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5 Results and Discussion

Classification

Table 3 presents the relationship of the identified associations, alliances, formations, and the frequency of each association. Appendix Table A-1 summarizes the dominant species constancy and mean vegetative cover in each association. Appendix Table A-2 summarizes elevation and slope for each of the identified associations.

Formations and Physiognomic Classes

This study identified four physiognomic classes (dwarf sparse woodland, dwarf woodland, shrubland, and herbaceous) and five formations (Table 3) at DPG. The SNVCS defines physiognomic classes based on vegetative cover at the 5-meter level. This definition was adapted from the United Nations Educational, Scientific and Cultural Organization (UNESCO 1973) physiognomic class definition. A strict interpretation of these SNVCS definitions proved ineffective at DPG for identifying woodlands because their short stature would have caused them to be classified as shrublands. This problem with classifying woodlands is common in the western United States (Moir and Carleton 1987). The dichotomous key to physiognomic classes developed in the initial data reduction step attempted to address this problem by defining woodland classes at the 2-meter level.

Alliances

This study identified 17 alliances at DPG (Table 3). The SNVCS names alliances based on the dominant species in the top most stratum (The Nature Conservancy 1994b). However, most of the associations at DPG were dominated by a single species in a single stratum, resulting in many plant communities having the same alliance and association name. Therefore, alliances were named based on the genera of the dominant species. This naming convention was used by Francis and Aldon (1987) in their approach to classifying semi-arid western plant communities.

Associations

This study identified 26 associations at DPG (Table 3). Of these 26 associations, 21 (80 percent) had previously been identified by other authors.

Table 3. Hierarchical classification and frequency of DPG plant communities.

Community Descriptions

The composition and ecological relationships of the identified alliances are discussed only in those cases where more than one association was identified within the alliance. Nomenclature follows Welsh et al. (1993).

Figure 5. Needle leaved evergreen dwarf open woodland formation.

Needle Leaved Evergreen Dwarf Open Woodland Formation

Juniperus Alliance

Throughout most of the Great Basin, Utah juniper (Juniperus osteosperma) co-occurs with pinyon pine (Pinus monophylla) to form a variety of pinyon-juniper associations (West 1988). However, Utah juniper (Juniperus osteosperma), the more xeric of the two (Tueller et al. 1979), was the only tree species found within the boundaries of DPG (not including species planted in the community of Dugway, Utah, and other contonment areas). The taller mountain ranges (i.e., Cedar Mountains and Granite Mountains) located within the boundaries of DPG were blanketed with pygmy Utah juniper woodlands. However, the ameliorating effects that elevation has on climate may not be sufficient to allow the establishment of the slightly more mesic pinyon pine. Utah juniper associations also occur on the undulating sand dunes that occur on the valley floors at DPG. Figure 6 presents the relationship of the three Utah juniper associations to several recognized environmental gradients at DPG (Vest 1962).

Juniperus osteosperma - Mixed Grass Association. The Utah Juniper-Mixed Grass Association was found at elevations ranging from 1403 to 1983 m with a mean elevation of 1566 m. Slopes ranged from essentially flat to very steep, with a mean slope of 15.6 percent. Utah juniper was the single tree species in this dwarf open woodland association. Total mean cover above 2 m averaged 12.8 Figure 6. Schematic of the distribution of the identified Utah juniper associations in relation to recognized environmental gradients at Dugway Proving Ground.

percent and ranged from 2.5 to 37.5 percent. Viscid rabbitbrush (Chrysothamnus viscidiflorus) and broom snakeweed (Gutierrezia sarothrae) were common, low cover, shrub associates. There was no single species that dominated the herbaceous strata in this association. Downy brome, with a constancy of 0.8, was the most common grass associate. Indian ricegrass (Stipa hymenoides), Sandberg's bluegrass (Poa secunda), and bluebunch wheatgrass (Elymus spicata) were other common grass associates.

J. osteosperma - Elymus spicatus/Artemisia nova Association. The Utah Juniper-Bluebunch Wheatgrass/Black Sagebrush Association occurred in a relatively broad elevation band that ranged from 1418 to 2018 m. The mean elevation (1581 m) of this association was the highest in the juniper alliance. Slopes ranged from 8 to 35 percent with a mean of 24.4 percent. Floristically and physiognomically, this association was a mosaic of the Utah Juniper-Mixed Grass Association and the Black Sagebrush Association described below. The elevational distributions of the three associations overlapped at DPG (Appendix Table A-2). Based on environmental data and literature reviews, it appears that these associations may be distributed along an available soil moisture gradient. The Utah Juniper-Bluebunch Wheatgrass/Black Sagebrush Association was likely a transitional community between the Utah Juniper-Mixed Grass and Black Sagebrush Associations. Additional information on the edaphic setting of these associations might prove useful in refining the classification.

The cover of Utah juniper above 2 meters ranged from 2.5 to 15 percent with a mean cover of 4.5 percent. Bluebunch wheatgrass, with a mean cover 9.7 percent, and black sagebrush, with a mean cover 8.5 percent, codominated the low vegetative stratum. Nevada ephedra (Ephedra nevadensis) and viscid rabbitbrush were common, but low cover shrub associates. Sandberg's bluegrass and carpet phlox (Phlox hoodii) were the most common herbaceous associates.

J. osteosperma L. Association. Unlike the previous two Utah juniper associations, this association was found along a relatively narrow elevation band at DPG that ranged from 1346 to 1484 m. The mean elevation (1386 m) was also substantially lower than the other two juniper associations. Slope ranged from 0 to 10 percent with a mean of 3.5 percent. This association occurred primarily on the valley floors at DPG, as indicated by the relatively low mean elevation and slope. Vest (1962) reported that low elevation juniper associations typically were found in and among the many dune systems on the valley floors at DPG. The dune systems provide an edaphic setting that allows Utah juniper to grow below its typical elevational range at DPG.

Utah juniper dominated this association. Cover of Utah juniper ranged from 5 to 62 percent, with a mean cover of 21.5 percent. Downy brome was the most common herbaceous associate with a cover that ranged from 15 to 62 percent and a mean cover of 28 percent. Broom snakeweed and four-wing saltbush (Atriplex canescens) were common low cover shrub associates. Indian ricegrass was a common herbaceous associate.

Great Basin Arid Shrubland Formation

Artemisia Alliance

Species of sagebrush (Artemisia) are the characteristic dominant of many communities in the Intermountain West (West 1988). Küchler (1970) and West (1979) recognize two broad sagebrush vegetation types in the western United States: the sagebrush steppe and the Great Basin sagebrush. The sagebrush steppe occurs primarily in the northern Colorado Plateau, Wyoming, Idaho, Oregon, and Washington. The GB type occurs largely in western Utah, Nevada, and northern New Mexico, and is similar to the sagebrush alliance identified at DPG. Physiognomically, the GB sagebrush communities are smaller in stature than the sagebrush steppe, with shrubs rarely attaining a height in excess of 1 meter (West 1988). This assertion was consistent with data collected at DPG. Floristically, diversity is generally lower in the GB sagebrush. West (1988) attributed the physiognomic and floristic differences between the two types to the aridity of the Great Basin. Soil salinity is a critical factor affecting the distribution of sagebrush-dominated communities. Sagebrush does not tolerate high soil salinity and therefore occurs more often in upland areas and foothills throughout the Great Basin (Blaisdell et al. 1982). At DPG there were four associations that comprised the Sagebrush Alliance (Table 3). Figure 8 presents the relationship of the four identified sagebrush associations to several environmental gradients at DPG (Vest 1962).

Figure 8. Schematic of the distribution of the identified sagebrush associations
in relation to recognized environmental gradients at Dugway Proving Ground.

A. nova Association. The Black Sagebrush Association occurred in a relatively broad elevational band, ranging from 1431 to 1965 m at DPG, with a mean elevation of 1644 m. The Black Sagebrush Association was found on slopes that ranged from 0 to 29 percent and was reported to be common on soils in the Checkett-Amtoft group at DPG (USSCS 1986). Black sagebrush has been reported to be associated with shallow, droughty soils (Vest 1962; Beetle 1979; McArthur 1981), and most abundant between 1500 m and 2400 m (Blaisdell, Murray, and McArthur 1982).

As previously discussed, the Black Sagebrush Association intergraded with the Utah Juniper-Mixed Grass Association, forming the transitional Utah Juniper-Bluebunch Wheatgrass/Black Sagebrush Association. Based on literature and environmental data from DPG, it seems likely that the three associations were distributed along a gradient of available soil moisture (Figure 9). The Black Sagebrush Association occurred at the sites with the lowest available soil moisture. The Utah Juniper-Mixed Grass Association occurred at the sites with the highest available soil moisture, and the Utah Juniper-Bluebunch Wheatgrass/Black Sagebrush Association at intermediate sites. Since the Black Sagebrush Association occurred at higher elevations, it would be expected to receive slightly higher moisture, but it typically occurred on more exposed ridges, and northwestern or western exposures. These site conditions lead to shallower soils and more xeric conditions at DPG.


Figure 9. Schematic of three closely related associations distributed
along an elevational and soil moisture gradient.

The cover of black sagebrush, the exclusive dominant, ranged from 15 to 37.5 percent, with a mean cover 24.8 percent. Viscid rabbitbrush and Nevada ephedra were common (constancies of 0.7 and 0.5, respectively) though relatively low cover shrub associates. Downy brome had a mean cover of 7.3 percent and was a very common herbaceous component (constancy 0.9). Sandberg's bluegrass was also a common (constancy 0.8) herbaceous component, though its mean cover was considerably lower than downy brome. In their study of habitat type of northern Nevada, Zamora and Tueller (1973) described habitat types in northern Nevada that are very similar in floristic composition to the Black Sagebrush Association.

Artemisia tridentata Var. wyomingensis/Poa secunda Association. The Wyoming Sagebrush (A. tridentata var. wyomingensis)/Sandberg's Bluegrass Association occurred in a narrow (1455 to 1530 m) elevational band at DPG, with a mean elevation of 1508 m. Slopes ranged from 16 to 46 percent with a mean slope of 27.6 percent. McArthur (1981) reported that Wyoming sagebrush was usually associated with shallow, poor soils underlain by a caliche or silica layer. At DPG these edaphic conditions usually occur in the foothills and valley outwashes (USSCS 1986). Blaisdell, Murray, and McArthur (1982) reported that Wyoming sagebrush was associated with the most xeric sites of the big sagebrush varieties.

The shrubs Wyoming sagebrush, viscid rabbitbrush, shadscale (Atriplex confertifolia), and Nuttall's horsebrush (Tetradymia nuttalli) all had a constancy of 1.0 in this association. However, Wyoming sagebrush was considered the dominant shrub in this association because it had a mean cover of 33 percent, as opposed to Nuttall's horsebrush, viscid rabbitbrush, and shadscale whose mean covers are 5.0 percent, 1.3 percent, and 1.2 percent, respectively. Sandberg's bluegrass had a mean cover of 11 percent and was the dominant herbaceous species. Downy brome and carpet phlox were both common, low cover herbaceous associates.

Artemisia tridentata Association. The Common Sagebrush Association occurred over a wide elevational range of 1321 to 1860 m, with a mean elevation of 1503 m. Slopes ranged from 0 to 25 percent and had a mean of 4.6 percent. Common sagebrush was typically associated with deep, alluvial soils throughout the Great Basin (Beetle 1979). These sites occur primarily in the alluvial fans of the foothills and piedmont at DPG (USSCS 1986). Common sagebrush (A. tridentata) also occurs on stabilized dunes and lake terraces throughout DPG (Vest 1962).

The cover of the dominant shrub, common sagebrush, ranged from 15 to 62.5 percent (mean 25.1 percent) in this association. Viscid rabbitbrush was a somewhat common (constancy 0.7) shrub associate with relatively low cover (mean 1.1 percent). Downy brome had the highest mean cover (22.9 percent) of the herbaceous species and was the dominant herbaceous species in this association. Indian ricegrass was a somewhat common but low cover herbaceous associate.

A. tridentata/J. osteosperma Association. This association was found from 1455 to 1546 m at DPG, with a mean elevation of 1498 m. Slopes ranged from 4 to 50 percent with a mean of 19 percent. The identification of the Common Sagebrush/Utah Juniper Association was tentative; it may simply be a variant of the Common Sagebrush Association. However, Moir and Carleton (1987) identified a very similar association. Further investigation of this association is needed to confirm its existence at DPG.

The high constancy (1.0) of Utah juniper distinguishes it from the preceding association, from which it was absent. Nevertheless, Utah juniper had a mean cover of only 2.5 percent, which precluded its designation as a woodland. Common sagebrush was the dominant species (mean cover of 35 percent) in this association. Viscid rabbitbrush had a constancy of 1.0, but relatively low mean cover of 4.5 percent. Downy brome was the dominant herbaceous associate with a mean cover of 19.7 percent. Sandberg's bluegrass was a common (constancy 1.0), though low cover herbaceous associate.

Chrysothamnus Alliance

Rabbitbrush (Chrysothamnus spp) usually occurred intermixed with sagebrush and halophytic shrub associations. Many species of rabbitbrush are considered less palatable and tend to dominate sites that have been degraded through overgrazing or other destructive land use activities (Blaisdell and Holmgren 1984). This alliance was also common on many of the vegetated dunes throughout DPG.

Chrysothamnus viscidiflorus Association. The Viscid Rabbitbrush Association occurred from 1349 to 1498 m at DPG with a mean elevation of 1451 m. Slopes range from 0 to 5 percent with a mean of 1.1 percent. Vest (1962) reported that viscid rabbitbrush occupied sandy areas and foothill outwashes. This association tends to intergrade with the Common Sagebrush Association. Viscid rabbitbrush was the exclusive shrub dominant in this association with a mean cover of 15 percent. Downy brome was a common (constancy 1.0), though relatively low cover (mean 5.6 percent) herbaceous constituent. Indian ricegrass was also a common low cover herbaceous associate.

Chrysothamnus nauseosus (Pallas) Association. The Rubber Rabbitbrush Association occurred at elevations ranging from 1424 to 1502 m with a mean elevation of 1471 m. Physiographically, this association was found in essentially flat areas with slopes ranging from 0 to 7 percent. Rubber rabbitbrush (Chrysothamnus nauseosus) and dune rabbitbrush (C. nauseosus var. turbinatus) were the dominant shrubs in this association. Dune rabbitbrush is a variety of rubber rabbitbrush; nevertheless, taxonomists consider it to be a distinctive taxon (Welsh et al. 1993). The two are readily recognizable in situ and often occur as associates. Rubber rabbitbrush had a constancy of 1.0 and mean cover of 16.4 percent, whereas dune rabbitbrush had a constancy of 0.6 and a mean cover of 11.6 percent.

Gutierrezia sarothae Association. The Broom Snakeweed Association generally occurred at the lower elevations of DPG ranging from 1321 to 1497 with a mean elevation of 1375 m. The slope ranges from 0 to 25 percent with a mean of 4.7 percent. Broom snakeweed was the dominant shrub in this association, ranging in cover from 15 to 37.5 percent with a mean of 17.3 percent. Broom snakeweed responds well to disturbance and can dominate on sites that have been degraded (Welsh et al. 1993). No other shrub associates had meaningful cover or constancy in this association. Downy brome was a recurring herbaceous associate (constancy 0.9) with relatively high cover of 19 percent. Indian rice grass was also a somewhat common, albeit low cover herbaceous associate.

Eriogonum nummulare Association. The Coin Buckwheat Association occurred between 1317 and 1424 m elevation at DPG. The mean elevation was 1370 m. The slope ranges from 0 to 10 percent with a mean of 3.5 percent. This association was dominated by the subshrub coin buckwheat (Eriogonum nummulare), which had a mean cover of 11.9 percent. Coin buckwheat typically occurs on stabilized sand dunes throughout much of Western Utah and Nevada (Welsh et al. 1993). Dune scurfpea (Psoralidium lanceolatum) was a common (constancy 0.8) associate having a mean cover of 5 percent. Downy brome and indian ricegrass were also common herbaceous associates, both having constancies of 1.0 and mean covers of 10.1 percent and 1.4 percent, respectively.

Psoralidium lanceolatum Association. The Dune Scurfpea Association occurred from 1314 to 1490 m with a mean elevation of 1395 m. The slope ranged from 0 to 10 percent with a mean slope of 3 percent. Dune scurfpea was the dominant species in this association and had a mean cover of 18.1 percent. Indian ricegrass was a common (constancy 1.0) herbaceous associate, with a relatively low mean cover of 1.4 percent. Wild tarragon (Artemisia dranunculus) had a high mean cover (18.1 percent), but occurred haphazardly throughout this association (constancy 0.5).

Ephedra nevadensis Association. The Nevada Ephedra Association occurred between 1434 and 1825 m, with a mean of 1558 m. The slopes ranged from 0 to 8 percent, with a mean of 5 percent. Nevada ephedra had a mean cover of 20.6 percent and dominated this association. Common sagebrush was the only common (constancy 1.0) shrub associate. Downy brome was the dominant herbaceous associate with a mean cover of 38.8 percent. Indian ricegrass was a common (constancy 0.8) herbaceous associate with a mean cover of 1.1 percent.

Great Basin Cold Desert Chenopod Shrubland Formation

The GB Cold Desert Chenopod Shrubland Formation was comprised of four alliances and six associations dominated by phreatophytic shrubs in the Chenopodiaceae family. This formation is similar to the saltbush-greasewood type described by West (1988). Salt desert scrub is a common name often used to describe Chenopod-dominated communities in the Great Basin. As the common name implies, these communities often occur on soils that are slightly to very saline. However, Chenopod-dominated communities also occur on non-saline soils where climatic conditions are too xeric to support sagebrush communities (Holmgren 1983). The distribution of Chenopod communities has been related to tolerance to flooding and poor soil aeration, water table depth, and soil texture (Roundy, Evans, and Young 1983). Therefore, the presence of a Chenopod-dominated community at a site does not necessarily imply saline soil conditions. Many valley bottoms within the GB are a mosaic of Chenopod-dominated associations. The associations are typically dominated by a single shrub species and are segregated based on edaphic conditions (Figure 11).


Figure 11. Schematic of the distribution of the Chenopod-dominated
associations identified at Dugway Proving Ground in relation to soil
salinity and depth to water table.

Atriplex Alliance

Three associations comprised the Atriplex alliance at DPG.

Atriplex confertifolia Association. The Shadscale Association occurred at DPG from 1342 to 1503 m. The mean elevation (1438 m) was the highest of the three identified Atriplex associations. Blaisdell and Holmgren (1984) report shadscale communities typically occurring in valley bottoms on deep, well drained soils of intermediate salt content. The slope ranged from 0 to 25 percent with a mean of 2.1 percent. Shadscale was the dominant shrub in this association. Vegetative cover of shadscale ranged from 15 to 62.5 percent, with a mean cover of 28.3 percent. Downy brome was the dominant herbaceous associate with a mean cover of 27.5 percent. Squirrel tail (Elymus elymoides) was a common (constancy 0.9) herbaceous associate with a mean cover 1.2 percent.

Vest (1962) reported that shadscale was dominant in three ecological communities at DPG: shadscale-budsage, shadscale-gray molly (Kochia americana)-greasewood (Sarcobatus vermiculatus), and shadscale-gray molly. This study identified three similar associations that were dominated by a single Chenopod shrub. Shadscale was dominant only in this association. The other two, the gray molly and greasewood associations, are discussed below.

Atriplex canescens Association. The Fourwing Saltbush Association occurred at elevations ranging from 1320 to 1483 m at DPG. The mean elevation was 1418 m. Slope ranged from 0 to 4 percent with a mean of 1 percent. Fourwing saltbush is often associated with sandy soils, but does occur on soils with high clay and silt content (USSCS 1986). Within the Great Basin, this association often occurs in a mosaic with the shadscale and greasewood associations (Blaisdell and Holmgren 1984). Within these mosaics, ecotones between each association were mixtures of several species with no single dominant. These broad zones were often classified in the mixed shrub association described below.

Fourwing saltbush was the dominant shrub in this association with a mean cover of 13.2 percent. Downy brome was the dominant herbaceous associate with a mean cover of 21.2 percent. Pale evening primrose (Oenothera pallida) and indian ricegrass were common (constancy 0.7) herbaceous associates, with mean covers of 3.4 percent and 3.1 percent, respectively.

Atriplex gardneri Association. The Gardner's Saltbush Association occurred at DPG on the level plains in the valley bottoms at low elevations (1415 m). This association was dominated by Gardner's saltbush (Atriplex gardneri) which had a mean cover of 15.0 percent. Shadscale and gray molly were common, low cover shrub associates. Herbaceous associates included: downy brome, Sandberg's bluegrass, and bur buttercup (Ranunculus testiculatus), though each had cover of less than 2 percent.

The Gardner's Saltbush Association was found at only two sites at DPG. Bourgeron and Engleking (1992) have identified an A. gardneri series and its composition has been described by Blaisdell and Holmgren (1984). However, further investigation is needed to understand the distribution of the Gardner's Saltbush Association at DPG.

Kochia americana Association. The Gray Molly Association was distributed in two distinct elevational bands at DPG. The high elevation band occurred from 1966 to 2036 m and the low elevation band from 1322 to 1574 m. Slope was minimal to nonexistent in this association. Gray molly has been reported to be associated with fine texture soils with little or no gravel (Blaisdell and Holmgren 1984). Gray molly was the single dominant in this association and had a mean cover of 18.0 percent. Greasewood and shadscale were common shrub associates with mean covers of 3.5 percent and 1.3 percent, respectively. Downy brome was the only common herbaceous associate.

Sarcobatus vermiculatus Association. The Greasewood Association occurred in two distinct elevational zones at DPG. The low elevational zone ranges from 1307 to 1473 m (mean of 1405 m); the high elevation zone ranges from 1571 to 2078 m (mean of 1926 m). Slope ranged from 0 to 4 percent in the low elevation zone. There was no slope on the high elevation sites. The greasewood community that Vest (1962) identified at DPG was found on deep soils made of lake deposits. Greasewood had a mean cover of 17.8 percent and was the single dominant shrub in this association. Downy brome was a common herbaceous associate with a mean cover of 19.4 percent.

Allenrolfea occidentalis Association. This association occurred on the flat valley floors at the lowest elevations (mean 1313 m) and therefore under the most saline soil conditions at DPG. Iodine bush (Allenrolfea occidentalis) had a mean cover of 5.3 percent and was the dominant species in this sparsely vegetated association.

Great Basin Mixed Shrubland Formation

Many of DPG's shrub communities were mixtures of species with no particular species being dominant. Vest (1962) described a similar mixed brush community at DPG that had no clear dominant. These mixed shrub communities were placed in the Great Basin Mixed Shrubland Formation. In many cases, these communities were transition zones or ecotones between associations previously described. The Great Basin Mixed Shrubland Formation occurred on a wide range of elevations (1304 to 1793 m), though more often at the low to mid elevations. Mean elevation was 1434 m and slopes ranged from 0 to 45 percent with a mean of 3.2 percent. Broom snakeweed and viscid rabbitbrush were the most common shrubs; both had a constancy of 0.6. Other common shrubs were: shadscale (constancy 0.5), fourwing saltbush (constancy 0.4), and gray molly (constancy 0.4). Vegetative cover of all shrubs was low. Downy brome was the most common herbaceous species (constancy 0.9) followed by indian ricegrass (constancy 0.7). As with the shrubs, cover of any one herbaceous species was generally low.

Great Basin Cold Desert Grassland Formation

Poa secunda Association. This association was found on only two sites at DPG: a valley bottom (1356 m, 0 percent slope) and a mid elevation (1510 m, 61 percent slope) site. As a result, this association requires additional investigation to confirm its existence and distribution at DPG. Sandberg's bluegrass, with a constancy of 1.0 and mean cover of 15.0 percent, was the dominant species in this grassland association. Shadscale was a common (constancy 1.0), though low cover (mean 1.8 percent) shrub. Downy brome was the most common herbaceous associate.

Elymus elymoides Association. This grassland association ranged in elevation from 1388 to 1498 m at DPG. Slopes ranged from 0 to 6 percent with a mean of 2.8 percent. This sparsely vegetated association is dominated by squirrel tail and downy brome. Overall vegetative cover was low compared to other associations in the Great Basin Cold Desert Grassland Formation. Shrub cover was sparse to nonexistent. Squirrel tail had a mean cover of 2.5 percent, whereas downy brome had a mean cover of 2.1 percent. Indian ricegrass was the only moderately common (constancy 0.5) herbaceous associate.

Elymus spicatus Association. The Bluebunch Wheatgrass Association was the most common native grassland association encountered in this study at DPG. It ranged in elevation from 1482 to 1679 m (mean of 1556 m), and occurred on moderate to steep slopes ranging from 15 to 34 percent (mean 27.8 percent). Bluebunch wheatgrass dominated this association with a constancy of 1.0 and a mean cover of 15.0 percent. However, there was a significant shrub component comprised of three shrubs: shadscale (constancy 0.8; mean cover 0.8 percent), viscid rabbitbrush (constancy 0.8; mean cover 0.9 percent), and black sagebrush (constancy 0.8; mean cover 12.5 percent).

The Bluebunch Wheatgrass Association closely resembled the Black Sagebrush Association described previously. Its relatively high cover and herbaceous dominance of bluebunch wheatgrass was the key feature that differentiated it from the Black Sagebrush Association. Further investigation may result in the identification and classification of an additional association in the Artemisia alliance, the Black Sagebrush/Bluebunch Wheatgrass Association.

Hilaria jamesii Association. This grassland association was dominated by the warm season grass H. jamesii (galleta), which had a constancy of 1.0 and a mean cover of 26.3 percent. Downy brome was the most common herbaceous associate (constancy 1.0) with a mean cover of 19.3 percent. Vest (1962) stated that galleta was common in the foothills and lower mountain slopes at DPG. Data from this study indicated that galleta was common in lower elevation sites. The Galleta Grassland Association was encountered at only two sites. Elevation was 1310 m and both sites were on flat valley bottoms. Further investigation into the distribution of galleta grasslands would benefit the DPG classification.

Stipa hymenoides Association. This grassland association was dominated by indian ricegrass, which had a mean cover of 15.0 percent. Other common associates, all with a constancy of 1.0, were Sandberg's bluegrass, squirrel tail, and downy brome. The Indian Ricegrass Grassland Association was encountered at only two sites; therefore, it is in need of further investigation to confirm its existence and distribution at DPG. The mean elevation and slope were 1545 m and 25.5 percent, respectively.

Bromus tectorum Association. This association ranged in elevation from 1306 to 1866 m (mean 1423 m) the largest continuous elevational band of all associations identified at DPG. Slopes ranged from 0 to 40 percent (mean 3.7 percent), which was also the largest range of any associations at DPG. The Downy Brome Association was encountered at more sites than any other association at DPG. Cover ranged from 15.0 to 97.5 percent, depending on the site. Downy brome occurs in virtually all associations at DPG. The sites classified as Downy Brome Associations had virtually no other species with cover more than 1.0 percent.

Downy brome, also known as cheatgrass, was believed to have been introduced into the United States around 1870. Downy brome is an annual invasive, non-native grass that can in many instances replace native vegetation. Downy brome now dominates vast areas of rangeland in the Intermountain West by outcompeting native grasses. Downy brome usually germinates in the fall and completes its life cycle in late June or early July (West 1988). Furthermore, downy brome increases the fuel load, which results in an increase in the fire frequency. The increased fire frequency kills many of the native shrubs and herbs, thereby increasing the available habitat for downy brome. This cycle repeats itself until many sites are almost exclusively dominated by downy brome. The large tracts of land that are exclusively downy brome at DPG were likely formed in this manner.

Sporobolus airoides Association. This grassland association was dominated by S. airoides (alkali saccoton) which had a mean cover of 37.5 percent. As its common name implies, alkali saccoton is most often found at DPG and elsewhere, in saline meadows, alkaline sand dunes, and alkali flats where it sometimes forms monospecific communities (Blaisdell and Holmgren 1984; USSCS 1986). Bourgeron and Engelking (1992) recognize a S. airoides series in their Western United States Classification System. Vest (1962) reported that alkali saccoton was most abundant in alkaline sandy loams adjacent to dunes at DPG. The Alkali Saccoton Grassland Association was encountered at only two sites. More information on its distribution would improve the DPG classification. Neither elevation nor slope data was available from either site.

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