© Mike Norton (Colorado)
Pinyon-Juniper Habitat
In Colorado, pinyon-juniper ecological systems may occur as shrublands or woodlands. These systems are typically found at lower elevations (ranging from 4,900 – 8,000 ft) on dry mountains and foothills of the Colorado Plateau region on the Western Slope. Pinyon-juniper also occurs in a limited distribution on the eastern plains near the Purgatoire River. This system is usually found on warm, dry sites on mountain slopes, mesas, plateaus, and ridges. Pinyon-juniper stands vary considerably in appearance and composition, depending on altitude and geographic location. These stands may be solely dominated by the pinyon pine, or they may be co-dominated by species of juniper. Juniper tends to be more abundant at the lower elevations, while pinyon pine tends to be more abundant at the higher elevations, and the two species share dominance within a broad middle-elevation zone. These woodlands often occur in a mosaic with other systems, including sagebrush shrublands, Gambel oak shrublands and semi-desert shrublands. The understory ranges from a relatively rich mixture of evergreen and/or deciduous shrubs, to a sparse to moderately dense herbaceous layer dominated by perennial grasses (with or without shrubs), to no vegetation at all. Severe climatic events occurring during the growing season, such as frosts and drought, are thought to limit the distribution of pinyon-juniper systems to the relatively narrow altitudinal belts that they occupy.
Pinyon-juniper woodlands are influenced by climate, grazing, fires, tree harvest, and insect-pathogen outbreaks. From the late 1800s to the present, distribution and density of pinyon and juniper and the accompanying native understory has been significantly altered. Pinyon-juniper, especially juniper, have encroached on adjacent shrublands and grasslands, thus changing the habitats available to wildlife, as well as the forage available to domestic cattle. In the past, fire and competition for resources from perennial grasses and other understory plants kept the spread and thickening of the pinyon-juniper woodland in check. We now see various densities of younger trees occurring on sites that were once shrublands or grasslands.
The effect of a fire on a stand is variable, depending on tree height and density, amount of fuel load on the ground, weather conditions, and season. Large trees generally survive unless the fire gets into the crown due to heavy fuel loads in the understory. In this system, fire acts to open stands, increase diversity and productivity in understory species, and create a mosaic of stands of different sizes and ages across the landscape while maintaining the boundary between woodlands and adjacent shrubs or grasslands. Altered fire regimes, overgrazing, and tree cutting can all affect stand quality and fire behavior. These factors can also lead to increased soil erosion and habitat/species loss.
Rarity in the Pinyon-Juniper System
Pinyon-juniper is the third most important habitat in the state for Colorado’s rarest plants, with 16 of Colorado’s rarest plants found in this ecosystem type. This is also an important habitat for birds, including the Gray Vireo, one of Colorado's rare birds. North American Breeding Bird Survey data for 1966 - 1996 show significant rangewide declines in Gray Vireo populations. Threats and reasons for decline are unknown, but loss and degradation of habitat in pinyon-juniper woodlands is a potential factor.
Conservation
The pinyon-juniper ecological systems are declining in both extent and quality compared to historic norms. Threats include urban development, recreation (especially motorized recreation), invasive species (most notably an increase in cheatgrass in the understory, which has led to increasing fire ignitions), and energy development. To access Colorado's Biodiversity Scorecard click here. To access Colorado's State Wildlife Action Plan click here.
Additional Resources
Colorado Natural Heritage Program ecological system descriptions
Click here for Colorado Plateau Pinyon-Juniper Woodland
Click here for Southern Rocky Mountain Pinyon-Juniper Woodland
NatureServe Explorer ecological system profiles
Colorado State Forest Service
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