Characteristics That Affect Management
Rangeland is fragile by nature because of limitations in climate, topography, and soil characteristics. These limitations alone or in combination can make an area unsuitable or less suitable for particular management practices. Important limitations are given in the section “Detailed Soil Map Units.” Certain characteristics that can affect management are described in this section.
Aspect is the direction in which a slope faces. In a given precipitation zone, the north-facing slopes are cooler, have deeper soils, and are more productive than the south-facing slopes. Depending on the elevation, north-facing slopes generally are well suited to grazing by livestock and wildlife late in spring and in summer. South-facing slopes provide excellent range in spring, but they are poorly suited to livestock grazing in summer. South-facing slopes are important to big game in winter because less snow accumulates on these slopes and they are the first to green up in spring. Southeast- and west-facing slopes have similar vegetative site characteristics as south-facing slopes.
The steepness of slope affects livestock use and the feasibility of applying improvement practices. Areas that have slopes of 30 percent or less are most preferred by livestock. Areas that have slopes of more than about 50 percent receive very little use, even if the forage is abundant. Limited livestock use on steep slopes normally is anticipated, and stocking rates are adjusted accordingly. Ground equipment use is impractical on slopes of more than 30 percent.
Droughtiness reduces the production of forage and limits the choice of species for reseeding. Droughtiness is a result of low annual precipitation or the low water-holding capacity of the soil. Soil characteristics such as coarse texture, shallow depth, or high rock fragment content limit the water-holding capacity. Cold temperatures limit the length of the growing season, suppress plant growth, and delay plant development.
The amount of stones and cobbles on the surface can influence both grazing management and the potential for revegetation. Some soils have so many stones and cobbles on the surface that livestock avoid them if possible. Stones on the soil surface affect the feasibility of mechanical seedbed preparation and seeding.
Soil surface textures can limit use or the season of use. Soils that have a sandy surface texture have a high hazard of wind erosion. Grazing should occur late in fall, in winter, and early in spring when the soils are moist and the potential for wind erosion is low. Soils that have a silty surface texture and low organic matter content are subject to crusting. A vesicular crust can form, which restricts infiltration and seedling emergence. Soils that have a clayey surface texture have a very slow infiltration rate and very slow permeability. In cold areas, soils that have a silty or clayey surface texture are subject to frost heaving. Vegetation is subject to trampling and crown damage during wet periods in winter and spring.
A high water table can occur seasonally or year round. Wetness, even if the root zone is saturated only briefly, impacts the composition and production of vegetation. This is readily apparent in soils that are ponded or have a water table at or near the surface. Under these conditions, grazing can cause soil compaction, soil displacement, and plant crown damage. Soils are not suited to mechanical site preparation during wet periods, and they are subject to erosion from concentrated flows. Seeding techniques should be tailored to the site conditions, and species that can tolerate seasonal wetness should be seeded.
Rock outcrop and escarpments occur throughout the park. They occur most typically on steep, south-, east-, and west-facing slopes. Most formed as a result of geologic faults or glacial action or are made up of exposed sedimentary and igneous rock. Areas of Rock outcrop and escarpments can be several hundred feet in length and ten to several hundred feet in height. They act as physical barriers to domestic livestock and many species of wildlife by preventing or restricting vertical movement. Some wildlife species, such as raptors and bighorn sheep, prefer habitat associated with areas of Rock outcrop and escarpments.
Loss of site potential in some of the soils in the park has occurred as a result of a significant loss of the surface layer from wind or water erosion. The loss of this layer can cause major changes in the composition of the plant community. This irreversible change in the plant community as a result of soil erosion is most evident in soils that are shallow or have a claypan and a thin surface layer and an underlying subsoil that has low permeability and restricts the growth of roots. Depending on the extent of the erosion, losses in total production can be as much as 25 to 50 percent or more.