SLOPE AND EXPOSURE EFFECTS ON RANGE SITE INTERPRETATIONS (ARIZONA).

Abstract

Vegetational responses to changes in exposures within a constant slope range were studied on the Shallow Upland Range Sites and Granitic Hills Range Site in the Chihuahuan semidesert grasslands in Central Arizona. Sixteen exposures with slopes between 11 and 17 degrees were chosen for subsample sites. Environmental, complete soil descriptions, and vegetational composition data were taken. All data were analyzed using analyses of variance, ordination programs, and regression analyses to determine climate, soils, and vegetational relationships among exposures. The resulting data indicated that the geological lithologic unit on which the soils formed was the most important factor affecting apparent vegetational type. In this study, the data obtained from the complete soil profile descriptions contributed little information to the understanding of vegetational responses. Soil surface characteristics and surface soil horizon properties influenced soil moisture relationships. The conservation of soil moisture appeared to be more important to plant communities than did the total moisture holding capacity of the soil continuum. Monthly precipitation reliability and soil surface reflectances were environmental factors affecting plant communities occurring on different exposures. Fall/spring, winter/spring, and spring soil temperature interactions were the most important environmental factors affecting vegetation on different sloping exposures. All exposures within each of the four sample locations had vegetational components that were similar to the vegetational components of other exposures but all exposures were found to have different plant communities. Each exposure within a given slope range is a phase and/or subphase of currently used range site descriptions. A range site that is based on a potential natural community at one type location cannot be extrapolated across broad geographical expanses to define vegetative potentials for other areas having similar vegetative aspects. Range site descriptions must be site specific for one geographical rangeland that has had the same historical uses.