a b s t r a c tDrylands typically have distinctive vegetative patterns that reflect the underlying physical landscape. We use a geopedological classification to organize the landscape into five categories from broad scale to fine scale: (1) physiographic divisions which describe regional topography, such as mountains and basin floors; (2) regolith type which identifies residuum versus transported sediments; (3) parent materials which describe mineralogy; (4) landforms which provide meso-scale descriptions of topography; and (5) soil texture, a fine-scale variable important for infiltration, erodibility, and available water holding capacity. The study was conducted in a 1753 km 2 area of the White Sands Missile Range and easternmost Jornada Experimental Range and revealed the following.(1) At the broadest scale, mesquite is dominant on the basin floor when the parent material is siliceous sand, but (2) biological soil crust becomes dominant in the basin floor when the parent material is gypseous. (3) Creosotebush is dominant on rocky soils of the piedmont slopes regardless of parent material, but (4) grasslands become dominant on the bajadas upslope in the semiarid zone and into the semiarid mountain uplands regardless of bedrock type. This method provides a way of supplementing ecologic-edaphic studies and provides a framework within which mechanisms can be explored.
True mountain mahogany (Cercowpusmontunus Raf.) and its habitats were studied in the canyons and foothills of the Wasatch Mountains of Central Utah. Twenty populations were selected and sampled for various biotic and abiotic environmental variables. All study sites contained true mountain mahogany as a dominant or subdominant plant. The communities are shrub dominated with other plant life forms contributing little to the total cover of the sites. The more northern exposed sites appear to be undergoing succession while the more southern exposures seem more stable. Native shrubs common to the Intermountain Region are important to the livestock and wildlife resources of the region. Increasing demands upon our shrub resources by a growing human populace necessitate more efficient range management. The efficacy of rangeland policies relating to shrub species depends upon knowledge of the ecological requirements of the species. Such information can best be gained through study of the natural ecosystems that support the shrub species in question, True mountain mahogany (Cercocarpus montanus Raf.) is widely recognized as a useful forage plant (Plummer 1969, Young and Bailey 1975) and knowledge of its habitat relations is essential for productive management. Martin (1950) revised the genus Cercocarpus and delimited 8 varieties. The species referred to in this present work is Cercocarpus montanus Raf. var. montanus. Martin described this variety and listed several synonyms. Members of this genus were first called mountain mahogany by early pioneers, but that name has since been restricted to the genus Sweitenia by the Federal Trade Commission. Following this ruling, the U.S. Forest Service check list has approved "Cercocarpus" as the common name (Hayes and Garrison 1960). Nevertheless, certain common names have become firmly entrenched in the literature: true mountain mahogany, birchleaf mahogany, and alder-leaf mountain mahogany. True mountain mahogany is a widely distributed browse species in the western United States (Fig. 1). The mean habitat type is in the Great Basin and Rocky Mountains, along bluffs and mountain slopes between 1,070 and 3,050 m (Medin 1960, Martin 1950, Pyrah 1964, Greenwood and Brotherson 1978). The Soil Conservation Service (1971) reports that true mountain mahogany is found within the 240-550 mm rainbelt on sites having a June deficiency of moisture. Medin (1960) found true mountain mahogany growing on sandstone and shale in Colorado and his data indicated that soil depth was the most important factor influencing annual shrub production. A soil conservation report for Utah indicated that true mountain mahogany was most abundant on sites with shallow soils and with 35% or greater coarse fragments. Other researchers have noted that the shrub can withstand high lime and prefers sandy soils, but it is occasionally found in shales or deep loams (Brotherson and Brotherson 1967, Plummer 1969, Ream 1964). True mountain mahogany is recorded on all aspects and in fertile canyon bottoms where the pH ranges...
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