Occurrence in North America
mountains of western Texas, New Mexico, Arizona, and southwestern
Colorado [15,16,50]. It extends south along the mountains to central
Mexico; most of its distribution is in Mexico [44,50,65].
Regional Distribution in the Western United States
This species can be found in the following regions of the western United States (according to the Bureau of Land Management classification of Physiographic Regions of the western United States):
7 Lower Basin and Range
11 Southern Rocky Mountains
12 Colorado Plateau
13 Rocky Mountain Piedmont
Southwestern white pine is a native, evergreen conifer with an open,
irregular crown. It can grow to 90 feet (27 m) tall with diameters to
3.2 feet (1 m) [15,53,64]. The bark is thin, rough, and furrowed
[15,54]. Branches are long and horizontal to pendant . Needles
are 2.4 to 4 inches (6-10 cm) long in fascicles of five . The cones
are 2.8 to 9.8 inches (7-25 cm) long with reflexed, thick scales; cones
are dehiscent when mature [15,46]. Seeds are essentially wingless and 0.4
to 0.5 inch (10-12 mm) long [15,53].
Arizona Mountains Forests Habitat
This taxon is found in the Arizona Mountain Forests, which extend from the Kaibab Plateau in northern Arizona to south of the Mogollon Plateau into portions of southwestern Mexico and eastern Arizona, USA. The species richness in this ecoregion is moderate, with vertebrate taxa numbering 375 species. The topography consists chiefly of steep foothills and mountains, but includes some deeply dissected high plateaus. Soil types have not been well defined; however, most soils are entisols, with alfisols and inceptisols in upland areas. Stony terrain and rock outcrops occupy large areas on the mountains and foothills.
The Transition Zone in this region (1980 to 2440 m in elevation) comprises a strong Mexican fasciation, including Chihuahua Pine (Pinus leiophylla) and Apache Pine (P. engelmannii) and unique varieties of Ponderosa Pine (P. ponderosa var. arizonica). Such forests are open and park-like and contain many bird species from Mexico seldom seen in the U.S.. The Canadian Zone (above 2000 m) includes mostly Rocky Mountain species of mixed-conifer communities such as Douglas-fir (Pseudotsuga menziesii), Engelmann Spruce (Picea engelmanni), Subalpine Fir (Abies lasiocarpa), and Corkbark Fir (A. lasiocarpa var. arizonica). Dwarf Juniper (Juniperus communis) is an understory shrubby closely associated with spruce/fir forests. Exposed sites include Chihuahua White Pine (Pinus strobiformis), while disturbed north-facing sites consists primarily of Lodgepole Pine (Pinus contorta) or Quaking Aspen (Populus tremuloides).
There are a variety of mammalian species found in this ecoregion, including the endemic Arizona Gray Squirrel (Sciurus arizonensis), an herbivore who feeds on a wide spectrum of berries, bark and other vegetable material. Non-endemic mammals occurring in the ecoregion include: the Banner-tailed Kangaroo Rat (Dipodomys spectabilis NT); Desert Pocket Gopher (Geomys arenarius NT). In addition, there is great potential for restoring Mexican Wolf (Canis lupus) and Grizzly Bear (Ursus arctos horribilis) populations in the area because of its remoteness and juxtaposition to other ecoregions where these species were formerly prevalent.
There are few amphibians found in the Arizona mountain forests. Anuran species occurring here are: Red-spotted Toad (Anaxyrus punctatus); Southwestern Toad (Anaxyrus microscaphus); New Mexico Spadefoot Toad (Spea multiplicata); Woodhouse's Toad (Anaxyrus woodhousii); Northern Leopard Frog (Lithobates pipiens); Chiricahua Leopard Frog (Lithobates chiricahuensis VU); Madrean Treefrog (Hyla eximia), a montane anuran found at the northern limit of its range in this ecoregion; Boreal Chorus Frog (Anaxyrus woodhousii); Western Chorus Frog (Pseudacris triseriata); and Canyon Treefrog (Hyla arenicolor). The Jemez Mountains Salamander (Plethodon neomexicanus NT) is an ecoregion endemic, found only in the Jemez Mountains of Los Alamos and Sandoval counties, New Mexico. Another salamander occurring in the ecoregion is the Tiger Salamander (Ambystoma tigrinum).
A number of reptilian taxa occur in the Arizona mountains forests, including: Gila Monster (Heloderma suspectum NT), often associated with cacti or desert scrub type vegetation; Narrow-headed Garter Snake (Thamnophis rufipunctatus), a near-endemic found chiefly in the Mogollon Rim area; Sonoran Mud Turtle (Kinosternon sonoriense NT).
Habitat and Ecology
Southwestern white pine is widespread in mesic sites on ridges, slopes,
and canyons of montane zones . The best growth of this species
occurs on moist, cool sites with deep soil [14,53]. The climate is
semiarid, characterized by mild winters and warm summers. There are two
wet seasons: July to September and December to March [2,10]. Average
precipitation ranges from 19.2 to 45 inches (487-1,143 mm) per year
Southwestern white pine occurs in ravines or on mesic lower slopes at
5,000 feet (1,525 m) in southeastern Arizona and goes up to timberline
in southwestern Colorado [52,76]. It is typically found at elevations
from 6,000 to 10,000 feet (1,830-3,048 m) [17,52]. Southwestern white
pine often occurs on north- to east-facing slopes, but it has been
reported on all aspects [1,11,17,57,71].
Southwestern white pine is found on sites with loamy soil textures
ranging from shallow, gravelly loams to deep, sandy loams to stony
silty clay loams [23,30,32,40].
Two to nine conifer species occur in the southwestern mixed-conifer
forest type. Their proportions vary depending on site characteristics
[37,48]. Some associated species not mentioned in Distribution and
Occurrence are listed below. Associated tree species are New Mexico
locust (Robinia neomexicana), Gambel oak (Quercus gambelii), and
silverleaf oak (Quercus hypoleucoides) . Associated shrubs are
mountain snowberry (Symphoricarpos oreophilus), bush oceanspray
(Holodiscus dumosus), Arizona honeysuckle (Lonicera arizonica), and
Fendler ceanothus (Ceanothus fendleri) [8,14,23,57]. Other associated
plants are Arizona fescue (Festuca arizonica), screwleaf muhly
(Muhlenbergia virescens), pine dropseed (Blepharoneuron tricholepsis),
Arizona wheatgrass (Elymus arizonicus), and western yarrow (Achillea
Key Plant Community Associations
Southwestern white pine normally occurs in low densities in southwestern
pine, mixed-conifer, and spruce-fir forests . It occurs as an
associate in habitat type series of the major conifers in these forests
[3,4,5,17,26,52]. Southwestern white pine infrequently forms small pure
stands; it is most likely to be dominant in high-elevation, cool
Southwestern white pine may be present as a minor component in riparian
community types in south-central Arizona and in the montane riparian
woodland zone of southwestern Colorado [9,70].
Southwestern white pine is listed as an indicator species in the
(1) Classification of the forest vegetation on the National Forests of
Arizona and New Mexico 
(2) A classification of forest habitat types of northern New Mexico and
southern Colorado 
(3) Forest habitat types in the Apache, Gila, and part of the Cibola
National Forests, Arizona and New Mexico 
(4) Forest and woodland habitat types (plant associations) of northern
New Mexico and northern Arizona 
(5) A classification of spruce-fir and mixed conifer habitat types of
Arizona and New Mexico .
This species is known to occur in the following ecosystem types (as named by the U.S. Forest Service in their Forest and Range Ecosystem [FRES] Type classification):
FRES21 Ponderosa pine
FRES23 Fir - spruce
Habitat: Cover Types
This species is known to occur in association with the following cover types (as classified by the Society of American Foresters):
206 Engelmann spruce - subalpine fir
210 Interior Douglas-fir
211 White fir
216 Blue spruce
219 Limber pine
237 Interior ponderosa pine
Habitat: Plant Associations
This species is known to occur in association with the following plant community types (as classified by Küchler 1964):
K018 Pine - Douglas-fir forest
K019 Arizona pine forest
K020 Spruce - fir - Douglas-fir forest
K021 Southwestern spruce - fir forest
Habitat & Distribution
Fire Management Considerations
Prediction of natural fuel loadings in southwestern mixed-conifer
forests is difficult; much variation exists within and between stands
. Sackett  reported average square diameter and specific
gravity for southwestern white pine and other conifers in different size
classes, making the planar intersect method of fuel evaluation
applicable to southwestern forests.
Regeneration by prescribed fire of mixed-conifer series for wildlife has
been discussed in detail .
Plant Response to Fire
While southwestern white pine is known to be an important seral tree
following fire, such as in white fir-Douglas-fir (Abies concolor-
Pseudotsuga menziesii)/Gambel oak habitat types, no information was
found in the literature about southwestern white pine rates of recovery
after fire .
Immediate Effect of Fire
usually die; older trees (more than 50 years) are susceptible to
No information was found in the literature about the fire susceptibility
of southwestern white pine seeds stored in caches. It is possible that
soil may sufficiently insulate cached seeds from fire damage.
Tree without adventitious-bud root crown
Secondary colonizer - off-site seed
Southwestern white pine is not fire adapted; it does not have
fire-induced sprouting, seed germination, or biomass increases .
Southwestern white pine is fire sensitive in young age classes. Older
trees with somewhat thicker bark are relatively more fire resistant
. The thin bark and horizontal or drooping branches increase its
susceptibility to fire.
Lightning-ignited fires occur in the southwestern forests during spring
or early summer before the rains begin . Arizona and New Mexico
mixed-conifer forests have the highest frequency of lightning fires in
the United States .
Fire-scarred cross sections of living and dead southwestern white pine
and other conifers were examined to determine the role of fire in
southwestern forests. Over time spans of 288 and 426 years, the average
intervals between fires in western Texas and east-central Arizona were
4.7 to 9 or 22 years [2,8,20]. These chronologies have been used with
climatic data to develop fire hazard forecasting models .
More info for the terms: climax, codominant, tree
Southwestern white pine is frequently a persistent, long-lived seral or
climax species in mixed-conifer forests [4,26,51]. It has been
classified as an early successional major tree . Southwestern white
pine infrequently is a climax dominant or codominant in open stands
Southwestern white pine is relatively shade intolerant compared to other
associated conifers in the southwestern mixed-conifer forests. It is
relatively resistant to damage from full sunlight . In 8 out of 12
sites in New Mexico, southwestern white pine was in the overstory, but
in only 2 of those 12 sites was it in the understory. Ahlstrand 
suggested that canopy closure prevented southwestern white pine
replacement at these sites.
Southwestern white pine reproduces sexually. It begins to produce cones
when pole sized or about 15 years [39,44]. The interval between seed
crops for any one tree is 3 to 4 years . Seed traps were placed in
clearcut mixed-conifer forest in Arizona to monitor regeneration over 3
years. No southwestern white pine seeds were found in the traps .
Southwestern white pine seeds require 2 weeks to 4 months stratification
before germinating [43,44,78]. Germinability can vary between 52 and 95
percent [43,44]. Seed collection and germination methods are discussed
in detail [36,44].
Seeds of the southwestern white pine ripen synchronously throughout a
forest and overwhelm the harvesting efforts of predators [11,34]. Red
squirrels clip entire cones and cache them . The wingless seeds are
dispersed by birds, primarily by the Steller's jay and Clark's
nutcracker [11,47,72]. Animal caches result in clustered stands .
Southwestern white pine seedlings root deeply (to about 8 inches [20.3
cm]) the first year, which increases their survival under drought
conditions [40,43]. All of the southwestern white pine seedlings died
in a greenhouse experiment that assessed the drought resistance of
conifers along an elevational gradient. Southwestern white pine died
after significantly (p less than 0.05) fewer days than other conifer species from
lower elevations .
With the initial deep root growth, southwestern white pine seedlings had
the slowest top growth rate of four conifer species measured. At about
6 years of age, the average height of southwestern white pine seedlings
was 13.3 inches (33.8 cm) . In another study, 2-year-old seedlings
were between 4 and 8 inches (10.2-20.3 cm) tall .
Growth Form (according to Raunkiær Life-form classification)
Life History and Behavior
Southwestern white pine flowers in June. Cones mature in September, and
seeds are dispersed from September to October . Southwestern white
pine seeds germinate either in the spring or in the summer after the
rains begin .
Molecular Biology and Genetics
Barcode data: Pinus strobiformis
Statistics of barcoding coverage: Pinus strobiformis
Public Records: 6
Specimens with Barcodes: 10
Species With Barcodes: 1
IUCN Red List Assessment
Red List Category
Red List Criteria
The range of this species is very extensive and it is common to abundant in pine forests in the Sierra Madre Occidental and elsewhere. There may be some decline due to exploitation of timber trees in some localities, but overall the population can be considered stable and is therefore assessed as Least Concern.
- 1998Lower Risk/least concern (LR/lc)
National NatureServe Conservation Status
Rounded National Status Rank: NNR - Unranked
NatureServe Conservation Status
Rounded Global Status Rank: G5 - Secure
Silviculture: Various silviculture systems have been used in the
mixed-conifer forests where southwestern white pine occurs.
The average annual growth of southwestern white pine was less than 0.1
inch (0.25 cm) per year in a virgin mixed-conifer stand in Arizona. The
initial volume of southwestern white pine was 1,097 board feet per acre,
and after 5 years, the volume was 1,163 board feet per acre .
Small patch clearcutting has been used for regeneration of southwestern
white pine and associated conifers in old-growth mixed-conifer stands
[24,32]. Regeneration density of southwestern white pine per acre was
low after 10 years .
Gray  classified southwestern white pine as a suitable seed tree.
Southwestern white pine regeneration by seed trees was not successful,
however, in communities at 9,500 to 10,000 feet (2,896-3,048 m);
revegetation was very slow .
Spot seeding yielded a low number of surviving southwestern white pine
seedlings in east-central Arizona [39,43]. Rodents were controlled, but
frost heaving and unsuitable tree ecotypes could have factored into the
seedling mortality . Heavy broadcast seeding of southwestern white
pine should be used instead of spot seeding .
Planting southwestern white pine seedlings at different elevations and
densities are discussed in detail .
Wildlife: Patch clearcuts were evaluated for forage production and
utilization by ungulates. Understory production significantly (p less than 0.05)
increased on clearcut areas; however, utilization by ungulates did not
differ significantly (p>0.05) . Equations are available for
predicting forage production in mixed-conifer forests [22,48]. Patch
clearcuts removed about 30 percent of the total basal area in an uneven-
aged virgin Arizona mixed-conifer forest, which did not adversely affect
the nesting or feeding of birds . This method reduced southwestern
white pine basal area by 47 percent on northern aspects and by 13
percent on southern aspects.
Some of the mixed-conifer forests that southwestern white pine is in
provide habitat for sensitive and threatened species such as Mexican
spotted owl, northern goshawk, and Sacramento Mountain salamander
Damaging agents: Southwestern white pine is the principal host of
Apache dwarf mistletoe (Arceuthobium apachecum). Infected trees have
reduced growth, increased susceptibity to other infections, insects,
and mortality [35,75]. Southwestern white pine is infected by red ring
rot and by root and butt rots . Southwestern white pine seedlings
received an average overall ranking of 8.8 in trials for resistance to
white pine blister rust; a score of 11 was the lowest resistance .
Other factors: Various methods of weed suppression made no significant
(p=0.10) difference in growth of southwestern white pine produced in
Southwestern white pine has good potential for Christmas tree production
in the eastern United States [36,78].
Southwestern white pine is planted along streets in urban areas. Its
foliage has an intermediate susceptibility to salt spray; medium foliar
injury with moderate growth reduction occurred .
Relevance to Humans and Ecosystems
Importance to Livestock and Wildlife
Seeds of southwestern white pine are eaten by small mammals and birds.
It is not generally browsed by game animals or livestock .
Southwestern white pine was seldom used by birds in either logged or
unlogged forest areas .
Southwestern white pine is often found in mixed-conifer forests that are
valuable summer habitat for big and small game animals, rodents, and
game and nongame birds [19,25]. Since natural regeneration of clearcut
mixed-conifer forests on south-facing slopes requires 50 to 100 years,
these clearcut areas can be a valuable long-term forage resource for
deer and elk .
Wood Products Value
slightly darker heartwood. It is used locally for cabinetry, doors, and
window frames . Crooked stems and coarse branches make it
undesirable for lumber .
Chihuahua white pine
Pinus strobiformis, commonly known as Southwestern white pine, Mexican white pine or Chihuahua white pine, is a medium-sized white pine tree whose native habitat is in southwestern United States and Mexico. It is typically a high-elevation pine growing mixed with other conifers (a Montane forest).
Pinus strobiformis, a member of the white pine group, Pinus subgenus Strobus, is a straight, slender tree growing to 30 m tall and 90 cm in diameter. The bark is smooth and silvery-gray on young trees, aging to furrowed and red-brown or dark gray-brown. The branches are spreading and ascending. Twigs are slender, pale red-brown, aging to smooth gray or gray-brown. Buds are ellipsoid, red-brown, and resinous. Leaves (needles) are five per bundle (fascicle), sometimes four, spreading to ascending-upcurved, 4-9 cm long (rarely 10), 0.6-1.0 mm in diameter, straight, slightly twisted, pliant, dark green to blue-green, and persist 3-5 years. The upper surface ('adaxial' - facing toward the stem of the plant) is conspicuously whitened by narrow stomatal lines. The lower surfaces ('abaxial' - facing away from the stem of the plant) are without evident stomatal lines. The margins are sharp, razorlike and entire to finely serrulate, apex narrowly acute to short-subulate. Each fascicle has a deciduous sheath 1.5-2.0 cm long which is shed early.
The cones are very large, 16–50 cm long and 9–11 cm broad, and have scales with a very characteristic prolonged and often recurved or S-shaped apex. The seeds are large, and with a very short wing; they are dispersed mainly by birds, particularly the Mexican Jay. It is a very drought tolerant tree but greater populations grow on moist and cool places living in association with Pinus hartwegii and Pinus rudis. 
The tree can natively be found in mountainous areas in Arizona, southwest Colorado, New Mexico, and western Texas. Most of the native pines are in Mexico, in the Sierra Madre Occidental mountains of Northern Mexico, from a short distance south of the US–Mexico border south through Chihuahua and Durango to Jalisco.  The pine rarely appears in pure strands, but grows along with other native conifers, such as Limber Pine, Ponderosa pine, Blue Spruce, Aspen, White Fir, Douglas Fir, and Engelmann Spruce.
The southwestern white pine can be grown as a Christmas tree, windbreak tree, or an ornamental tree. It is popular as a replacement in drier areas for the Eastern white pine. It can be used in cabinet making, but it is poor as a lumber tree. The seeds were used as a food by Native Americans in the present day Southwestern United States.
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Names and Taxonomy
Mexican white pine
border white pine
Pinus ayacahuite var. strobiformis Lemmon
Pinus strobiformis Engelm. . There are no recognized subspecies,
varieties, or forms. Southwestern white pine hybridizes with limber
pine (P. flexilis James) where their ranges overlap [6,17,53].