Pinyon–juniper woodland



Pinyon–juniper woodland, also spelled piñon–juniper woodland, is a biome found mid-elevations in arid regions of the Western United States, characterized by being an open forest dominated by low, bushy, evergreen junipers, pinyon pines, and their associates. At lower elevations, junipers often predominate and trees are spaced widely, bordering on and mingling with grassland or shrubland, but as elevation increases, pinyon pines become common and trees grow closer, forming denser canopies. Historically, pinyon-juniper woodland has provided a vital source of fuel and food (particularly piñon nuts) for peoples of the American Southwest.

Both pinyon pine and juniper species reproduce exclusively through seed production, and produce most of their seeds during mast years, which occur about every three to five years. Birds and small mammals in the ecosystem are the main distribution method for these seeds, and the woodlands support a number of native species like the Pinyon mouse and the Pinyon jay. These animals often at least partially subsist on their seeds, spreading them by caching them in underground holes.

Range
Today, pinyon–juniper woodland range spans from New Mexico, to the eastern Sierra Nevada, the Colorado Plateau, the Great Basin, and higher elevations of Mountain ranges of the Mojave Desert. The woodland's range includes the Mogollon Rim in the south, to its northern extent in the Snake River Plain. It typically occurs at between 4,500 and 7,500 feet. Historically, however, the range and elevations of pinyon-juniper woodland have shifted based on differences in climate. On the order of 10,000 years ago during the Wisconsin glaciation, pinyon-juniper woodlands occurred in areas that today are the Chihuahuan, Mojave, and Sonoran desert lowlands, and since then pinyon-juniper altitude ranges have continued to change based on changing moisture and temperature ranges over time.

In the last 200 years specifically, the area occupied by pinyon-juniper woodlands has increased by two to six times. This is attributed to a number of factors, but especially the direct and indirect effects of climate, overgrazing and altered fire regimes. Specifically, the wet period between the 1800s and 1900s boosted tree establishment, and livestock grazing both eliminated perennial grass cover that would hinder tree establishment, and removed fine fuels that could start large fires.

Arizona and New Mexico
The pinyon–juniper woodland is one of the most prevalent types of coniferous woodland in northern Arizona and New Mexico. In Arizona the great basin woodland species are Juniperus arizonica, Juniperus californica in western Arizona, Juniperus deppeana, Juniperus monosperma, Juniperus osteosperma, Juniperus scopulorum at higher elevations, Pinus monophylla var. fallax below the Colorado plateau, and Pinus edulis on the Colorado plateau. In Arizona the great basin woodland includes many species of oaks: Quercus turbinella, Quercus gambelii at higher elevations, Quercus grisea, Quercus arizonica, and Quercus emoryi. In Southern Arizona, Pinus discolor, Juniperus deppeana, and Pinus leiophylla make up the conifer woodland with many oak species.

Utah and Canyonlands region
The pinyon–juniper plant community covers a large portion of Utah and the Canyonlands region. Singleleaf ash (Fraxinus anomala), and Utah serviceberry (Amelanchier utahensis) are codominants of pinyon pine and Utah juniper. In this region, the community occurs on rocky soils or jointed bedrock.

Mojave Desert
In the steppes adjoining the Mojave Desert, this vegetation type can be found in areas receiving 12–20 inches of annual precipitation, and between 4,500 and 8,000 feet. Associates include bitterbrush (Purshia glandulosa), Apache plume(Fallugia paradoxa), desert sagebrush (Artemisia tridentata), green ephedra (Ephedra viridis), mountain mahoganies (Cercocarpus spp.), and buckwheats (Eriogonum spp.). In the Mojave, pinyon–juniper woodlands are generally above the Joshua Tree woodlands vegetation type, and requires more annual precipitation.

Sierra Nevada
In the eastern Sierra Nevada, the elevation range is 4,000-5,500 feet in the north, and 5,000 to 8,000 feet in the southern reaches of the range. Pinyon–juniper woodland requires 12–20 inches of annual precipitation, so is generally located above the sagebrush scrub vegetation type, which can survive on an average of 7 inches per year. It is located below the alpine zone. There is often an understory dominated by sagebrush (Artemisia tridentata) and its associates. Co-dominants include Jeffrey Pine (Pinus jeffreyi) and an understory of sagebrush scrub or rabbitbrush scrub (Ericameria spp.).

Subtypes
Pinyon–juniper woodland has three major subtypes based on vegetation: pinyon-juniper savannah, pinyon-juniper wooded shrubland, and pinyon-juniper persistent woodland.
 * Pinyon-juniper savannahs are dominated by grasses and forbs, with dense undergrowth and savannah-like sparser trees. Monsoon summer rains are common in pinyon-juniper savannahs, since they favor the growth of warm-season grasses. Common grass species are blue grama, other species of grama grass, new mexico muhly, curlyleaf muhly, and needle and thread grass.
 * Pinyon-juniper persistent woodlands are dominated by trees, with a variety of tree ages, often in areas with winter or bi-modal precipitation. Persistent woodland sites are usually unproductive, with thin soils and sparse ground or shrub cover, often occurring on rocky or rugged terrain.
 * Pinyon-juniper wooded shrublands are transition zones where pinyon-juniper woodland is expanding or contracting, often bordering other vegetation types. In absence of fire, they shift from grass and forb-dominated, to shrub-dominated, to tree-dominated communities over time. Common shrub species are big sagebrush, other species in that genus, antelope bitterbrush, rabbitbrush, mountain mahogany, and snakeweed.