Pinyon-pine and juniper woodlands are widespread on the Colorado Plateau between about 5000 feet to 7000 feet in elevation. 11.4% of New Mexico, or about 3.6 million hectares, consists of pinyon-juniper habitat. In Arizona there are over 5 million hectares of "P-J," as this "elfin" or "pygmy" woodland is often called.
While the U.S. Forest Service distinguishes 32 pinyon and 23 juniper plant communities, Colorado pinyon pine (Pinus edulis) is the most common pine species in this woodland type, and Utah juniper (Juniperus osteosperma) is the most common juniper. One-seed (J. monosperma), Rocky Mountain (J. scopulorum), and alligator (J. deppeana) junipers can be abundant in different areas of the Plateau.
Annual precipitation is typically from 10 to about 15 inches in pinyon-juniper woodlands, and tree species in these communities have evolved both drought and cold resistance. Pinyons dominate at higher elevations, and tend to form more closed-canopied stands that exhibit forestlike dynamics and species composition, commonly including a significant shrub component of oaks and alderleaf, mountain mahogany and limited grasses. Juniper tends to grow at lower elevations and in more arid areas as its scaled foliage allows it to conserve water more effectively than pinyon pine. Juniper-dominated woodlands tend to include open savannas of scattered trees without a significant shrub component, except in areas where big sagebrush has become dominant as a consequence of overgrazing. There are relatively few vertebrates endemic to pinyon-juniper woodlands, but there are significant levels of biodiversity in less prominent organisms such as herbaceous vegetation and soil organisms.
The long history of livestock grazing in many pinyon-juniper woodlands on the Colorado Plateau has both diminished and altered herbaceous vegetation, leading to widespread desertification of understory conditions. Although there is little firm documentation of woodland fire histories, many researchers believe that year-round grazing since the late 1800s suppressed former fire regimes; surface fires could no longer spread though the bare interspaces between the trees. Accelerated precipitation runoff and soil erosion commonly occur in these areas, leading to significant, permanent losses of site productivity and erosive watershed conditions. Major vegetative changes include decreases in cool-season grasses, and increases in grazing-resistant plants such as snakeweed and big sagebrush.
Woodland communities have expanded considerably over the course of this century in many parts of the Colorado Plateau. Tree densities have increased, and junipers and pinyon pines have expanded upslope into ponderosa pine forests and downslope into grass and shrub communities. Densities have increased in some areas to the point that larger proportions of pinyon-juniper woodland can now support crown fires. For a discussion of the causes of this type of woodland expansion, see the research essay, Where Have All the Grasslands Gone?
"Chaining," or mechanical removal of woodlands by land management agencies, has been commonly used in an effort to convert woodlands to grasslands for livestock. For example, about 600,000 hectares of pinyon-juniper woodlands were mechanically treated in Arizona during the 1950s and early 1960s, as were about 223,000 hectares of woodland on U.S. National Forests alone between 1950 and 1985. This process, though effective in uprooting and killing the trees, is damaging to other community species and may be less beneficial than other management strategies. Its use has been greatly reduced since the 1970s. Harvests for fuelwood also peaked in the 1970s, when harvest levels proved unsustainable in some areas. Current management policy by the U.S. Forest Service focuses more attention on active management of woodlands to restore less erosive watershed conditions.
Where have all the grasslands gone? Numerous ecological studies across the Southwest have documented the decline in herbaceous vegetation (grasses and non-woody flowering plants) while forests thicken and brush invades. Documenting the changes in the Jemez Mountains of northern New Mexico, ecologist Craig Allen considers the evidence that these patterns are tied to changes in land use history, primarily livestock grazing and fire suppression.
Packrat Midden Research in the Grand Canyon. On the Colorado Plateau the ice age (Pleistocene) vegetation of the Grand Canyon has been determined through the analysis of plant fossils preserved in caves and fossil packrat middens. Large changes occurred as the most recent ice age ended and the Holocene era began. Adapted by Kenneth L. Cole from his journal article.
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