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 meter, rarely 25 meter tall, but on exposed tree line sites only 5-10 meter tall. It is also called Rocky Mountain White Pine. 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.