Pinus flexilis

Limber pine

Pinaceae

The Basics

Taxonomy: Kingdom - Plantae (plants). Subkingdom - Tracheobionta (vascular plants). Superdivision - Spermatophyta (seed plants). Division - Coniferophyta (conifers). Class - Pinopsida. Order - Pinales. Family - Pinaceae (pine). Genus - Pinus L. Species - Pinus flexilis James

Ecology: Limber pine is a slow growing, long-lived species, sometimes taking several hundred years to reach maturity. Mature trees may exceed 1000 years of age. Limber pine stands are broadly even-aged, though populations also occur in uneven-aged stands and on very harsh sites as widely spaced, isolated individuals. Trees often have an irregular or multi-stem growth form, and rarely reach over 15 m. At high elevations they sometimes form krummholz. Trunks may reach 2 m in diameter. The species is cold and drought tolerant. Trees are ectomycorrhizal, have deep taproots, and are very windfirm. Limber pine (and whitebark pine) are pioneering species that are either seral or topoedaphic climax species under different environmental conditions. In fact, seed dispersal by Clark's nutcrackers to outlying sites, treeline, and other harsh environments essentially increases the ecological niche breadth of these species. Clark's nutcrackers can maintain climax communities, colonize previously unforested sites, or initiate succession. The later stages of succession in xeric subalpine forests vary due to differences in sites and seed availability.

Identification

Trees to 26m; trunk to 2m diam., straight to contorted; crown conic, becoming rounded. Bark gray, nearly smooth, 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, smooth. Buds ovoid, light red-brown, 0.9--1cm, resinous; lower scales ciliolate along margins. Leaves 5 per fascicle, spreading to upcurved and ascending, persisting 5--6 years, 3--7cm 1--1.5mm, 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. Pollen cones broadly ellipsoid-cylindric, ca. 15mm, pale red or yellow. Seed cones maturing in 2 years, shedding seeds and falling soon thereafter, spreading, symmetric, lance-ovoid before opening, cylindro-ovoid when open, 7--15cm, straw-colored, resinous, sessile to short-stalked, apophyses much thickened, strongly cross-keeled, umbo terminal, depressed. Seeds irregularly obovoid; body 10--15mm, brown, sometimes mottled darker, wingless or nearly so.

Threats

White pine blister rust: Limber Pine is highly susceptible to infection and death from the disease white pine blister rust (WPBR) caused by the non-native pathogen Cronartium ribicola   Limber Pine populations in the northern US Rocky Mountains have been infected by WPBR for over 50 years and mortality is high in many sites The disease has continued to spread south and now infects Limber Pine in MT, ID, WY, CO, SD, NM and CA with heavily infected stands in each of these states. New locations of infected stands are being found yearly Limber Pine has not evolved in the presence of C. ribicola; as a result it only has very low frequencies of resistance to the disease. WPBR kills Limber Pine trees of all ages and young trees are especially susceptible. Even before the disease kills the larger trees it kills the cone bearing branches and severely restricts seed production. WPBR threatens the sustainability of Limber pine populations by compromising the regeneration cycle with high mortality of all aged trees, reduced seed production of the mature trees and high susceptibility of young seedlings and saplings These impacts are being seen in many Limber Pine forests now and are expected to expand to other landscapes as the disease continues to spread. Until natural selection results in an increase in durable WPBR resistance, ecosystems impacts will continue  Initial modeling suggests that the slow regeneration time, delayed maturation (>50 years for a tree to become reproductive and over 100 years before producing large cone crops), and low initial frequency of heritable resistance in Limber pine will lead to high mortality in native populations for centuries to come While this mortality is selection against susceptible individuals, it is likely to reduce populations to below a sustainable threshold. Limber Pine is largely dependent on corvid species to disperse its seeds and studies suggest that once populations are impacted to the point that seed production is very low, the mutualism with the corvids may fail, as the birds do not visit such stands Extirpation of populations is likely to occur which will lead to further genetic isolation of remaining populations

Mountain pine beetle: The recent epidemic of Mountain Pine Beetle (MPB; Dendroctonus ponderosae) is further threatening the sustainability of Limber Pine populations. Limber Pine is an excellent host for MPB and larger beetle broods are often produced on Limber Pine compared to other pine hosts. MPB generally only attack larger trees (those above 10 cm d.b.h.) and do not attack the smaller advanced regeneration trees However, MPB further reduces the seed production of the population and in the presence of WPBR the rust will continue to kill the susceptible seedlings that are able to get established. While MPB epidemics are not unprecedented in Limber Pine forests, the current epidemic is more extensive than in the past and the presence of WPBR severely compromises the recovery capacity of the populations after the MPB epidemic passes. In areas of high WPBR incidence, MPB is killing the remaining live reproductive trees which are often those that possess some genetic resistance to the disease. This is setting back the natural selection for resistance in Limber Pine that has already occurred. In those populations that have more recently been invaded by C. ribicola, MPB is killing the seedtrees from which seeds must be collected to test for resistance, slowing the identification of resistance trees and estimates of resistance in populations. MPB in all locations is affecting the availability of seed for collections for restoration plantings

WPBR alone is a serious threat to Limber pine populations and will continue to threaten the species for centuries The combination of WPBR with the MPB epidemic compromises the regeneration cycle of the pine populations further and reduces the populations’ resiliency to recover from this and any other disturbance Extirpation of populations is probable and imminent. The extirpation of populations in the high mountain and tree line habitats will likely transition those lands from forests to non-forested landscapes. Limber Pine is often the only tree species that can tolerate those harsh habitats and therefore there will be no replacement tree species to occupy the site. Loss of the trees will also affect snow accumulation and snow melt on the wind-exposed high elevation sites

Reproduction

Seed production: The minimum seed-bearing age of limber pine ranges from 20 to 40 years. There are 2 to 4 years between large seed crops. Seeds from krummholz trees have low germination potential. Cones of limber pine are cylindrical, 8-15 cm long. They release their seeds if not preyed upon. The seeds are large (7-12 mm long) and sometimes have a vestigial wing. Small mammals and birds, especially Clark's nutcrackers and pinyon jays, disperse limber pine seeds. Clark's nutcrackers have co-adapted an important mutualism with limber pine and are the primary harvester and disperser of its seeds. Limber pine regeneration on burns is largely from germinants of Clark's nutcrackers seed caches. Clusters of seedlings germinating from a single cache may generate multi-stemmed growth forms that contain 2 or more distinct genotypes. A consequence of this growth form is a tendency toward clumped stand structure.

Vegetative regeneration: Limber pine reproduces entirely from seed; it does not layer lower branches in the soil. Seeds are not effectively dispersed by wind.

Species Distribution

Citation

IUCN Red List
Schoettle, A. & Stritch, L. 2013. Pinus flexilis: least concern. The IUCN Red List of Threatened Species.

USDA Plants Database
USDA, NRCS. 2016. The PLANTS Database. National Plant Data Team, Greensboro, NC 27401-4901 USA.

USFS Plant Database
Habeck, R. J. 1992. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory.

Flora of North America
Flora of North America Editorial Committee, eds. 1993+. Flora of North America North of Mexico. 19+ vols. New York and Oxford.

Silvics of North America
Burns, R.M., and B.H. Honkala. 1990. Silvics of North America (Volume 1: Conifers, Volume 2: Hardwoods). USDA Forest Service Agricultural Handbook 654.

The Jepson Herbarium
The Jepson Manual: Vascular Plants of California. B.G. Baldwin, D.H. Goldman, D.J. Keil, R. Patterson, T.J. Rosatti, and D.H. Wilken [editors]. 2012. 2nd edition, thoroughly revised and expanded. University of California Press, Berkeley, CA.

USGS Plant Species Range Maps
Critchfield, W.B., and Little, E.L., Jr., 1966, Geographic distribution of the pines of the world: U.S. Department of Agriculture Miscellaneous Publication 991, p. 1-97. Little, E.L., Jr., 1971-1978, Atlas of United States trees, volume 1,3,13,17, conifers and important hardwoods: U.S. Department of Agriculture Miscellaneous Publications.