Potosí piñon (Lanner 1981).
Malusa (1992) showed this piñon to be closely related to P. johannis and P. discolor, all of which share similar bark, habit, and leaf stomatal distribution (and thus a very distinctive appearance), with P. quadrifolia also allied but less closely so. P. orizabensis (not examined by Malusa) also belongs to this group. Silba (1986) reduced P. johannis and P. discolor to varieties of P. culminicola, but this is doubtful; for instance, there is no evidence of hybridisation where P. johannis and P. culminicola are sympatric (see under P. johannis).
Type. Mexico. Nuevo León: Cerro Potosí, near summit of mountain, Beaman 2675 (holotype, MSC; isotypes, A, MONT, US). The plant had actually been collected much earlier on that mountain by C. H. Mueller in 1934 and 1935. A still older collection of the species (Ellareal & Carunza 4242), from the Sierra de Arteaga in Coahuila, had likewise been overlooked. Mueller referred his collections to P. flexilis, and Villareal and Carranza identified their material as P. cembroides (Farjon and Styles 1997).
A shrub 1-5 m tall and up to 15-25 cm diameter, multistemmed or very low-branched, with a low, dense, rounded crown. Often forms extensive mats up to several meters thick and tens of meters across ("matorral"). The bark is thin, scaly, with peeling shaggy grey-brown plates exfoliating to expose fresh bright orange bark beneath (similar to P. johannis); bark on young stems smooth and grey. Branches numerous, the first-order branches often prostrate to assurgent, the higher-order branches assurgent to erect, short, thick, rigid but resilient, forming a dense, rounded to flat crown, usually matted with other individuals. Shoots short, thick, rough; the bases of the bract leaves (pulvini) decurrent, persistent, light brown turning grey. Bract leaves small, narrowly triangular to subulate, apex caudate, margins erose, persistent in part, light brown, weathering blackish grey. Vegetative buds broadly ovoid, the terminal bud 6-10 mm long, the laterals smaller, slightly resinous; the scales imbricate, free at apex, subulate-caudate, light brown. Fascicle sheaths initially 6-8 mm long, consisting of 4-6 imbricate, translucent, ciliate-margined scales; in mature fascicles the scales separate to form a rosette at the base of the fascicle, straw-coloured to grey, semi-persistent but mainly falling before the leaf fascicles. Foliar units in dense tufts on the ultimate branches. Leaves five in a fascicle (very rarely 4 or 6), 3-5 cm long, 0.9-1.3 mm thick, obtuse; stiff, curved toward the shoot apex, margins entire, stomata present on both adaxial surfaces, the abaxial surface dark green and the adaxial surface glaucous with 4-5 stomatal lines on each leaf face; retained 2-3 years. Leaf anatomy: Cross section triangular, with a convex abaxial side; hypodermis monomorphic, with 2 layers of cells; resin ducts 1-2, external, on the abaxial side; stele terete; outer cell walls of the endodermis not thickened; vascular bundle single. Pollen cones crowded on the proximal part (ca. 1/2) of a new shoot, ovoid-oblong when mature, 5-8 mm long, yellowish, turning yellowish brown. Seed cones are subterminal, solitary or paired, sessile or on a stout, short peduncle covered with subulate-caudate cataphylls. Immature cones subglobose, resinous, purple-brown, maturing in 2 years. Mature cones subglobose when closed, opening with a flattened base and remote, spreading fertile seed scales, then 3-4.5 × 3-5 cm; soon deciduous. Seed scales ca. 45-60, parting and spreading wide except the smaller, infertile proximal scales, irregular, often curved, thin, coarsely wrinkled, orange-brown, with 1-2 deep, cup-like depressions holding the seeds, bordered by membranous seed wing remnants, margins irregular, erose, yellowish on both sides, seed cups brown. Apophysis slightly raised, transversely keeled, rhomboid with a pointed apex, 10-15 mm broad, with a 4-5 mm wide blackish umbo, flat or slightly raised. Umbo dorsal, slightly raised, rhombic in outline, up to 5 mm wide, with or without a minute prickle, dark brown, often resinous. Fully developed cones in the upper crown have 10-15 fertile scales; small cones from low branches (with poor pollination and nutrient supply) are often lopsided with 1-10 fertile scales. Seeds dark orange-brown, 5-7 × 4-5 mm, with a rudimentary 0.5-1 mm wing that remains in the cone on seed release; the seed shell 0.5-1.0 mm thick, hard; endosperm white. Most seed is dispersed by mid-November, suggesting cone maturity in late October. Cotyledons 8-11 (Perry 1991, M.P. Frankis field notes 1991.11, Farjon and Styles 1997).
In the exposed sites where it is most common, it primarily assumes a krummholz form. This habit is very similar to other mountain dwarf pines, e.g., P. mugo in Europe and P. pumila in NE Asia, and is also assumed by normally erect pines such as P. albicaulis when they grow at alpine timberline sites. Adaptation to leaf cuticle abrasion by wind-blown ice is responsible for this habit.
Mexico: Coahuila and Nuevo León. It was first found in the summit area of Cerro Potosí in Nuevo León, 65 km W of Linares at 24.8729°N 100.2334°W. This is the most extensive known population. Three or four small populations are also recorded on the high ridges about 50 km northwest of there on the Nuevo León-Coahuila border; see Riskind and Patterson (1975) for details. Soils are rocky and calcareous. It grows at 2965-3700 m, the highest mean elevaton of any pine. Since its current distribution includes only the highest elevations on summits within its range, it must be regarded as severely at risk of extinction due to global warming. Existing climate parameters are not well-known due to a lack of weather stations at these summits (Perry 1991, M.P. Frankis field notes 1991.11, Farjon and Styles 1997, C.J. Earle field notes 2007.02.19). Hardy to Zone 7 (cold hardiness limit between -17.7°C and -12.2°C) (Bannister and Neuner 2001).
On Cerro Potosí, I found the species at its lower elevational limits as an understory shrub with Agave and grasses in an open P. hartwegii-P. ayacahuite forest. (As a point of interest, these may be P. ayacahuite-P. flexilis hybrids.) With increasing elevation and site exposure, the erect pines yield dominance to P. culminicola, and at the summit it forms extensive mats interspersed with occasional gnarled and stunted P. hartwegii. I also observed that the pine mats carry fire well; areas of up to several dozen hectares had burned on various sides of the mountain, the overall pattern consistent with an hypothesis of stand replacing fire at timescales of 50 to 200 years, although this would obviously be responsive to anthropogenic ignitions, which are quite possible at this easily accessible site.
Farjon and Styles (1997) report that at lower elevations, in the Sierra La Marta, Coahuila, P. culminicola grows in a scrub community with Quercus spp., Arctostaphylos, Ceanothus, Agave, and grasses; on the Cerro La Viega and the Sierra de Arteaga, Coahuila, they grow in a similar community that also includes Abies and Pseudotsuga (species not specified).
Pollen dispersal has been reported on Cerro Potosí to occur in late July, at 3690 m, which indicates a late fertilisation and short growing season (Farjon and Styles 1997).
Best growth, to 5 m tall, is at around 3400-3450 m on Cerro Potosí; plants near the summit are shorter, mostly 1-1.5 m tall (M.P. Frankis field notes, 1991.11). All other species can attain larger sizes, so this can be described as the smallest of all pines.
There are no reported data. However, during a field visit to Cerro Potosí in 2007.02, I observed evidence of traveling krummholz trees. This phenomenon, previously observed among Picea engelmannii in Colorado, occurs when a krummholz tree layers on its downwind side while succumbing to frost abrasion at its upwind side. Over time the entire tree travels downwind. The phenomenon obviously occurs slowly; radiocarbon dating has been used to ascertain travel rates of 0.9 to 2.6 centimeters per year for Picea engelmannii at Niwot Ridge, Colorado (Benedict 1984). The trees I saw had left dead wood trails 1 to 2 m long, suggesting that they were at least a couple of hundred years old.
As of 2007.11.06, no studies have been reported.
Perhaps the only piñon with no history of aboriginal use, as it grows at formerly uninhabited altitudes. Like the other piñons, it has edible seeds and no doubt the microwave station staff on Cerro Potosí now collect them for local use on occasion. It may also receive some use as firewood.
USDA hardiness zone 7. With its very attractive blue-green foliage, it is potentially a valuable slow-growing ornamental species for small gardens in cool dry areas, but it is still very rare in cultivation.
It is very easy to see on the top of Cerro Potosí, being the dominant species from 3300 m up to a few metres short of the summit (3670-3713 m, depending on which map you use!). There is a steep dirt road paved with large and angular rocks that zig-zags up the NE slopes right to the top, where it serves a microwave station. The road is passable in a 2WD vehicle with decent ground clearance and sturdy tires, and along it there are many fine campsites, but no reliable water. The road begins at the foot of the mountain in the village of Dieciocho del Marzo, and is prominently signed. Be prepared to sign in and pay an entrance fee (10 pesos in 2007) at the entrance station a couple of kilometers up this road. By the way, about a kilometer past the entrance station on this road is one of the easiest places to see the rare piñon P. nelsonii, which forms a small stand to the right (north) of the road. Other species visible on the drive up the mountain include Pseudotsuga lindleyana, Abies (not sure which species), P. arizonica var. stormiae, P. ayacahuite, and P. hartwegii. See Perry (1991) for directions to this road and advice on travel.
In November 1991, after a hot dry two years in 1989-1990, the summit trees had a heavy cone crop of well-formed large (4 cm) cones, but lower trees had poorer crops of mostly somewhat smaller cones, presumably due to drought stress. In normal years, the reverse probably applies with conditions at the summit too cool for good crops. The seeds are sought out and dispersed by gray-breasted jays and Clark's nutcrackers, which spread the seeds widely, hiding them in the ground for a winter food resource; as they hide more than they need, the surplus are left to germinate safe from marauding rodents (Lanner 1981).
Andresen and Beaman. 1961. J. Arnold Arbor. 42: 437.
Benedict, J.B. 1984. Rates of tree-island migration, Colorado Rocky Mountains, USA. Ecology 65:820-823
Riskind, D.H. and T.F. Patterson. 1975. Distributional and ecological notes on Pinus culminicola. Madroño 23: 159-161.
M.P. Frankis, 1999, assisted with preparation of this page.
Last Modified 2014-03-29