Two varieties, P. leiophylla var. leiophylla and P. leiophylla var. chihuahuana (Engelm.) Shaw 1909. The var. chihuahuana was separately described by Engelmann in Wislizenius 1848 as Pinus chihuahuana, and the relationship between that taxon and Pinus leiophylla was discerned by Shaw (1909). Some later authors (e.g. Martínez 1948) continued to discriminate the two species
I puzzled about the differences between leiophylla and chihuahuana for several days as I criss-crossed the Sierra Madre Occidental, first heading east on Mex-16 from Hermosillo to Chihuahua and then back west on Mex-40 from Durango to Mazatlán, stopping in dozens of places to examine the two varieties. They form a cline. The endpoints of the cline, taken alone, would appear to be quite different trees, and it's usually possible to assign a given tree to leiophylla or chihuahuana. However, in places with intermediate environmental conditions, there are intermediate forms, and the two species are essentially identical except for a few characters that may be expected to vary in response to environmental factors. Var. chihuahuana tends to be shorter, to grow more towards the country's interior, where sites are drier and climate is more continental, has somewhat stiffer needles, 3 per fascicle, and somewhat larger cones, and from a distance the needles have a sort of "brushed" appearance, as if the tree was in a painting. P. leiophylla has 5 needles per fascicle, doesn't have the brushed appearance, and tends to occur as bigger trees, closer to the ocean, in stands with higher basal area. These are trees of the mountains, which means that the transition from maritime to continental climate happens very quickly, within a few kilometers of the crest of the range. Thus there is a relatively small area within which intermediate-form trees can be found. This is a pattern more characteristic of introgression than of clinal variation. I am inclined to the clinal explanation, though, because both varieties have many characters in common. Both varieties have early-deciduous fascicle sheaths, and are the only Mexican hard pines to have this character. Both varieties also have a three-year cone maturation, unique in Latin American pines (but also seen in Pinus pinea); the ability to coppice (resprout from a cut stump), shared with Pinus rigida; and the common production of epicormic shoots, shared with a variety of species (photo on the Pinus canariensis page) (pers. obs. 2007, Perry 1991, and Farjon and Styles 1997).
Typical of early-collected pines (this species was described in 1831, along with Pinus patula), the holotype consists of a single sheet with two sterile branchlets.
Tree, 15-35 m tall and 20-85 cm dbh, usually with a single round trunk often clear of branches for 60% of height, and an open, rounded crown. Bark dark grey-brown, very thick, rough, scaly, divided into elongate irregular scaly plates with deep longitudinal fissures; on young trees and branches scaly, flaking, red-brown to grey-brown. Branches long, slender, spreading or ascending, the lower ones curved downwards. Shoots scaly with short, decurrent pulvini, red-brown or glaucous turning grey-brown. Fascicle sheaths 12-20 mm long with 5-6 early-deciduous scales, so that mature fascicles have no sheath. Leaves in fascicles of (2-)3-5(-6), spreading in lax or rigid tufts near the ends of branches, persisting 2-3 years, (4-)6-15(-17) cm × 0.5-1.3(-1.5) mm, margins minutely serrulate, light green, stomata on all faces in 4-8 lines on the convex abaxial face, 3-4 lines on the adaxial faces. Resin ducts 2-6, medial, sometimes 1-2 internal, stele round with 2 vascular bundles. Pollen cones crowded near end of shoots in a short dense cluster, cylindrical, 10-15(-20) × 4-6 mm, yellow-pink maturing light brown. Seed cones subterminal, solitary or in whorls of 2-5 on stout peduncles 10-20 mm long, persisting several years after opening. Immature cones ovoid to subglobose, pink turning purple-green to purple-brown, maturing in three seasons, falling 2-3 years after maturity. Mature cones ovoid, nearly symmetrical, (4-)5-7(-8) × (3-)4-5.5 cm when open. Seed scales 50-70, straight or recurved near base of cone, purple-brown with lighter grey marks of seed wings. Apophysis raised, transversely keeled, with the second season's growth showing as a narrow band around the umbo; rhombic, up to 15 mm wide, brown. Umbo dorsal, pyramidal, with a blunt prickle, darker than the apophysis. Seeds ovoid, 3-4(-5) mm long, dark grey-brown with black spots. Seed wings articulate, oval, 10-18 × 4-8 mm, yellow-brown, translucent. Adventitious shoots may sprout after fire or cutting (Martínez 1948 cited by López-Upton 2003, Farjon and Styles 1997). The wood has a specific gravity of 0.44 to 0.51, and is relatively dense, heavy, hard, and very resinous with a pale brown heartwood (Echenique-Manrique and Díaz-Gómez 1969, Murillo 1988, Zobel 1965, all cited by Lopez-Upton 2003).
Var. leiophylla has leaves chiefly in fascicles of 5, 0.5-0.9 mm wide, with 4-6 lines of stomata on the abaxial face and 2-3 resin ducts. Var. chihuahuana has leaves chiefly in fascicles of 3, 0.9-1.3 mm wide, with 5-8 lines of stomata on the abaxial face and 4-6 resin ducts (Farjon and Styles 1997).
The early-deciduous fascicle sheaths on a hard pine represent a simple, reliable character that makes it easy to identify this species in the field.
Mexico: Sonora, Chihuahua, Durango, Nayarit, Zacatecas, Jalisco, Michoacán, México, Distrito Federal, Hidalgo, Morelos, Tlaxcala, Puebla, Veracruz, Guerrero, and Oaxaca; USA: Arizona and New Mexico; at 1500-3300 m elevation. From Zacatecas northward var. leiophylla is gradually replaced by var. chihuahuana (Farjon and Styles 1997). As described above, in this overlap zone, var. leiophylla is primarily a tree of maritime/warm/mesic sites west of the Sierra Madre Occidental crest, while var. chihuahuana is characteristic of continental/cold/xeric sites east of the crest.
Bannister and Neuner (2001) say it is hardy to Zone 9 (cold hardiness limit between -6.6°C and -1.1°C), but they must only be considering var. leiophylla; var. chihuahuana is substantially more hardy.
Found at (1600)-2200-2750-(3000) m elevation, which corresponds to temperate to warm-temperate climates (Perry 1991 cited by López-Upton 2003) having an annual temperature range of -15 to 38°C and annual rainfall of 700 to 1500 mm with a May-October wet season (Eguiluz-Piedra 1978 cited by López-Upton 2003). It grows best on deep, well-drained soils of volcanic origin (Martínez 1948 cited by López-Upton 2003).
The species rarely forms pure stands, and is commonly associated with other pines including P. engelmannii, P. arizonica, P. teocote, P. lumholtzii, P. montezumae, P. ayacahuite, P. pseudostrobus (both vars.), P. devoniana, and P. patula, as well as Abies religiosa, A. guatemalensis, Juniperus sp., Arbutus sp., Quercus sp., and several hardwoods (Eguiluz-Piedra 1978, Perry 1991, both cited by López-Upton 2003). See also Thompson et al. (1999).
I only have data for variety chihuahuana, which see.
There is no record of dendrochronological use of this species (as of May 2005).
I have no data on its aboriginal uses. Since the mid-20th Century, exploitation of this species has increased throughout much of its range, particularly in the Sierra Madre Occidental. It is tapped for resin and cut for general construction, railway ties, and fuelwood (Eguiluz-Piedra 1978, Perry 1991, both cited by López-Upton 2003; Farjon and Styles 1997). Var. chihuahuana is particularly exploited for resin production (Farjon and Styles 1997).
See the pages for each variety.
Pinus leiophylla typically produces some cones every year, masting at intervals of 3 to 5 years (Zamora-Serrano et al. 1993, cited by López-Upton 2003). Cones are susceptible to the fungus Caeoma conigenum Heds. et Hunt (Martínez 1948, Perry 1991, both cited by López-Upton 2003). Important insect pests are Conophthorus conicolens Wood, which can destroy 16 to 60 percent of conelets, cones, and seeds, and Leptoglossus occidentalis Heidemann (Cibrián-Tovar et al. 1995 cited by López-Upton 2003).
López-Upton, Javier. 2003. Pinus leiophylla. Species description in the Tropical Tree Seed Manual. Available http://www.rngr.net/Publications/ttsm/Folder.2003-07-11.4726 (accessed 2007.08.31).
Schlechtendal, D. F. L. von, and A. von Chamisso. 1831. Plantarum mexicanarum a cel. viris Schiede et Deppe collectarum (recensio brevis). Addenda (1). Linnaea 6: 352-384 (p. 354).
Last Modified 2017-12-29