Pinus flexilis (limber pine) description

Pinus flexilis

E. James 1823

Common names

Limber pine, limbertwig, Rocky Mountain pine (Peattie 1950), pino (Perry 1991), white pine (Little 1980), pin blanc de l'ouest (Kral 1993).

Taxonomic notes

Section Cembra. Syn: Apinus flexilis (E. James) Rydberg (Kral 1993). One variety, Pinus flexilis var. reflexa Engelmann 1879 (syn: P. reflexa (Engelmann) Engelmann 1882; P. ayacahuite var. strobiformis Lemmon 1892; P. ayacahuite var. reflexa (Engelmann) Voss 1907; P. flexilis subsp. reflexa (Engelmann) Murray 1982; P. stylesii Frankis ex Businský 2008) (Farjon and Styles 1997).

Typical P. flexilis is quite uncontroversial. It its northern ranges it is quite clearly distinct from all other white pines and really shows little variation throughout its range. Var. reflexa, due to its hybrid origin, has had a history steeped in controversy, and remains the subject of much debate. Farjon and Styles (1997) regard P. flexilis and P. strobiformis as forming a polymorphic cline, while Perry (1991) regards them as distinct taxa with introgression in their shared range. I have not had a chance to study var. reflexa in its western ranges, where the hybridization with P. strobiformis is most complex and widespread. I have seen it on Cerro Potosí, where it has recently been described as a new species Pinus stylesii, and will comment on that area.

Cerro Potosí is a remarkable place in many ways. It is a gigantic limestone monolith in westernmost Nuevo León. There are a couple of minor peaks to the west and northwest that approach it in altitude (over 3,700 meters), but apart from these, it is the tallest thing for several hundred kilometers in any direction. Biogeographically, it is an extraordinarily isolated island, and contains a goodly number of endemic plant species. Among these is Pinus culminicola, a pinyon, arguably the world's smallest pine (up to 3 m tall). The top thousand meters or so of the mountain are dominated by a conifer forest of, from the top down, Pinus culminicola, Pinus hartwegii, Pinus flexilis var. reflexa, Pinus strobiformis, Pseudotsuga menziesii, Abies vejarii, and Pinus arizonica var. stormiae. Still lower are Pinus cembroides, Pinus greggii, and Pinus nelsonii. It is a Mecca for rare pines.

Jesse Perry (1991) described Pinus flexilis from the summit of Cerro Potosí, and in evidence provides a photograph of a P. flexilis cone collected at 3,400 m near the summit. P. flexilis at its current closest approach is a thousand kilometers to the west in the mountains of New Mexico. Data on its previous range in this vast desert land are scanty, but based on reconstructions of temperature and precipitation during the Pleistocene, say 10,000 to 18,000 years ago, it is quite plausible that the species could have been common on Cerro Potosí and on many other ranges in Texas and northern Mexico. 18,000 years ago was an eyeblink in evolutionary time, 50 generations for a long-lived species like P. flexilis, and it is highly implausible that P. flexilis living around Cerro Potosí 18,000 years ago might have evolved through classical Darwinian selection into a new species. It is certainly plausible, though, that hybridization may have been at work over such a time scale.

The white pines of northern Mexico and the southwestern US form quite an incestuous family. Noticable variations in morphology occur along nearly continuous gradients extending from southern Arizona and New Mexico south along both the Sierra Madre Occidental and Oriental and extending along the full breadth of the Central Volcanic Plateau. Various authors have described an array of species and subspecies in these white pines, including P. flexilis, P. strobiformis, P. brachyptera, and P. ayacahuite, and now P. stylesii. I have examined each of these taxa in several portions of its range and find that every one of them can be confused with at least one of the others in some part of its range. Such a pattern of variation is consistent with an hypothesis of hybridization and/or introgression on species margins, and the pattern is quite common in conifers of the pine family, occurring for instance among the spruces of Sichuan, the Caucasian firs, or the cedars of Turkey.

Based, then, upon the paleoclimate history of the area and the biogeographic setting of Cerro Potosí, it is my belief that the taxon lately described as Pinus stylesii arose initially from the hybridization of P. flexilis and P. strobiformis in the mountains of Nuevo León, probably during late Pleistocene climate changes that caused relatively rapid range changes for plants growing in that area. P. flexilis may have persisted for a time on Cerro Potosí, perhaps even into the historic era, but introgression to the tiny remnant population on the summit of Potosí, coupled with the high incidence of disturbance (chiefly by fire) on that mountaintop, doomed it to eventual extinction. The remnant hybrid pines also constitute a very small island of flexilis-like pines amongst a sea of P. strobiformis, and their distinctive genetic composition may in the future continue to become more like that of typical P. strobiformis as hybridization events continue to accumulate (to say nothing of the effects of projected climate change). Thus, the populations described as P. stylesii are biologically an artifact, a snapshot of the final events in extinction of an isolated remnant population of P. flexilis.

There is one interesting consequence of this interpretation: although these trees are here described as P. flexilis var. reflexa, they likely contain rather more genes from P. strobiformis than from P. flexilis, and in the future continued backcrossing with P. strobiformis will only continue that trend. Thus it would probably be more genetically accurate to call them a P. flexilis-like variety of P. strobiformis. However, no such variety has been described.

Three photographs presented here show cones of limber pine in northeast Oregon, in extreme southern California, and on the summit of Cerro Potosí. A fourth photograph shows the Cerro Potosí cone along with several P. strobiformis cones collected at the same locale. All cones appeared to have developed normally and produced a large fraction of normally developed seed (based on the observaton that all had been thoroughly gleaned by corvids), and all cones appeared representative of their local populations. Based on this evidence, supported by evidence from foliage, bark and general tree form amongst the Cerro Potosí trees, I am satisfied that the trees I found on Cerro Potosí can be described as P. flexilis × strobiformis hybrids, i.e., P. flexilis var. reflexa.

Description

Trees 12-15(26) m tall; 60-90(200) cm diameter, straight to contorted; crown conic, becoming rounded; growth form may be substantially altered near timberline (krummholz form occurs) or on very dry sites. Bark light grey, nearly smooth, becoming dark brown and cross-checked in age into scaly plates and ridges. Branches spreading to ascending, often persistent to trunk base; twigs pale red-brown, puberulous (rarely glabrous), slightly resinous, aging gray, tough and flexible, smooth. Buds ovoid, light red-brown, 0.9-1 cm, resinous; lower scales ciliolate along margins. Needles 5 per fascicle, spreading to upcurved and ascending, persisting 5-6 years, 3-7 cm × 1-1.5 mm, pliant, dark green, abaxial surface with less conspicuous stomatal bands than adaxial surfaces, adaxial surfaces with strong, pale stomatal bands, margins finely serrulate, apex conic-acute to acuminate; sheath 1-1.5(-2) cm, shed early. Staminate cones broadly ellipsoid-cylindric, ca. 15 mm, pale red or yellow. Ovulate cones maturing in 2 years, shedding seeds and falling soon thereafter, spreading, symmetric, lance-ovoid before opening, cylindro-ovoid when open, 7-15 cm long, yellow-brown, resinous, sessile to short-stalked, apophyses much thickened, strongly cross-keeled, umbo terminal, depressed. Seeds irregularly obovoid; body 10-15 mm, brown, sometimes mottled darker, wingless or nearly so. 2n=24 (Little 1980, Kral 1993).

In the absence of cones, it strongly resembles P. albicaulis. However, limber pine twigs become roughened at a smaller size, usually <10 cm diameter, vs. >10 cm diameter in P. albicaulis. On older trees (>30 cm dbh), limber pine bark is usually composed of longitudinal reddish-brown plates with intervening fissures, while whitebark pine bark becomes light brown and thinly platy without conspicuous fissures. When in flower, whitebark pollen cones are a striking red color, while limber pine pollen cones are reddish or yellow. Saplings are very difficult to distinguish; Kral (1993) contends that bud scale margins are entire in whitebark pine, whereas lower bud scales have ciliolate margins in limber pine. I have not tested this assertion in the field.

Range

Rocky Mountains and Intermountain Ranges from Canada: SE British Columbia and SW Alberta, S through USA: Oregon, Idaho, Montana, North Dakota, South Dakota, Nebraska, Wyoming, Colorado, Utah and Nevada to N New Mexico and W through N Arizona to S California at (1000)1500-3700 m, preferring dry, rocky slopes and ridges of high mountains up to timberline, often occurring in pure stands (Little 1980, Kral 1993); perhaps in Mex: Nuevo León (see Taxonomic Notes). See also Thompson et al. (1999). Var. reflexa may occur in the USA: Arizona, New Mexico and W Texas (populations identified as P. strobiformis), and is known from a few locales in Mexico (Chihuahua, Coahuila and S Nuevo León) but may have a more extensive range in that country (Farjon and Styles 1997).

Big tree

Height 18 m, dbh 222 cm, crown spread 14 m. Locality: On a ridge S of Snowbird, Utah (American Forests 1996).

Oldest

Crossdated ages of 1,670 years from site ERE in New Mexico, collected by Swetnam and Harlan; and 1659 years for specimen KET3996 from Ketchum, Idaho collected by Schulman in 1956 (Brown 1996). Given the fact that crossdated tree ages are always underestimates because of the near-impossibility of sampling the tree's seedling growth years, either of these trees could have been the older, particularly since KET3996 was sampled about 30 years before the ERE tree. During a 1994 visit to Craters of the Moon National Monument, I believe I located KET3996; it was dead, and had been for many years.

Dendrochronology

Limber pine is an important species due to its longevity and widespread occurrence in the arid U.S.

Ethnobotany

There is some evidence that the seeds were used as a food source by certain Great Basin tribes, such as the Northern Shoshone. Numerous grinding stones at Alta Toquima Village, a high elevation prehistoric site in central Nevada, also suggest use of pine nuts as food, with limber pine the likely source (Lanner 1996). I have observed that miners, sheepherders, and other rural residents if its range (ca. 1850-1950) used it for cabins, fencing, mine timbers, and doubtless, firewood. However, the wood is too contorted and resinous, and the trees generally too small, to warrant commercial exploitation.

Observations

The most memorable stands I have seen have been:

Craters of the Moon National Monument in Idaho, which has numerous groves and an interesting history. Many ancient limber pines in the Monument were killed by the Park Service in the early 1960s in a poorly conceived attempt to eradicate a mistletoe infestation.
The timberline grove along the trail up Borah Peak, Idaho. This site is a particularly worthy objective for photographers, as in contains many ancient and extremely gnarled trees. These limbers are the ones, in my experience, that most closely resemble the extremely ancient and gnarled growth form typically associated with P. longaeva.
The forest near the summit of Cave Mountain, southeast of Ely, Nevada. A steep road passable for light trucks and sturdy sedans climbs to a communications complex on the summit of this peak and affords access to an extensive stand of P. flexilis and P. longaeva. The east-facing slopes of the peak support a closed-canopy stand of very large P. flexilis, the only closed-canopy stand of this species that I have seen, and in it grow the largest trees of this species known from Nevada.
Along the hiker's trails up Thomas and Lamoille Canyons in the Ruby Mountains of Nevada, where it grows with P. albicaulis. It is interesting to see how these very similar white pines share the habitat in the Ruby Mountains. Limber pine generally occurs at somewhat lower elevations. Where the two species co-occur, limber pine typically occupies relatively rocky granitic substrates, while whitebark pine is usually found in areas with deep soils. Below the elevation limit of whitebark, limber pine moves onto deep soils, often in mountain mahogany (Cercocarpus ledifolius) woodlands. Above the elevation limit of limber, whitebark pine occurs on some extremely rocky sites.

Remarks

The common name "limber" refers to the tough, flexible twigs. The seeds are an important food source for rodents and certain birds (Little 1980).

The fresh-cut wood has the odor of turpentine (Kral 1993).

White pine blister rust (Cronartium ribicola), an introduced fungal disease, has afflicted this and certain other white pines (Elias 1987).

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