Picea mexicana Martínez

Trees to 20–30 m tall, <1 m dbh. Bark yellowish-brown at first, turning grey (paler than in P. engelmannii), becoming rough and fissured, breaking into thick plates, freshly exposed bark yellowish-brown. Crown narrowly conical, somewhat columnar and open in old trees. First order branches short, slender, spreading horizontally, tips assurgent, lower branches on older trees downswept; second order branches short, dense, spreading or variably pendent. Branchlets slender, stiff, pendent except for leading shoots, greenish-yellow and finely pubescent at first, maturing yellowish-brown, becoming more or less glabrous, ridged and grooved; pulvini c. 2 mm long. Vegetative buds ovoid-conical, to 5–6 mm long, resinous at apex; bud scales obtuse-triangular, appressed at first, later spreading, red-brown, persisting several years. Leaves spreading radially around leading shoots, crowded and directed forward along the shoot upper side, weakly parted beneath shoots on older wood and side shoots, (12–)18–30(–36) × 1–1.2 mm, base minutely truncate, linear, straight or slightly curved, more or less square in cross section, apex acute; amphistomatic, with 2 narrow bands of stomata on each surface; leaf colour on youngest shoots strikingly glaucous, becoming dull glaucous-green after a year or more; odourless when crushed. Pollen cones 1–1.5 cm long. Seed cones ovoid-cylindric, sessile, apex obtuse, (4.5–)5–8 × 2.5–3.5 cm, reddish-green at first, pale reddish- to yellowish-brown at maturity. Seed scales obovate-obtrullate, thin, flexible, (1.2–)1.5–2 × 1–1.6 cm at mid cone; upper surface smooth or finely striated, glabrous, upper margin rounded or truncate, undulate, entire or erose-denticulate; base cuneate. Bract scales broadly ovate, cuspidate, 4–6 mm long, entirely included, Seeds ovoid, 2–3 mm long, dark blackish-brown; seed wings ovate-oblong, 10–12 × 4–5 mm, yellowish-brown. (Farjon 2017; Debreczy & Rácz 2011; Ledig et al. 2004; Rushforth 1986).

Distribution  Mexico S Chihuahua, Coahuila, Nuevo León United States Arizona (Chiricahua Mountains); Texas (?)

Habitat Restricted to the north facing slopes and nearby ravines of the highest peaks in the Mexican Sierras on limestone substrate at 3000–4000 m asl. Associate species on Sierra de la Marta include Pinus arizonica, P. strobiformis and Pseudotsuga menziesii var. glauca.

USDA Hardiness Zone 7

RHS Hardiness Rating H6

Conservation status Endangered (EN)

Taxonomic note The Mexican botanist Maximino Martínez described Picea mexicana in 1961. It was amended first to a variety of P. engelmannii, distributed to the north in the Rocky Mounains, in 1984 (P. engelmannii var. mexicana (Martínez) Silba) and then in 1988 to a subspecies (Picea engelmannii subsp. mexicana (Martínez) P.A.Schmidt). It has been treated at the rank of subspecies in most major works ever since (e.g. Farjon 1990; 2001; 2017; Debreczy & Rácz 2011; Grimshaw & Bayton 2009) but some authorities and studies advocate treating it at species rank (e.g. Rushforth 1987; Ledig et al. 2004; Eckenwalder 2009; Thomas 2019). In one of the most complete phylogenetic anaylses of the genus to date, Lockwood et al. (2013) concluded that P. mexicana is not particularly closely related to P. engelmannii after all; they place P. mexicana sister to P. pungens and P. engelmannii sister to P. glauca, with P. sitchensis isolated between these two pairs. Few would argue that P. mexicana most closely resembles P. engelmannii, but in light of the molecular evidence it is better treated as a distinct species.