The frequency of two-needled fascicles increases following wet years and decreases following dry years. The internal anatomy of both these needle types are identical except for the number of needles in each fascicle suggesting that Little's 1968 designation of this tree as a variety of Pinus edulis is more likely than its subsequent designation as a subspecies of Pinus monophylla based entirely upon its single needle fascicle. It is an aromatic species. Essential oil can be extracted from the trunk, limbs, needles, and seed cones. Prominent aromatic compounds from each portion of the tree include α-pinene, sabinene, β-pinene, δ-3-carene, β-phellandrene, ethyl octanoate, longifolene, and germacrene D. Ecology The seeds are dispersed by the pinyon jay, which plucks them out of the open cones. The jay, which uses the seeds as a food resource, stores many of the seeds for later use, and some of these stored seeds are not used and are able to grow into new trees. The seeds are also eaten by wild turkey, Montezuma quail, and various mammals. History Colorado pinyon was described by George Engelmann in 1848 from collections made near Santa Fe, New Mexico on Alexander William Doniphan's expedition to northern Mexico in 1846. It is most closely related to the single-leaf pinyon, which hybridises with it occasionally where their ranges meet in western Arizona and Utah. It is also closely related to the Texas pinyon, but is separated from it by a gap of about so does not hybridise with it. An isolated population of trees in the New York Mountains of southeast California, previously thought to be Colorado pinyons, have recently been shown to be a two-needled variant of single-leaf pinyon from chemical and genetic evidence. Occasional two-needled pinyons in northern Baja California, Mexico have sometimes been referred to Colorado pinyon in the past, but are now known to be hybrids between single-leaf pinyon and Parry pinyon. Uses The edible seeds, pine nuts, are extensively collected throughout its range; in many areas, the seed harvest rights are owned by Native American tribes, for whom the species is of immense cultural and economic importance. They can be stored for a year when unshelled. Archaeologist Harold S. Gladwin described pit-houses constructed by southwestern Native Americans 400–900 CE; these were fortified with posts made from Pinyon trunks and coated with mud. Colorado pinyon is also occasionally planted as an ornamental tree and sometimes used as a