Forests of the semiarid and arid zones of the interior western United States (US) are some of the most unique in North America. They occupy 11 to 34% of the landscape at mostly higher elevations (USDA Forest Service, 1981). These forests are characterized by a high diversity of flora, fauna, climates, elevations, soils, geology, hydrology, and productivity. Within the space of a few dozen kilometers, forests can change from desert shrublands to spruce and fir, the equivalent of going from northern Mexico to the Arctic. Because of the hydrologic cycle in the Interior West of the US, these forests are generally not known for their high productivity or contribution to the nation’s wood supply. Some exceptions do exist in the ponderosa pine ecosystem. However, these forests have a high value due to other resources and amenities that they supply. In much of the western US, the primary source of municipal drinking water is runoff that emanates from the high elevation portions of these forests (Dissmeyer, 2000). These forests also provide habitat for wildlife, including many threatened and endangered species. Arid zone forests are now being utilized for recreation to such a degree by a rapidly expanding western US population that the value of the recreation amenities they supply exceed those of extractive resources (i.e., wood and minerals; Lyons, 1998).
Forests in the dry regions of the interior western US are characterized by an overriding system driver, water availability, which controls the type, amount, and productivity of vegetation. This driver controls key ecosystem processes and ultimately carbon (C) reserves. The arid region forests that are examined in this chapter are characteristically ones in which annual water losses through evapotranspiration exceed or are slightly less than annual precipitation. The presence of water, interacting with soil processes, soil properties, soil biota, and vegetation ultimately determines the amount, quality, and state of C.
The main objective of this paper is to describe the types of dry forests that occur in the Intermountain West of the US, the soils that they occupy, the interactions with current and future land management activities, and the potentials for additional C sequestration. High elevation forests that are part of the larger intermountain forest ecoregions are discussed in Chapter 17.