The Limber Pine (Pinus flexilis; family Pinaceae) is a species of pine tree that occurs in the mountains of the Western United States and Canada, specifically the subalpine areas of the Rocky Mountains from southwest Alberta south to the Mexican border; the Great Basin mountains of Nevada and Utah; and the White Mountains, the east slope of the Sierra Nevada and the San Bernardino Mountains in California with a small disjunct population in the Black Hills of South Dakota. It is typically a high-elevation pine, often marking the tree line either on its own, or with Whitebark Pine, either of the bristlecone pines, or Lodgepole Pine. In favourable conditions, it makes a tree to 20 m, rarely 25 m tall, but on exposed tree line sites only 5-10 m tall.
Limber Pine is a member of the white pine group, Pinus subgenus Strobus, and like all members of that group, the leaves ('needles') are in fascicles (bundles) of five, with a deciduous sheath. This distinguishes it from the Lodgepole Pine, with two needles per fascicle, and the bristlecone pines, which share five needles per fascicle but have a semi-persistent sheath.
Distinguishing Limber Pine from the related Whitebark Pine, also a white pine, is very much more difficult, and can only easily be done by the cones. In Limber Pine, the cones are 6-12 cm long where the species overlap, green when immature, and open to release the seeds; the scales are not fragile. In Whitebark Pine, the cones are 4-7 cm long, dark purple when immature, and do not open on drying, but are fragile and are pulled apart by birds (see below) to release the seeds. A useful clue resulting is that Whitebark Pines almost never have intact old cones lying under them, whereas Limber Pines usually do.
In the absence of cones, Limber Pine can also be hard to tell from Western White Pine where they occur together in the northern Rockies and the Sierra Nevada east slope. The most useful clue here is that Limber Pine needles are entire (smooth when rubbed gently in both directions), whereas Western White Pine needles are finely serrated (feeling rough when rubbed gently from tip to base). Limber Pine needles are also usually shorter, 4-7 cm long, to Western White Pine's 5-10 cm (though note the overlap).
In Arizona and New Mexico, Limber Pine differs from the populations further north. These populations, often known as Southwestern White Pine, are sometimes treated as a variety, Pinus flexilis var. reflexa, but more often as a distinct species, either (accurately) under the name Pinus reflexa, or (through confusion with the Mexican Chihuahua White Pine), erroneously under the name Pinus strobiformis (which correctly applies to the Mexican species). The Southwestern White Pine differs from typical Limber Pine in being a larger tree, to 25-35 m tall, with longer needles, 6-11 cm long, which have strongly white stomatal bands on the inner faces of the needles (inconspicuous in the type), and are slightly serrated towards the tips of the needles. The cones are also larger, typically 10-20 cm long. It differs from true Mexican Pinus strobiformis in that the needles are not fully serrated, and the cones being smaller (15-25 cm in P. strobiformis), the cone scales shorter and the seeds smaller. It is possible that Pinus reflexa is a natural hybrid between Pinus flexilis and Pinus strobiformis. Type localities of the three taxa are:
Limber Pine is an important source of food for several species, including Red Squirrels and Clark's Nutcrackers. American Black Bears may raid squirrel caches for Limber Pine nuts. Squirrels, Northern Flickers, and Mountain Bluebirds often nest in the trees.
Unfortunately, Limber Pine is afflicted with White Pine Blister Rust (Cronartium ribicola), a fungus that was introduced accidentally from Europe. Limber Pine mortality is high in many areas throughout its range, except Arizona, where it has not yet been found. However, there is no known way of controlling the blister rust in existing trees. Research is under way, locating and breeding from the occasional naturally resistant Limber Pines, and by studying the resistance mechanisms of the European and Asian white pines (e.g. Swiss Pine, Macedonian Pine), which are strongly resistant to the disease.
The popular cultivar 'Vanderwolf's Pyramid', widely sold as an ornamental tree, derives from Pinus reflexa, though it is usually listed in nursery catalogs under Pinus flexilis.