seed width did not change 

significantly with elevation. As a result, seeds were less elongate at higher elevations. Length of seed plus wing (C20) also decreased as elevation increased. Width 

of seed (C19) decreased northward, whereas the opposite was true for the Rocky 

Mountains race (located to the north and having narrowest seeds of all races). The 

narrowest seeds for ponderosa pine must be found somewhere in the transition zone 

between the Southwestern and Rocky Mountains races.

Smith (1977) characterized monoterpenes of southwestern ponderosa pine as 

having “large amounts of alpha-pinene and (delta-3-carene) and small amounts 

of beta-pinene, myrcene and limonene.” The southern part of this race, Smith’s 

Coronado Region and Chiricahua Zone, have “about 90 percent alpha-pinene with 

small or trace amounts of the other four major components.”

Discussion

Racial differences and transition zones delineated here help clarify the taxonomy of 

ponderosa pines north of Mexico (fig. 3). Major morphological differences among 

races documented here were sufficient to cause early taxonomists to apply a unique 

epithet to each race except Central High Plains ponderosa pine.

Using seed collected for this study, Wells’ provenance study (1964a, 1964b) 

confirmed four of the five races identified here. Significant genetic barriers to 
36

research paper psw-rp-265

Figure 3—Distribution of ponderosa pine and boundaries of its taxonomic entities.
37

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

crossing among three races are evident or likely (Conkle and Critchfield 1988, 

Critchfield 1984,). Smith (1977) has established strong genetic differences among 

four of the five races in proportions of five monoterpenes in xylem oleoresin. 

Niebling and Conkle (1990) have established differences in allozymes between 

three of the five races.

In light of these collected evidences, ponderosa pine includes five subspecies 

and at least three, perhaps five, transition zones (Callaham 2013). These subspecies 

closely coincide with:

• Geographic races proposed by Weidman (1939)

• Ecotypes proposed by Wells (1964a) and Haller (1965)

• Regions and zones proposed by Smith (1977)

• Geographic clusters proposed by Read (1980, 1983).

Numerous, significant differences among subspecies are contrasted in the 

following sections.

Pacific vs. Columbia Ponderosa Pine

The Pacific race, designated here as Pacific ponderosa pine, differs significantly 

from the North Plateau race, designated here as Columbia ponderosa pine. Data 

shown here (tables 2, 3, and 5) and findings by others establish that these are two 

different taxa.

Critchfield (1984) reported from preliminary data that these taxa are partially 

isolated by reproductive barriers that reduce crossability to 50 percent; however, he 

went on to write: “Although these data indicate the probable existence of a genetic 

barrier, they are too limited for a reliable estimate of its magnitude.”

Smith (1977) found significant differences between these taxa in xylem turpentine chemistry. Monoterpenes of Pacific ponderosa pine are characterized by high 

amounts of both beta-pinene (27 to 45 percent) and delta-3-carene (35 to 41 percent, 

plus considerable myrcene (20 to 21 percent) and limonene (18 to 22 percent), 

whereas monoterpenes of Columbia ponderosa pines are characterized by very high 

delta-3-carene (48 to 53 percent), less myrcene (19 to 21 percent), and little to very 

little limonene (7 to 12 percent). Both taxa produce very little alpha-pinene (12 to 19 

percent). Smith proposed (1977) and confirmed by later research10 that amount of 

each of the five primary monoterpenes is controlled by a single gene with two additive alleles at the locus. Smith also noted that color of resin exuding from Pacific 

ponderosa pine “tends toward yellow and dark amber,” but he did not comment on 

color of resin from Columbia ponderosa pine.

10 Personal communication from Richard H. Smith (retired entomologist).
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research paper psw-rp-265

Smith (1981) made cogent observations of differences in the color of immature 

cones. Those of Pacific ponderosa pine were green to yellow-green. Those of 

Columbia ponderosa pine were red to purple on the majority of trees; less than 50 

percent of the trees had green to yellow-green cones. Mixtures of immature cone 

colors occurred in northeastern California and south-central Oregon, where Smith 

identified the transition zone between Pacific and Columbia ponderosa pines. He 

also observed for Columbia ponderosa pine “that higher elevation stands generally 

will have higher frequencies of darker (red to purple immature) cones.”

Niebling and Conkle (1990) estimated, on the basis of differences in allozymes, 

that average genetic distance between these two taxa was small; however, they 

recognized that trees they sampled, and called Pacific race, actually were growing 

in the transition zone between Pacific and Columbia ponderosa pines. Therefore, 

their published estimate of genetic distance between these two taxa probably is 

conservative.

Two plots in southern California did not differ significantly from the rest of 

Pacific ponderosa pine. Haller (1962), after studying morphology of ponderosa 

pines growing at several locations in southern California, concluded: “They are 

about as variable as in the western Sierra Nevada ... (and most characters) are 

within the usual ponderosa range on nearly all individuals ... (except that) the cones 

are notably smaller and denser ... .”

There is no basis for dividing Pacific ponderosa pine into northern and southern 

forms as was proposed initially by Weidman (1939). Wells (1964b) proposed that his 

North Plateau ecotype included the Willamette Valley, but he concluded: “The two 

Willamette Valley sources (actually my two sources) were different from the (North 

Plateau) sources in several characters ... .” He then included my two plots from the 

Willamette Valley of Oregon with all other Pacific sources. Haller’s (1965) coastal 

ecotype, based on observations of trees growing in situ, is synonymous with Pacific 

ponderosa pine.

An isolated but significant stand of ponderosa pine at southeastern corner 

of Puget Sound (Fort McClellan, Washington, see plot 79, fig. 1) at first analysis 

was considered to be associated with the North Plateau race. Subsequent analyses 

showed it belongs with Pacific ponderosa pine and may be the northernmost stand 

of that taxon. Haller11 supports its being Pacific ponderosa pine but knew of a population growing still farther north in the Skagit River Valley in British Columbia, 

just north of the international boundary. He wrote: “ ... at 1,700 ft elevation, that 

appears to be closer to Pacific ponderosa pine than to Columbia ponderosa pine.”

11 Personal communication from J.R. Haller, March 1992.
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

These taxa are separated sharply by the crest of the Cascade Range in Oregon 

and Washington. A very short, narrow Columbia Gorge transition zone, near Hood 

River, Oregon, must occur but was not studied. Discontinuous populations growing 

along the Columbia River provide stepping-stones for gene transfer through the 

Cascade Range. Sorenson (1994), studying growth responses in a provenance study, 

found that nine populations from this transition zone differed significantly from 

immediately adjacent Columbia ponderosa pines.

The combined barriers of the crest of the Sierra Nevada and the Mojave and 

Sonoran Deserts in southern California and Arizona cleanly separate Pacific ponderosa pine from ponderosa pine growing in the Great Basin and in the Southwest.

Smith (1977) presented evidence—less delta-3-carene and more beta-pinene—

for a southern California race (his San Jacinto Region) south of a narrow transition 

zone in the Transverse Range and Tehachapi Mountains. Critchfield (1984) and 

Conkle and Critchfield (1988), without further data or analysis, mapped Smith’s 

region and transition zone. Detailed study is needed of the characteristics of foliage, cones, and resins from trees growing in this transition zone and in southern 

California.

Transverse/Tehachapi transition zone—

The Transverse Range system crosses California south of the Central Valley. 

Included is the Tehachapi Mountain Range extending easterly and southward separating the San Joaquin Valley to the northwest and the Mojave Desert to the southeast. This zone is designated here only because of Smith’s (1977) findings of significant differences in oleoresin monoterpenes north and south of this zone. Neither 

this author nor Haller found differences between ponderosa pines growing north 

and south of this zone.

Umpqua-Tahoe transition zone—

Further intensive study will be necessary to delineate the long, sinuous, but relatively narrow, Umpqua-Tahoe transition zone. It separates these taxa in southern 

Oregon, northern California, and the eastern side of California in the rain shadow 

of the Sierra Nevada. Data presented here plus maps provided by Sturgeon (1979) 

and Smith (1981) show the zone meandering southerly through Oregon.

North of Mount Shasta, in far northern California, it turns east-southeasterly to 

Adin.12 East of the summit of the Sierra Nevada it turns southerly and terminates 

near Lake Tahoe.

12 Smith in a personal communication: The line passes through this area on the border 

between Modoc and Lassen Counties. Trees on these plots located only 10 mi to the north 

and east of Adin differed significantly from Pacific ponderosa pine.
40

research paper psw-rp-265

This transition zone has not been extended to include six isolated stands 

growing in the rain shadow of southern Sierra Nevada near Bishop, California. 

Smith (1977) extended the transition zone to include these stands because he found 

monoterpenes from xylem oleoresin of these pines had relatively more (46 vs. 41 

percent) delta-3-carene as was the case for pines growing farther north in the transition zone. Haller (1962) studied trees in one of Smith’s stands and observed: “They 

grow only along the courses of perennial streams ... .” He described their morphology as “nearly typical and similar to that from his Ebbetts Pass transect (west of 

that pass on the Pacific slope of the Sierra Nevada), except for its highly variable 

bark.” Conkle13 found the size, shape, and color of ripening cones on trees growing 

on Haller’s transect along Rock Creek in Inyo County and in five nearby stands to 

be indistinguishable from these characteristics of cones found on the western slopes 

of the Sierra Nevada.

Helena Transition Zone—

A relatively narrow transition zone, between meridians 110° and 112.5° near Helena 

in central Montana, separates Columbia ponderosa pine from Rocky Mountains 

ponderosa pine. The existence and location of this transition zone was first suggested by Weidman (1939), later supported by Wells (1964a) and Haller (1965), and 

confirmed with detailed data by Read (1980).

Rehfeldt,14 from observations of seedlings grown in common gardens, writes: 

“All of western Montana is a transition zone. Exactly how far it extends into central 

and eastern Montana will have to await the results of our new studies, but my guess 

is that the transition extends all the way to the Black Hills.” Rehfeldt’s contention 

that genetic variation across Montana is continuous and distinctive is accepted, but 

his genetic variation was not expressed as significant morphological differences 

among trees growing in situ.

Smith (1977) also identified this Helena transition zone from a minor, but 

significant shift in monoterpenes occurring in xylem oleoresin. He placed the 

shift “at the Missouri River.” It seems to relate to the barrier created by the Big 

Belt Mountains, the first significant range on the eastern flank of the northern 

Rocky Mountains. His plots 58 and 59—located only a few miles west of the 

longitude of Helena—differed from his plot 60—located just east of the Big Belt 

Mountains, about 40 mi east-northeast of Helena. Turpentine of plots to the west 

had significatly less of both alpha-pinene and beta-pinene and significantly more 

of delta-3 carene.

13 Personal communication from M.T. Conkle, January 1992.

14 Personal communication from Gerald E. Rehfeldt, January 27, 1992.
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Rocky Mountains Ponderosa Pine

Both names, Pinus ponderosa var. scopulorum Engelm. and P. ponderosa subsp. 

scopulorum (Engelm.) E. Murray (1982), should be applied to Rocky Mountains 

ponderosa pine (Callaham 2013). The taxon others have called the Rocky Mountain 

ponderosa pine includes the Rocky Mountains, Central High Plains, and Southwestern races as they are described here. Findings about these subspecies lead to a 

smaller range for subsp. scopulorum. It encompasses (fig. 3):

• Northern interior and central interior ecotypes proposed by Wells (1964a)

• The “race found east of North Plateau” by Weidman (1939)

• Three or five of Read’s (1980, 1983) delineated clusters

Morphological data disclosed here were much too variable to warrant further 

subdividing the Rocky Mountains ponderosa pine. Well’s (1964a) nursery provenance study, from seed collected for the study reported here, supported existence 

of two to four ecotypes. Read (1980, 1983), using his growth performance data and 

Wells’ maps, suggested existence of several races (fig. 3).

Rehfeldt (1990) mapped patterns of clinal genetic variation within Utah 

and western Colorado. Within the Rocky Mountains race of ponderosa pine, he 

separated Uinta Mountains (my plots 174 and 179 = Wells’ 2258 and 2274) from 

Wasatch Plateau (my plots 164 and 165 = Wells’ 1211 and 2116) and each of these 

from three overlapping populations on Fishlake Plateau, Tushar Mountains (my plot 

180 = Wells’ 2284), and Pine Valley Mountains (my plot 175 = Wells’ 2274).

Smith (1977), studying five monoterpenes in xylem oleoresin, found no compelling evidence for subdividing Rocky Mountains ponderosa pine or for distinguishing it from either Columbia or Central High Plains ponderosa pines. However, 

monoterpenes of Rocky Mountains ponderosa pine differed significantly from those 

of both Pacific and Southwestern ponderosa pines.

Read,15 after plotting Smith’s data, concluded there were slight but recognizable differences in the amount of beta-pinene in monoterpenes between the Rocky 

Mountains (as mapped here) (21 to 28 percent) and Columbia ponderosa pine races 

(14 to 22 percent). After plotting Smith’s data, I drew four similar conclusions:

• Northern Rocky Mountains ponderosa pine growing in central Montana, 

Wyoming, and South Dakota (Smith’s plots 21 to 23, 60, and 61) differed 

significantly from Columbia ponderosa pine to the west [more alphapinene, (12) 14 to 16 percent vs. 12 to 13 percent, more beta-pinene, 23 to 

32 percent vs. 14 to 25 percent, less delta-3-carene, 45 to 48 (53) percent vs. 

50 to 53 percent, and less myrcene, 17 to 19 percent vs. 18 to 21 percent].

15 Personal communication to W.B. Critchfield, September 1982.
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research paper psw-rp-265

• Western Rocky Mountains ponderosa pine growing in Utah and Nevada 

(Smith’s plots 1, 42, 47, 62, 64, and 66) differed significantly from 

Columbia ponderosa pine growing to the north [more alpha-pinene, 17 

to 26 percent vs. 12 to 13 percent, less delta-3-carene, 34 to 37 percent 

in Nevada, 44 to 49 (53) percent in Utah, vs. 51 to 53 percent, and less 

myrcene, 14 to 17 percent vs. 18 to 21 percent].

• Southeastern Rocky Mountains ponderosa pine growing in Colorado 

(Smith’s plots 20, 48 to 49, 60) differed significantly from northern Rocky 

Mountains ponderosa pine [less beta-pinene, 19 to 20 percent vs. 27 to 28 

percent, more delta-3-carene, 48 to 55 percent vs. 44 to 47 (52) percent, and 

less myrcene, 12 to 15 percent vs. 17 to 18 percent].

• Western Rocky Mountains ponderosa pine growing in Utah and Nevada 

(plots above) differed significantly from southeastern Rocky Mountains 

ponderosa pine growing in Colorado (plots above) [less delta-3-carene, 34 

to 37 percent in Nevada, 44 to 49 (53) percent in Utah, vs. 48 to 55 percent, 

and more myrcene, 14 to 17 percent vs. 12 to 15 percent, and more limonene, 12 to 16 percent vs. 8 to 10 percent].

Rocky Mountains ponderosa pine differed from Pacific ponderosa pine by 

having less beta-pinene (21 to 28 percent vs. 27 to 45 percent), more delta-3-carene 

(37 to 55 vs. 35 to 41 percent), less myrcene (12 to 18 vs. 20 to 21 percent), and less 

limonene (8 to 19 vs. 18 to 22 percent). It differed from Southwestern ponderosa 

pine by having less alpha-pinene (12 to 29 vs. 38 to 74 percent), more beta-pinene, 

more delta-3-carene (37 to 55 vs. 15 to 42 percent), and more myrcene (12 to 18 vs. 

5 to 15 percent).

Critchfield (1984) presented evidence that Rocky Mountains ponderosa pine is 

reproductively isolated from Pacific ponderosa pine. He estimated crossability of 

these taxa to be only 35 percent.

Niebling and Conkle (1990) determined genetic distances based on differences 

in allozymes. They concluded: Rocky Mountains ponderosa pine was separated by 

major genetic distances from both Pacific ponderosa pine (actually trees growing in 

the Umpqua-Tahoe transition zone as noted earlier) and Columbia ponderosa pine.

Lacking information about ponderosa pine growing in the mountains of southern Nevada west of Las Vegas, I accepted Smith’s (1977) conclusion. He found that 

this population was related to typical Rocky Mountains ponderosa pine found to the 

north on scattered mountain in the Great Basin. Frank Hawksworth,16 an authority on mistletoes, found the mistletoe infecting stands on Spring Creek Mountain, 

northwest of Las Vegas, Nevada, is the species infecting Pacific ponderosa pine

16 Personal communication from Frank Hawksworth, March 1992. 
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Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

rather than the species infecting Rocky Mountains ponderosa pine. Despite this 

anomalous host-parasite relation, I continue to believe that Rocky Mountains 

ponderosa pine found elsewhere in the Great Basin reaches its southern limit in 

southern Nevada. Haller17 added “My data on needle length and needles per fascicle 

(Haller 1965), as well as other morphological characters, support your conclusion. 

The Rocky Mountains variety of white fir [Abies concolor (Gord. & Glend.) Lindl. 

ex Hildebr. var. concolor] also occurs in southern Nevada.”

Central High Plains Ponderosa Pine

The Central High Plains ponderosa pine race differed significantly from Rocky 

Mountains ponderosa pine and is recognized as a separate taxon. The common 

name recognizes that stands of Rocky Mountains ponderosa pine occur on the high 

plains both north and south of this taxon.

Distribution of Central High Plains ponderosa pine cannot be mapped accurately. My five plots from this subspecies define an area south and east of the Black 

Hills and east of the Front Range in Wyoming but not extending southward into 

Colorado. My area for this taxon extends substantially farther to the west and south 

in Nebraska than the “east-low elevation” and “north-central Nebraska” clusters 

found by Read (1980, 1983). According to Read:18 “The distribution should include 

White River that begins in northwest Nebraska and flows into South Dakota.” Wells 

(1964a) mapped this taxon as being indistinguishable from subspecies scopulorum 

in the Black Hills and Montana, but distinct from that subspecies in Colorado.

Examination of data for Wells’ plots (2172, 2180, 2190, and 2029 = respectively, 

my plots 22, 23, 24, and 26) lead to two conclusions:

• The Central High Plains race differed significantly from Read’s Central 

Rockies race (Wells’ 2137 and 2164 = respectively, my plots 4 and 21) and 

Read’s Colorado Plains race (Wells’ 2145 and 2155 = respectively, my plots 

6 and 19) in seed weight (40 to 54 mg vs. 33 to 36 mg), foliage color in 

August (blue-green vs. fewer trees blue-green), foliage color in October of 

first growing season (green, some light purple vs. most trees light purple), 

foliage color in October of second growing season (gray vs. light gray to 

gray).

• Central High Plains differed significantly from Read’s High Plains and 

Black Hills races (Wells’ 2024, 2028, 2095, and 2197 = respectively, my 

plots 1, 15, 116, and 114) in seed weight (40 to 54 mg vs. 30 to 41 mg) but 

not in other characteristics.

17 Personal communication from J.R. Haller, March 1992.

18 Personal communication, 1992.
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This taxon needs further detailed study of trees growing in situ and in provenance plantations established by Read and by Wright et al. (1969).

Smith (1977) had only one plot (= my plot 24) in the area of Central High Plains 

ponderosa pine. Monoterpenes in xylem oleoresins from that plot did not differ 

significantly from those of Rocky Mountains ponderosa pines growing nearby in 

either the Black Hills of South Dakota or the Front Range of Wyoming.

Southwestern Ponderosa Pine

Southwestern ponderosa pine differed significantly from both Rocky Mountains 

and Pacific ponderosa pine and is worthy of designation as a separate taxon. Specimens of it were collected by Wislizenus in 1846 and identified as Pinus brachyptera 

Engelmann (1848). An interesting point to note: Read’s (1980) seed source 864 is 

nearly the same location as Wislizenus’ discovery of this taxon.19

Read (1980, 1983) presented strong evidence for including the type location for 

P. brachyptera Engelm. in Southwestern ponderosa pine rather than in a transition 

zone. This pine was first collected south of Las Vegas, near what is now Romeroville, New Mexico, on the southeastern flank of the Sangre de Cristo Mountains. 

The cluster analysis from his seedling study would include the entire east flank of 

the Sangre de Cristo Mountains in Southwestern ponderosa pine. Later a cluster 

analysis of data from his 10-year-old plantation-grown trees indicated only the 

southernmost of these stands should be so included.

Read, feeling his seedling data were more meaningful, drew the northeastern 

bulge of Southwestern ponderosa pine to include:

• Continuous stands on the east flank of the Sangre de Cristo Mountains.

• Eight outlying stands from Romeroville north to Raton, New Mexico.

• A single stand of about 4,000 trees southeast of Kenton, Oklahoma.

• Continuous stands growing west of Trinidad, Colorado.

Smith (1977), without data from trees growing in this area, erroneously 

included it in his Escalante-Rio Grande transition zone. He would divide Southwestern ponderosa pine into two regions and a transition zone. Trees in his Mogollon Region had relatively high alpha-pinene (38 to 74 percent), a moderate amount 

of delta-3-carene (15 to 42 percent), and relatively small amounts of beta-pinene 

(6 to 14 percent), myrcene (5 to 15 percent), and limonene (6 to 16 percent). He also 

noted that xylem oleoresin in this region and his southernmost region (discussed in 

the next paragraph) “is typically colorless to light amber ... .” This contrasts with

19 Personal communication from R.A. Read, January 30, 1992. 
45

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

his observation that color of oleoresin exuding from Pacific ponderosa pine “tends 

toward yellow and dark amber.”

Smith’s smaller Coronado Region includes stands on isolated mountains in 

southeastern Arizona. It is based on data from only two plots. Trees from these 

plots had “about 90 percent alpha-pinene with small or trace amounts of the other 

four major components.” Using data derived from a third plot, he proposed a Chiricahua Zone where slightly more delta-3-carene (12 percent vs. 1 to 2 percent) is 

present. He related his findings about this region and zone to findings in Peloquin’s 

doctoral dissertation in 1971, subsequently published in 1984.

Peloquin studied cones, seeds, needles, and oleoresin from Southwestern ponderosa pine and two related species growing on six mountain ranges within Smith’s 

Coronado Region and Chiricahua Zone. He did not find evidence for existence of 

either Smith’s Coronado region or Chiricahua zone. Descriptors for Southwestern 

ponderosa pine fall almost exactly at midpoints of ranges independently established 

by Peloquin for Southwestern ponderosa pine. Morphological data from my three 

plots in southeastern Arizona plus Peloquin’s nine plots do not support recognition 

of a different taxon either in Smith’s Coronado region or his Chiricahua Zone.

Rehfeldt (1990) clearly showed strong genetic differences between his Kaibab 

provenance of my Southwestern ponderosa pine and his Pine Valley Mountains, 

Tushar Mountains, and Fishlake Plateau of my Rocky Mountains subspecies 

scopulorum. Rehfeldt (1993) demonstrated heritable differences in needle length 

among provenances and decreasing needle length at higher elevation within each 

provenance. Two of his provenances (Pinaleno Mountains and Chiricahua Mountains) were within Smith’s Coronado Region and Chiricahua Zone. Rehfeldt did 

not recognize any differences from Southwestern ponderosa pine growing a short 

distance to the north. He did note that the Pinaleno Mountains’ disjunct population 

is “so unique genetically that gene conservation programs may be desirable ... .”

Questions have been raised about my single plot from the Chisos Mountains, 

Big Bend National Park, southwestern Texas. The collector there was instructed 

to establish a plot in typical three-needled ponderosa pine, and that is what he 

provided. Data for plot 195 grouped without exception with data for all other plots 

of Southwestern ponderosa pine. Little20 and Haller21 both report finding three- to 

five-needled pines and typical P. arizonica in those mountains. Probably both 

20 Personal communication from Elbert L. Little, March 1992.

21 Personal communication from J.R. Haller, March 1992.
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research paper psw-rp-265

species occur there, and intermediate forms may be found as well [as has been the 

case elsewhere in the southwest (Peloquin 1984, Rehfeldt 1993)].

All who have studied Southwestern ponderosa pine agree that it:

• Is strongly influenced by a bimodal rainfall pattern—heavy rains in both 

winter and summer, but dry in spring and fall

• Differs morphologically and physiologically from all other ponderosa pines.

Data from physiological research (Callaham 1959, 1962; Rehfeldt 1993) and 

from provenance trials—Weidman (1939), Squillace and Silen (1962), Wells (1964a), 

Read (1980, 1983), and Rehfeldt (1993)—support existence and uniqueness of this 

distinctive fast-growing taxon. The only major differences in opinion pertain to its 

northern limits.

Four Corners transition zone—

A very broad transition zone of morphological intergradation exists at the northern boundary of Southwestern ponderosa pine. The center of this zone led to the 

name “Four-Corners” transition zone. The zone’s northwestern boundary starts at 

an undetermined point southwest of Cedar City, Utah, and extends east-northeast to 

Moab. Crossing into Colorado, the northeastern boundary extends east-southeast to 

the vicinity of Walsenburg, Colorado. Read aided in drawing the eastern boundary 

southward through the San Isabel National Forest and along the crest of the Sangre 

de Cristo Mountains. The zone’s southern boundary:

• Starts a short distance southwest of Cedar City, Utah

• Turns abruptly eastward along Arizona’s northern boundary to Four 

Corners

• Thence southeasterly it passes south of Farmington, New Mexico, and Los 

Alamos, New Mexico.

• Its junction with the eastern boundary is about 30 mi southeast of Santa Fe, 

New Mexico.

Findings of several authors support the existence and boundaries of this zone. 

Wells’ (1964a) delineation of a less distinct southern ecotype within his central interior ecotype is almost synonymous with this transition zone in Colorado but not in 

Utah. Read (1980), lacking data from this region, adopted the line drawn by Wells 

(1964a); therefore, the southern boundary of his Central Rockies race coincides 

with my northeastern boundary in Colorado. The inverted V-shape of the northern 

boundary of this transition zone is nearly identical to the line proposed by Haller 

(1965) and to Smith’s (1977) line delimiting the northern boundary of his Mogollon 

Region.
47

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Smith’s plot 46 (about 7 mi southwest of Cedar Breaks, Utah) and 50 (near 

South Fork, Colorado) resemble Southwestern plots. They have somewhat higher 

proportions of alpha-pinene and low to very low proportions of beta-pinene. Therefore, I would reposition Smith’s line slightly northward to include his plots 46 and 

50 in my transition zone.

Rehfeldt’s (1990 and 1993) patterns of genetic variation support our drawing:

• The northwestern boundary of this transition zone to include the 

Markagunt, Paunsaugunt, and Aquarius Plateaus and Boulder Mountains 

but to exclude (as Rocky Mountains subspecies) the Tushar and Pavant 

Mountains and Fishlake and Wasatch Plateaus

• The northeastern boundary to include the Abajo and LaSal Mountains, 

Uncomphagre Plateau, and San Juan Mountains

• The southern boundary on the border between Utah and Arizona

• Its southeasterly orientation in northwestern New Mexico to include the Rio 

Chama watershed and Sangre de Cristo Mountains

Rehfeldt’s (1990) data on growth and performance clearly placed Pine Valley 

Mountains (my plot 175 = Wells’ 2267) in the southwestern corner of Utah within 

Rocky Mountains ponderosa pine; whereas, my morphological data placed it within 

Southwestern ponderosa pine.

This skewed diamond-shaped region is one in which powerful adaptational 

forces must be contributing to observed patterns of inherent variation. Northern 

limits of monsoonal summer rain in the southwest coincide closely with the northern boundary of this transition zone. This transition zone’s southern boundary 

closely corresponds to the limits of southward movement of very cold continental 

winter storms. These dominant climatic features appear to be determinants of the 

Four Corners transition zone.

This transition zone seems to have significance also for bark beetles in the 

genus Dendroctonus (Smith 1977). Critchfield22 wrote:

The genus is afflicted with a dual nomenclature due to revision by a Utah 

taxonomist named Wood, but it happens that species interpretations don’t 

affect this particular problem. There are four (actually three) species of 

Dendroctonus in the southwest whose northern limits seem to be somehow 

related to the transition from scopulorum to southwestern ponderosa:

• D. barberi (in brevicomis: Wood): northern limits in southern and 

southwestern Colorado and southern Utah (one exception: Ashley 

National Forest).

22 Letter to R.A. Read, Dec. 29, 1982.
48

research paper psw-rp-265

• D. convexifrons (in adjunctus: Wood): northern limits in southern 

Colorado (north to Rye) and southern Utah.

• D. arizonicus (in frontalis: Wood) northern limits at latitude of 

Flagstaff and Santa Fe (totally within the range of southwestern ponderosa pine).

• D. approximatus (in parallelocollis: Wood) northern limits at latitude 

of Salt Lake City and Estes Park (far north of the transition zone).

Conclusion

Morphological and climatic data lead me to conclude that ponderosa pine in 

Western States and adjacent Canadian provinces comprises five races deserving 

of subspecific taxonomic identities. Proposed subspecific epithets and races are:

• Pinus ponderosa subsp. ponderosa, Columbia ponderosa pine, southward 

from southern British Columbia, east of the Cascade Range in Washington, 

Oregon, and northeastern California and eastward to the Continental 

Divide.

• Pinus ponderosa subsp. critchfieldiana Callaham, subsp. nov., Pacific 

ponderosa pine, from Puget Sound, Washington, southward to southern 

California.

• Pinus ponderosa subsp. scopulorum (Engelm. in S. Watson) E. Murray, 

Rocky Mountains ponderosa pine, from the continental divide in Montana 

eastward to the Black Hills of South Dakota and southward into Utah and 

Colorado.

• Pinus ponderosa subsp. readiana Callaham, subsp. nov., central High 

Plains ponderosa pine, restricted to Nebraska and adjacent areas in South 

Dakota, Wyoming, and Colorado.

• Pinus ponderosa subsp. brachyptera (Engelm. in Wislizenus) Callaham, 

comb. nov., Southwestern ponderosa pine, in Arizona, New Mexico, 

Oklahoma, and southwestern Texas, and, probably, extending into Mexico.

Four transition zones occur between subspecies:

• Helena Transition Zone

• Umpqua-Tahoe Transition Zone

• Four Corners Transition Zone

• An unnamed transition zone between ssp. scopulorum and ssp. readiana
49

Pinus Ponderosa: Geographic Races and Subspecies Based on Morphological Variation

Acknowledgments

Peter P. Feret (deceased) contributed significantly to enlargement of this manuscript while working at the Institute of Forest Genetics, Placerville, California 

(1983–1984) during his sabbatical leave as professor, Department of Forestry, 

Virginia Polytechnic Institute and State University, Blacksburg, VA 24061, USA.

Kenneth E. Diebel conducted the sophisticated discriminant statistical analyses 

while a post-doctoral research associate (1984), Department of Forestry, Virginia 

Polytechnic Institute and State University, Blacksburg, VA 24061, USA. He coauthored technical publications in 1988 and 1993.

This study was made possible, in part, by a grant from the Forest Genetics 

Research Foundation to the Institute of Forest Genetics, Pacific Southwest Research 

Station, USDA Forest Service, Placerville, California.

Metric Equivalents

When you know: Multiply by: To find: 

Inches (in) 0.0394 Millimeters (mm)

Inches 0.394 Centimeters (cm)

Feet (ft) 3.28 Meters (m)

Miles (mi) .621 Kilometers (km)

Acres (ac) .405 Hectares (ha) 

Milligrams (mg) 0.001 Grams (g)

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