THE STUDY AREA 



The study site is on the lower part of a stable and ancient, gently sloping (2 per- 

 cent) alluvial fan, 0.4 km. from the headquarters of the Desert Experimental Range in 

 southwestern Millard County, Utah. Elevation is 1,600 m. 



The area has not been grazed by domestic animals since 1934, and it is thought that 

 livestock use prior to that time did not materially change the natural composition of 

 the plant cover. The period of exclusion of livestock now exceeds the average lifespan 

 of all perennial grasses and shrubs of the cover type, except possibly Eurotia lanata. 



The largest native herbivores are jackrabbits, which are rare in this habitat. 

 The most abundant mammals are kangaroo rats and pocket mice, both primarily seed eaters, 

 and deer mice, which are more omnivorous. Pocket gophers are present but infrequent. 

 Mammalian herbivores probably consume no more than 1 or 2 percent of the plant growth. 

 Small omnivorous birds are in the area yearlong, but they are not abundant. Invertebrate 

 consumers of live vegetation are mostly found in the roots. Termites are occasionally 

 found in woody litter. Ants are rare. 



Winters are cold (average temperature for January is -3.5° C.) and summers are 

 warm (average temperature for July is 23.3° C). Precipitation, which averages 154 

 mm. annually, is sporadic, and intervals of 1, 2, or more months without rainfall can 

 occur at any season. Early spring is the only season in which soil moisture is consist- 

 ently adequate year after year for some plant growth. The accumulated v;inter moisture 

 usually wets the soil to a depth of at least 25 cm. ; occasionally it penetrates to twice 

 and, rarely, to three times that depth or deeper. More than half the moisture from 

 summer rains is retained near the surface, where it evaporates quickly. However, in 

 most years, storms at one or two times during the growing season put moisture to depths 

 favorable for renewal of plant growth. In 1965, the wettest summer on record (38 years], 

 the dry soil was recharged five times, to depths of 38, 15, 18, 38, and 23 cm. (Brewster 

 1968), In a typical year, the soil is dry most of the time after the short period of 

 spring moisture. The water table lies 145 m. below the ground surface. 



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