In Oregon, vegetation associated with the extensive basaltic uplands of Owyhee Province is a shrub–grassland climax type, i.e., with 10% or more natural canopy cover of shrubs. On the extensive sloping stony plains where soils are very shallow, low sagebrush is the dominant shrub. On buttes and mountain slopes where soils are shallow to moderately deep, Wyoming big sagebrush is the dominant shrub. Bluebunch wheatgrass, Sandberg bluegrass, and squirreltail are prominent grasses.
Native vegetation associated with the well-drained bottoms in Cow and Jordan creek basins is a natural grassland climax type, i.e., with less than 10% canopy cover of shrubs. Originally, these bottomlands likely produced a dense, vigorous stand of basin wildrye (Fig. 35).43 Poorly drained bottomlands likely produced basin wildrye, sedges, rushes, sod bluegrasses, and other such species that tolerate alternating wet and dry periods, which would occur in this arid climate.
Figure 35: A dense, vigorous stand of basin wildrye and Nevada bluegrass on a bottomland in the vicinity of Jordan Valley in Owyhee Province, Oregon. The man standing in the grass is Bud Town, SCS Range Conservationist.
Riparian areas in Owyhee Province in Oregon are primarily along the perennial streams in the upper Owyhee River system. These streams run in very extensive basalt-walled canyons and are relatively inaccessible from contiguous uplands. Based on the USGS 1:250,000 topographic quad map, these canyon-walled perennial streams extend about 120 miles collectively in Oregon’s portion of Owyhee Province. The significance of these riparian areas to wildlife is obvious even though there is a myriad of manmade water holes throughout Owyhee Province in Oregon. Vegetation in these perennial-stream riparian areas includes a variety of species which signify different degrees of wetness.
Scattered, small, clayey semiwet meadows in areas such as Mahogany Mountains are typified by species such as Nevada and Kentucky bluegrasses, meadow sedges, timothy, redtop, meadow barley, iris, cinquefoil, and yampa. These are natural grasslands.
The 1936 Forest Type Map of Oregon54 shows no stands of western juniper or other trees in Owyhee Province in Oregon. This indicates that, if junipers were on mountain slopes or buttes in the province at that time, they likely were scattered. However, a map that covers the north portion of Owyhee Province in Oregon, shows about 2,500 acres of shrubs and trees on Mahogany Mountains southeast of Malheur Reservoir.55
The shrubs and trees indicated on this map represent groves of curlleaf mountain-mahogany growing at about 5,700 feet elevation (Fig. 36). This is an interesting ecological phenomenon. Plant species growing here and the deep, black silty soils on north exposures in this isolated mountainous terrain are comparable to other isolated high-elevation mountains such as Hart and Steens mountains in High Desert Province of Oregon.
Figure 36: Groves of curlleaf mountain-mahogany on Mahogany Mountains in the northern portion of Owyhee Province, Oregon
In Mahogany Mountains, some of the oldest mahogany plants are huge. They grow in crevices of basalt bedrock outcrops on ridgetops. One mahogany tree measured 18 inches in diameter at 10 inches above ground level and was 20 feet high with a crown 25 feet in diameter. One old stump, probably cut for firewood by old-time sheepherders, was about 22 inches in diameter at the top of the stump, 18 inches above ground level. Most of the mahogany trees were cut a long time ago, and the remaining stumps show that the original stand of mature trees, 12 to 24 inches in diameter, were spaced about 10 feet apart.32 No western juniper trees were recorded in these studies.
By 1962, it was apparent that this high-elevation, well-watered area had been severely grazed by sheep and cattle over many years.41 It is an ideal summer range in an otherwise arid area and likely attracted the greatest concentration of grazing animals, including deer. The dense stands of younger mahogany and reproduction on north-facing slopes in the area suggested that continued close use over the years resulted in the mahogany’s encroaching, or increasing, on these shrub-covered north exposures, which are snowdrift locations that typically have deep, black silty soils.
In 1962, the snowdrift areas had produced an overstory of mahogany and an interesting array of plant species in the understory.41 These included shrubs such as bitter cherry and chokecherry, mountain snowberry, snowbrush, dwarf rock spirea, low Oregon-grape, big sagebrush (the variety was not noted at that early date but likely was mountain big sagebrush), and mahogany reproduction. Distinctive grass species on these sheltered sites included pinegrass, blue and basin wildrye, big, Canby, and Wheeler bluegrasses, a dryland sedge, Columbia needlegrass, and Idaho fescue. This is an interesting shrub–grassland climax type in an isolated high-altitude location. It also proved to be a haven for rattlesnakes.
The 1962 studies of Mahogany Mountains area41 indicated that the upland slopes are a shrub–grassland climax type with big sagebrush and green and gray rabbitbrushes as the major shrubs. Predominant grasses include bluebunch and beardless wheatgrasses, Idaho fescue, Columbia and Thurber needlegrasses, Sandberg and Canby bluegrasses, oniongrass, and squirreltail. Arrowleaf balsamroot was the most prominent forb.