Rangelands, semiarid, and arid regions unsuitable for cultivation or forests, are intrinsically prone to erosion at rates that exceed the rate of soil development. Subject to patterns of erratic or infrequent rainfall, ground cover consisting of microbiotic crusts, forbs, grasses, and shrubs can vary over the course of a year from nearly complete to virtually nonexistent. Droughty conditions and low biomass production restrict development of soil rich in organic matter and nutrients, characteristics that contribute to soil stability. Plant communities in rangelands flourish after rain, but ground cover is typically sparse before and during rainstorms. Under these conditions, the soil surface is exposed to raindrop impact, soil surface sealing, and overland flow. For example, rain in the Sonora and Mojave Deserts, and much of the Great Basin of North America, arrives as convectional storms from monsoonal weather patterns, bringing short-duration, high-intensity and -energy rainfall that dislodges and washes large amounts of soil into dry channels but quite often over relatively short distances in these channels. In regions where precipitation is in the winter and as snowfall, the risk of erosion is less than at lower elevation and latitudes, unless snowmelt is rapid and accompanied by rainfall.
Despite the vast difference in rangeland resources across continental and regional scales, these lands are generally populated by pastoralists who depend on the resources for subsistence or commercial survival. For soil conservation, the common management problem is one of managing the number, timing, frequency, and duration of livestock in a manner that leaves sufficient vegetation to protect and hold the soil in place.
Limiting soil erosion in these systems conserves the meager resources available for plant production. When soil erodes from these systems, the first soil to be washed away is the soil that is richest in organic matter and nutrients. The link between the soil resource and the vegetational communities capable of providing cover and protection against soil erosion, and productive forage for livestock, is intricately connected to the biochemical and biophysical needs of the component species of the vegetational community. Loss of the necessary soil resource for support of productive communities is described by state and transition models, where vegeta-tional communities go through periods of transition as soil resources are accumulated or lost, and temporarily stabilize (quasi-stability) in composition when a state of no net gain or loss in physical and chemical resources is reached. On a local scale, these processes may occur rapidly as soil is washed from one area and is deposited in a lower site, replenishing or adding to the fertility of the deposition site. On a landscape scale, once the nutrient pool is lost by erosion, decades or centuries may be required before it is replenished and with development of microbiological and vascular plant communities unrelated to the preerosion event communities. An example of this type of outcome is the conversion of the temperate rangelands, or shrub steppe, in North America that were once populated by shrubs and cool season perennial grasses, but are now monocultures of annual grasses.
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