Pinus discolor

D.K.Bailey & Hawksw. 1979

Common names

Border piñon (Lanner 1981).

Taxonomic notes

Pinus section Parrya, subsection Cembroides. Type: USA, Arizona, Coronado National Forest, Santa Rita Mountains, Madera Canyon, E.L. Little 23011 (holo US). Syn: P. cembroides subsp. cembroides var. bicolor Little 1968; P. culminicola var. discolor (Bailey and Hawksworth) Silba 1985; P. cembroides subsp. bicolor (Little) Silba 2009; P. culminicola var. bicolor (Little) Eckenw. 2009.

For a long time the validity of this taxon was hotly disputed. At one extreme, Kral (1993) summarily dismissed its existence altogether, while Farjon and Styles (1997) regarded it as merely a variety of P. cembroides, but a number of other authorities (Perry 1991, Price et al. 1998) saw it as a valid species distinct in its own right. Multiple molecular studies finally established the validity of P. discolor as a good species, as discussed on the P. cembroides page.

There have been no reports of any natural hybridization with P. cembroides.

Description

Small tree to 15 m tall, multi-stemmed, rarely with a single trunk. Crown low, dense and rounded, spreading, with branches extending outward as much as 3-4 m. Bark in young trees smooth and gray; in older trees rough and scaly, but not deeply furrowed or ridged. Twigs dark gray, rough, the bases of the leaf bracts somewhat decurrent. Leaves (2-)3(-4) per fascicle, 3-5 cm long, 0.9-1.2 mm thick; flexible, margins entire, stomata present only on the ventral surfaces, with a distinctive variegated appearance because the dorsal surface is dark green and the ventral surfaces glaucous white. Resin canals 2, external, dorsal; vascular bundle single. Fascicle sheaths orange-brown, 3 mm long, fading gray, and curled backward into a rosette; later occasionally deciduous. Cotyledons (6-)9(-11). Seed cones in first year brown, 9-11 × 5-7 mm, borne singly or in pairs on short slender peduncles. Cones at maturity orange-brown, resinous, oblong, up to 3.5 × 2-3 cm when closed, 4-5.5 cm wide when open, opening at maturity and soon deciduous. The peduncle is 3-4 mm long, falling with the cone. Cone scales are slightly glossy, thin and stiff, the apophyses irregularly rhomboid, small, thin, flat, 10-15 mm wide, with a weak transverse ridge; umbo dorsal, depressed, the prickle minute and soon deciduous. Only the central (3-)8-15 scales are seed-bearing, those at the base and apex of the cone generally very small and sterile. Seeds dark orange-brown with a rudimentary 0.5-1 mm wing that remains in the cone after seed release; about 10-12 × 10 mm, the seed coat or shell 0.5-1.0 mm thick, hard; about 2,200 seeds/kg; endosperm white; edible. Wood pale yellowish brown, used only for fuel (based on Perry 1991 description of P. johannis modified per M.P. Frankis).

Pinus discolor differs from P. cembroides in having fascicles of 3(-4) needles (rather than 2-3), a lack of stomata on the external needle surfaces, longer needle retention (4-7 vs. 2-5 years), smaller cones, and a tendency toward dioecy. Also, in the words of Bailey and Hawksworth, "The bark of P. discolor is somewhat thinner than that of otherwise comparable specimens of P. cembroides and consists of ragged, concave, grayish platelets of variable size and shape, typically 2-5 cm wide, with intervening, more or less longitudinal fissures, some of which exhibit a conspicuous orange to yellow color. In contrast, the bark of old trees of P. cembroides tends to exhibit thick, roughly polygonal plates of charcoal black, giving no impression of raggedness and having obvious transverse as well as longitudinal fissuring or cross-checking, and without the orange to yellow color deep in the furrows as in P. discolor."

The taxon is extremely similar to P. johannis, but the two species have disjunct distributions. Some of the few differences (in stature, cone size, and seed size) are based on small sample sizes, and may prove inconclusive with further research. Differences in terpene composition have also been reported (Zavarin and Snajberk 1986), but the validity of this research has been questioned (Farjon and Styles 1997).

Distribution and Ecology

Mexico: Chihuahua, Durango, Mexico, Nuevo León, Queretaro, San Luis Potosi, Sonora; USA: Arizona, New Mexico (based on locations of collections). In the US, it is notable on the eastern Mogollon Rim, Kitt Pk. and the Santa Catalina, Santa Rita, and Rincon Mountains (Jeff Bisbee email 2013.11.17, Lanner 1981, pers. obs.). Occurs at elevations of 1500-2400 m (Perry 1991). Hardy to Zone 8 (cold hardiness limit between -12.1°C and -6.7°C) (Bannister and Neuner 2001).

Pines of the Pinus cembroides complex. Blue is P. discolor. Purple diamonds are P. cembroides ssp. cembroides, purple stars are P. cembroides ssp. orizabensis, green is P. culminicola, brown is P. johannis, red is P. lagunae, and gold is P. remota. Based on GBIF.org downloads https://doi.org/10.15468/dl.pfuj4k (P. culminicola), https://doi.org/10.15468/dl.vvju45 (P. remota), and https://doi.org/10.15468/dl.5k7x9y (all other taxa), accessed 2023.03.09, with location uncertainty <10 km, and location duplicates removed.

The IUCN has not assessed the conservation risk for this species. However, in view of its very large range, it would likely be assessed as "Least Concern".

This species is one of the principal hosts for the dwarf mistletoe Arceuthobium pendens (Hawksworth and Wiens 1996).

Remarkable Specimens

No data as of 2023.03.10.

Ethnobotany

Potentially a valuable slow-growing ornamental species for small gardens in drought-prone areas, but it is scarcely in cultivation yet. USDA hardiness zone 8.

Observations

Jeff Bisbee (email 2013.11.17) reports that it occurs north of Clifton, AZ along Highway 191. "It very interesting because [P. × kohae] is growing with it, some trees having entirely single needles, and the typical larger cones of [kohae] with thicker cone scales and apophysis. These contrast with the very small, thin-scaled cones of discolor and the bi-colored needles. Intermixed with these two pinyon pines are also many two needled pinyons, which also have the larger cones. Some of them appeared to have somewhat bi-colored needles, others were more like edulis with evenly distributed stomata. Farther up the rim, were good edulis. So three pinyon pines in a small area." The same area has Hesperocyparis arizonica, and at somewhat higher elevations along the highway are populations of P. reflexa, P. ponderosa, Picea engelmanii, and Picea pungens.

Remarks

The epithet discolor refers "to the unusually conspicuous difference in color between the green dorsal and glaucous ventral needle surfaces" (Bailey and Hawksworth 1979).

Citations

Bailey, D. K. and F. G. Hawksworth. 1979. Pinyons of the Chihuahuan Desert region. Phytologia 44:129-133. Available: Biodiversity Heritage Library, accessed 2023.03.09.

Zavarin, E. and K. Snajberk. 1986. Monoterpenoid differentiation in relation to the morphology of Pinus discolor and Pinus johannis. Biochemical Systematics and Ecology 14: 1-11.

Much of this page was prepared by M.P. Frankis, 1999.02.

See also