Pinus monophylla
Pinaceae

Single-leaf Pinyon Pine

The Basics
(Plant Database, USFS)

Mature singleleaf pinyon is typically a short tree 6-12 m tall, with a rounded to flat-topped crown and multiple, upswept branches due to lack of self-pruning. It may occasionally be multi-stemmed from simultaneous establishment from seed caches. It generally forms open woodlands. Bark is smooth and thin on young trees, forming deep, irregular fissures and ridges with thin scales, and is up to an inch thick with age. The wood is light and soft, not strong or resinous...The seed or "nut" of singleleaf pinyon has a lower fat content than that of Colorado pinyon, giving it a more starchy or mealy taste by comparison.

The needles of singleleaf pinyon are solitary, rigid, and 2.5-3.5 cm long. The sheaths are deciduous. The single needle is unique among pines of the world. Singleleaf pinyon needles are long-lived 5-12 years. Their longevity is considered an extreme expression of "evergreenness," giving the tree the ability to conserve nutrients to take advantage of short favorable conditions within a generally unfavorable period. The allelopathic effects of singleleaf pinyon needle litter on germination and growth of herbaceous plants has been documented, and is attributed to terpene hydrocarbons. These compounds also impede decomposition, are highly flammable, and are readily volatilized by fire.

The pinyon-juniper woodland is generally a climax vegetation type throughout its range, reaching climax about 300 years after disturbance, with an ongoing trend toward increased tree density and canopy cover and a decline in understory species over time. Woodlands may also expand into adjacent grass and shrublands over time...Singleleaf pinyon has an extensive lateral root system, giving it the ability to penetrate into open areas between tree canopies and extract water and nutrients. Its ability to invade adjacent shrublands may also be related to the tree's ability to maintain a seasonally stable xylem water potential and thereby to endure drought better than the associated shrub.

Threats
(Plant Database, USFS)

Because mature singleleaf pinyon trees are short with thin bark, open crowns, full of long-lived, highly flammable foliage, and do not self-prune their dead branches, individual trees are susceptible to fire...Mature singleleaf pinyon can survive low-severity surface fires but is killed by more severe fires. Most tree seedlings are killed by fire, but cached seeds may survive...The effectiveness of tree kill depends on fire severity, which is largely determined by the amount of grass that carries the fire...Singleleaf pinyon occurs in many xeric sites with infertile, shallow, rocky soils, where fires are infrequent and unpredictable and depend on exceptional rainfall years that lead to herbaceous growth sufficient to carry fire.

Pinyon mice, deer mice, woodrats, squirrels, chipmunks, deer, black bears, and desert bighorn sheep eat singleleaf pinyon seeds, as do scrub, Steller's, and pinyon jays and Clark's nutcrackers. Many of these animals cache seeds for winter use and are critical for regeneration of singleleaf pinyon...The inner bark is a major food of porcupines, and is also eaten by squirrels, raccoons, rabbits, ringtails, coyotes, and gray foxes, as well as the larvae of the mountain pine beetle and the fungus causing pinyon blister rust. Limbs are attacked by pinyon dwarf-mistletoe (Arceuthobium divaricatum). The pitch is a staple food of pitch midges and is used by Dianthidium bees to build their nests.

Reproduction
(Plant Database, USFS)

Reproduction in singleleaf pinyon is by seed and does not occur naturally by vegetative means. It is monoecious and wind pollinated. Cone and seed development require 3 seasons and about 26 months...Singleleaf pinyon generally begins bearing cones at about 35 years of age, begins producing good seed crops at about 75 to 100 years, and reaches maximum production at about 160 to 200 years. Singleleaf pinyon exhibits region-wide synchrony in cone production, masting every 2 to 3 years.

After fertilization, seeds develop rapidly, mature, and disperse about 6 months later. With so much time between so many stages, cone and seed crops are exposed to a number of variables that affect the seed crop, such as weather, predation, and internal competition for resources between the previous and current year's cones; therefore, seed production is highly variable from tree to tree, year to year, and place to place.