relative cover) and at 2,600 m pinyon is completely dominant (fig. 7). As elevation 

 increases the relative percent cover of pinyon increases, while the relative percent 

 cover of juniper decreases. At an elevation of 2,000 to 2,200 m the average relative 

 percent cover of both species is about equal. 



When these relative cover data are divided on the basis of slope exposures 

 (table 4), the result show some small differences probably due to the modification of 

 climate encountered on different exposures. On the south and east exposures the rela- 

 tive percent cover contributed by juniper is slightly higher at high elevations, althoi 

 the difference is not statistically significant. The slight difference is probably 

 due to warmer temperatures and effectively drier soils. On north and west aspects 

 the average relative percent cover contributed by pinyon is slightly increased at 

 lower elevations, apparently due to cooler, moister sites encountered. An exception 

 is the White Mountain Range of California where the relative percent cover of pinyon 

 is high even at the lower elevations. This apparently results from the combination of 

 warmer temperatures and of overall aridity of this mountain range, particularly at 

 lower elevations (St. Andre and others 1965). 



16 18 20 22 24 26 28 

 ELEVATION CLASSES (x 100 IVl) 



Figure 7. — The average relative cover ('percent) of pinyon and juniper for plots of 

 each etevational interval. Vertical bars represent one standard error. There are 

 no bars for 1,600 and 2,800 meters because only one plot was available at these 

 elevations. 



In the central and southern Great Basin, where low temperatures are not likely to 

 be a major controlling factor (West and others 1978), relative composition of tree 

 species varies with longitudinal changes in seasonal moisture distribution (fig. 8). 

 As the amount of summer precipitation (July-September) increases from west to east 

 (Stidd 1967), the relative amount of juniper increases. Where summer preciptation 

 exceeds about 3 inches (8 cm), single-needle pinyon is gradually replaced by true 

 pinyon. 



19 



