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Tree to 30 m, trunk straight and erect, 0.8–1 m dbh. Bark thin, smooth, silvery grey; in older trees thick, rough, scaly, dark brown to grey, breaking into small, irregular plates. Crown initially conical or pyramidal, becoming rounded or irregular in mature trees. Branchlets reddish brown, smooth, glabrous; vegetative buds not or slightly resinous. Leaves in fascicles of five, in dense tufts near the ends of ultimate branches, persisting for three to five years, straight or slightly curved, sometimes slightly twisted, dark green or glaucous, triangular in cross-section, (5–)7–11(–12) × 0.06–0.11 cm, margins serrulate, apex acute. Fascicle sheaths 2–2.5 cm long, orange-brown, rapidly disintegrating. Cataphylls 0.5–1 cm long, brown. Male strobili yellow to orange-brown, ovoid-oblong to cylindrical, 0.6–1 cm long. Female cones subterminal and axillary, solitary or rarely in pairs or in whorls of three to four; peduncles stout, 1.5–2.5 cm long, erect then pendulous, falling with the cone. Cones 12–30(–60) × 7–11 cm, greenish purple to light brown, mature in about 18 months; mature cones variable, mainly cylindrical, often slightly curved. Scales 70–120, opening obliquely; scales thick, woody; apophysis usually rather resinous, light yellowish brown; umbo terminal, greyish brown, resinous. Seeds reddish brown to brown, 1.2–1.8 × 0.8–1.1 cm; wings vestigial, forming a fringe around the seed. Thieret 1993, Farjon & Styles 1997, Farjon et al. 1997, Farjon 2005a. Distribution MEXICO: Chihuahua, Coahuila, Durango, Jalisco, western Nuevo León, San Luis Potosí, Sinaloa, Sonora, Zacatecas; USA: Arizona, New Mexico, Texas (Trans-Pecos). Habitat Typically occurs on north-facing slopes and along mountain streams between 1900 and 3500 m asl. USDA Hardiness Zone 5. Conservation status Lower Risk. Illustration Farjon & Styles 1997, Farjon 2005a; NT580. Cross-references K237 (B213, S373, K210 as P. ayacahuite var. brachyptera).
Pinus strobiformis is little known in cultivation but in the wild forms a substantial tree, with a relatively narrow crown in its youth, although this becomes more rounded with age, with far-reaching branches. It is related to P. ayacahuite and resembles it in many characters, but its leaves are shorter and the average sizes of its handsome cones are smaller. The seeds of P. strobiformis are functionally wingless (Perry 1991). Northern populations (United States, northernmost Mexico) are intermediate between P. strobiformis as described by Engelmann, from southern Chihuahua, and P. flexilis, with smaller cones (12–22 cm) and needles with only a few apical serrations, and have been distinguished as P. flexilis var. reflexa Engelm. or P. reflexa (Engelm.) Engelm. (Farjon & Styles 1997, Farjon 2005a). These northern trees are commonly cultivated in the United States and seem to have a wide tolerance, growing even in the Chicago area (for example, at the Morton Arboretum). In Europe P. strobiformis is much less familiar, although there are a few dwarf cultivars in circulation, and ‘Vanderwolf’s Pyramid’ is widely available (usually sold as P. flexilis). TROBI records a few individuals scattered through the United Kingdom, the recognised champion being an 11 m specimen at the Manor, Walton-in-Gordano, Somerset, measured by Owen Johnson in 2006. Two trees of 11 m and 10.8 m grow in the Hillier Gardens, where they were received in 1985 as P. ayacahuite (Priest 107), and it is possible that other specimens from this collection exist elsewhere (A. Coombes, pers. comm. 2008). It is also present in some collections in continental Europe, but could be grown much more widely. It is, however, very susceptible to white pine blister rust; there has been extensive mortality in New Mexico recently, since the disease invaded the state in 1990 (Conklin 2004), and a mature specimen at Hørsholm Arboretum in Denmark also died of it in 1986 (M. Frankis, pers. comm. 2007).
Pinus veitchii Roezl from central Mexico has traditionally been included in P. ayacahuite as a variety, P. ayacahuite var. veitchii (Roezl) Shaw (see Bean and Krüssmann: B213, K210) – though one recent author (Businský 2004) treats it as a distinct species. It is much closer to P. strobiformis, however, matching it in cone scale thickness and seed size, and differing from it only in seed wing length and minor detail of curvature of cone apophysis.
[Editorial Note: The combination Pinus strobiformis subsp. veitchii (Roezl) Frankis was published in 2009, the same year New Trees went to press.]