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By Mark E. Miller, National Park Service
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Area actively grazed until 1975.
Latitude: 38° 09.700' NCLIM-MET Site #3 at the Needles District housing area is located near the northeastern end of a southwest-to-northeast trending topographic basin. The basin measures approximately 1500 m in length and slopes gently toward Salt Creek which flows northward at the northeastern end of the basin about 400 m from the CLIM-MET station. Width of the basin between the parallel-trending Cedar Mesa outcrops varies from 100 to 200 m. Alignment of the basin with the predominant wind direction from the southwest likely has implications for the comparison of wind, sedimentation, and temperature data between Site #3 and Site #2 in Virginia Park. Elevation of the site is approximately 1497 m.
The basin where CLIM-MET Site #3 is located, as well as most other readily accessible grassland basins and pockets in the Needles District, has a long disturbance history of grazing by domestic livestock. Cattle first were introduced to the area during the 1880s, and the Needles area received very high livestock usage during the 1880s and 1890s (Loope, 1972). The nearby Abajo Mountains (Blue Mountains) provided good summer range for livestock, whereas the lower elevation grasslands in and near the Needles were used primarily during winter. Proximity to Salt Creek and the cowboy encampment at Cave Spring suggest that the basin where CLIM-MET Site #3 is located was used as intensely as any grassland in the area. After the 1890s, stocking rates were lower but generally constant until grazing was eliminated in the District in 1975 (Loope, 1972).
Figure 1. Cheatgrass (Bromus tectorum)
Figure 2. Hilaria jamseii (galleta)
Current conditions at the site and similar areas in the District reflect this history of livestock use. The extant plant community at the site is dominated by the exotic (nonnative) grass Bromus tectorum, an annual species native to Eurasia and known commonly as "cheatgrass" (fig. 1). Cheatgrass accounts for greater than 75 percent of the plant cover at this site with densities estimated to range from 1000 to 1500 plants per m2. The perennial native grasses Stipa hymenoides (Indian ricegrass), Hilaria jamesii (galleta) (fig. 2), and Sporobolus cryptandrus (sand dropseed) each are represented only by a few scattered plants. In contrast with live individuals, dead clumps of native bunchgrasses (fig. 3) can commonly be found extending 1-2 cm above the soil surface at the site. Two annual broad-leafed plants, Lappula occidentalis (western stickseed) and Cryptantha crassisepala (plains cryptanth), are the most common native plants on the site. Together, these account for about 20 percent of the total plant cover. Each of these species is considered to be a native "weed" that tends to increase with disturbance. Other plants occurring in small numbers at the site include the native shrubs Atriplex canescens (four-wing saltbush) and Ceratoides lanata (winterfat), the exotic annual mustard Sisymbrium altissimum (tumble mustard), and Opuntia sp. (prickly pear). The cryptobiotic soil crust (Harper and Marble 1988; Belnap and Gardner, 1993) is poorly developed and dominated by cyanobacteria and moss spp.; soil lichens are not present. Numerous networks of rodent burrows occur on the site, representing another form of soil disturbance. The burrow systems probably are attributable to Dipodomys ordii (Ord's kangaroo rat), a nocturnal seed-eating rodent known to include cheatgrass among the suite of seeds it collects and caches (Armstrong, 1982).
Figure 3. Native bunchgrasses
Livestock grazing would have favored establishment and performance of cheatgrass over native species in several ways. Trampling by livestock can destroy or badly damage cryptobiotic soil crusts, thereby leading to soil destabilization and the burial and subsequent improved establishment success of cheatgrass seeds. In addition, cheatgrass is much more tolerant of repeated grazing and trampling than most perennial bunchgrasses native to the Colorado Plateau and Intermountain West (Mack 1986, 1989). Thus although cheatgrass is able to invade pristine (ungrazed) native grasslands under some circumstances (Young and Evans, 1985; Belnap unpub. data), repeated grazing enhances the ability of cheatgrass to invade communities and displace native species. Once cheatgrass is established and the vigor of adult native grasses is reduced by grazing and/or other environmental stressors, cheatgrass can maintain site dominance by easily outcompeting native seedlings.
Although there is no known documentation of when cheatgrass first became established in the Needles area, it likely has been dominant in heavily disturbed grasslands for over 60 years. One of the first botanical collections of cheatgrass in North America was made in Provo, Utah, in 1894 (Mack 1981; Welsh and others, 1993). Mack (1981) estimated that cheatgrass had achieved dominance throughout most of its current range by 1930. By 1941, the naturalist Aldo Leopold (1941) noted that westerners simply woke up one day to find that their rangelands had been taken over by a new weed.
Figure 4. Cheat dominated basin
At the margins of the cheat-dominated basin (fig. 4) where CLIM-MET Site #3 is located, there is a shift in plant community composition associated with changes in soil depth and greater availability of water due to runoff from rock outcrops. Within 10-15 m of the rocks, native shrubs such as Atriplex canescens, Artemisia tridentata (big sagebrush), and Sarcobatus vermiculatus (greasewood) dominate the vegetation. Also abundant in these shrublands is the exotic annual Salsola iberica (Russian thistle, or tumbleweed). Although tumbleweed also co-occurs with cheatgrass at the center of the basin, it currently is uncommon there due to an exceptionally dry spring. Its relative abundance at the basin margins (especially along the northwestern side) probably is attributable to the species' wind-dispersed nature and the fact that the sandstone outcrops act as tumbleweed catchments. Thin, lithic soils of sandstone outcrops are dominated by Juniperus osteosperma (Utah juniper), Pinus edulis (Colorado piñon), and several shrub species.
Armstrong, D.M., 1982, Mammals of the canyon country: A handbook of mammals of Canyonlands National Park and vicinity: [Moab, Utah], Canyonlands Natural History Association.
Belnap, J., and Gardner, J.S., 1993, Soil microstructure in soils of the Colorado Plateau: the role of the cyanobacterium Microcoleus vaginatus: Great Basin Naturalist v. 53, no. 1, p. 40-47.
Harper, K.T., and Marble, J.R., 1988, A role for nonvascular plants in management of arid and semiarid rangelands in Tueller, P.T., ed., Vegetation science applications for rangeland analysis and management: Dordrecht, Kluwer Academic Publishers, p. 135-169.
Leopold, A.S., 1941, Wilderness as a land laboratory, Living Wilderness, v. 6, no. 3.
Loope, W.L., and Gifford, G.F., 1972, Influence of a soil microfloral crust on select properties of soils under pinyon-juniper in southeastern Utah: Journal of Soil and Water Conservation, v. 27, no. 4, p. 164-167.
Mack, R.N., 1981, Invasion of Bromus tectorum L. into western North America: an ecological chronicle: Agro-Ecosystems, v. 7, no. 2, p. 145-165.
Mack, R.N., 1986, Alien plant invasion into the Intermountain West: a case history in Mooney, H.A., and Drake, J., eds., Ecology of biological invasions of North America and Hawaii: New York, Springer-Verlag, 321 p.
Mack, R.N., 1989, Temperate grasslands vulnerable to plant invasions: Characteristics and consequences in Drake J.A., Mooney H.A., di Castri, F., Groves, R.H., Kruger, F.J., Rejmanek, M., Williamson, M., eds., Biological Invasions: A Global Perspective: Chichester, John Wiley & Sons, p. 155-179.
Welsh, S.L., Atwood, N.D, Goodrich, S., and Higgins, L.C., eds., 1993, A Utah flora, 2d ed.: Provo, Utah, Brigham Young University, 986 p.
Young, J.A. and Evans, R.D., 1985, Demography of Bromus tectorum in Artemisia communities in White, J., ed., The population structure of vegetation: Dordrecht, Dr. W. Junk Publishers, p. 489-502.
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