Semi-arid Grasslands and Shrublands
Below 6,000 feet in the cool-temperate region of southern Utah, northern Arizona, and northwestern New Mexico several different types of native grasslands were once common. The two most dominant were a Great Basin grassland, typical of the more western, central, and northern regions, and a Plains grassland, which is confined to the southeastern part of the region. Plains grasslands are commonly dominated by Blue Grama or other gramas that extend into the area from southern Colorado and northwestern Texas. Great Basin grasslands are dominated by Galleta Grass and Indian Rice Grass and reach down to the Colorado Plateau from the northwest. Both types intergrade downslope with semi-arid scrub communities and upslope with pinyon-juniper woodlands.
A large transitional area between the two types occurs in north-central Arizona and extreme southern Utah. Most of the cold-tolerant, cool-season bunch grasses that are native to these grasslands are most productive during spring and early summer, and once existed in a mosaic with deep-rooted shrubs. Until the late 1800s and the coming of the railroad and the cattle industry, the only large animals to graze these lands were pronghorn antelope. It took only 10-15 years of overgrazing by cattle near the end of the last century to extensively alter these ecosystems. The native bunchgrasses, not generally tolerant of grazing, sustained high mortality when grazed heavily in spring. Wildfires, once common in these grasslands, are far less frequent today as grazing has left less residual grass to carry fires and land management agencies maintain fire suppression policies. Both grazing and fire suppression favored shrub species over grasses and accelerated soil erosion. Site conditions have been permanently altered, and Eurasian annual grass species such as cheatgrass have aggressively colonized vast areas.
No visitor to these cold-temperate regions can fail to be impressed by the omnipresence of big sagebrush, a hardy, cold-tolerant shrub that shapes the ecosystems it dominates. Its expansion on the Colorado Plateau has been remarkable. Though big sagebrush tends to be widely spaced with herbaceous plants and grasses living beneath them, the intershrub spaces are barren or contain microphytic crusts composed of lichens and algae. The shrubs concentrate water and nutrients to form islands of fertility that are not easily altered. The current mosaic of shrublands dominated by big sagebrush, and grasslands dominated by cheatgrass, is in large part a reflection of continuing desertification of the Colorado Plateau. The pre-settlement mosaic of cool-season bunch grasses and deep-rooted shrubs may now be one of the rarest ecosystems in the Southwest.
Grazing continues to be widespread in these grasslands, and the colonization by cheatgrass and expansion of big sagebrush at the expense of native perennial grasses is expected to continue. Extensive amounts of land are also being converted to agricultural production, especially in the Four Corners area where Arizona, Colorado, New Mexico and Utah meet. Once these ecosystems are converted, there is only limited potential for conversion to native grasslands, either mechanically or by removal of livestock.
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