FLOWERING PLANTS AND FERNS OF ARIZONA 17 



and Pinus edvlis t here found at the lower edge of their altitudinal 

 ranges. 



It is characteristic of the Great Basin Microphyll Desert that the 

 smaller drainageways are without distinctive plants and do not have a 

 marginal fringe of upland plants growing more densely than elsewhere 

 Fort stiera nt omexicana and Sarcobatus vt rm icvlatus are sometimes found 

 in such situations, but there are no perennials that assume the role 

 played in southern Arizona by Populus, Prosopis, Cercidiumfloridum, 

 and Baccharis. In the flood plains of large streams Sarcobat n s , 

 Forestijra, and Atriplex canescens are characteristic. The first of these 

 forms dense thickets in favorable places. 



Desert-Grasslaxd Transition. — Between elevations of 4,000 and 

 6,000 feet are found large and small areas of this vegetation, perhaps 

 forming one-tenth of the area of the State. The desert grassland is the 

 best grazing land in Arizona, and has great potential value under more 

 careful management than has commonly been practiced in the past. 

 Over this region there is an annual rainfall of 12 to 18 inches, which is 

 intermediate between the precipitation of the desert and that of the 

 forest. Although the vegetation is favored by somewhat higher rain- 

 fall, there is a lower range of temperatures than in the desert, winch 

 serves as a limiting condition to exclude the great majority of desert 

 plants. 



In the desert-grassland areas there is, in general, a good soil of 

 sufficient depth to favor root development in deep-rooting types of 

 plants. There are a number of areas, especially in the central part of 

 the State, which occupy the same elevations as the desert grassland 

 and have similar climatic conditions, but possess a scanty and shallow 

 soil. Some of these are limestone areas, whereas others are volcanic, 

 locally known as "mallapy" (mal pais). 



The largest area of desert grassland is found in the drainage of the 

 Little Colorado River in Apache, Navajo, and Coconino Counties. 

 Other large areas occur in Yavapai, Graham, and Cochise Counties. 



The typical communities of the desert grassland are made up of 

 perennial grasses, these commonly occurring as separate bunches with 

 intervening bare ground. The dominant grasses are species of Boute- 

 loua, Sporobolus, Aristida, MuJdenbt rgia, Hilaria, and Stipa. With one 

 or more grasses forming the matrix of the vegetation there are asso- 

 ciated with them a large number of species of root perennials and a few 

 annuals. Cacti are by no means absent, although the number of 

 species is small. Several species of Yucca are a characteristic feature 

 of this vegetation, Y. data and Y. baccata being the most common. 

 Dasylirion and Nolina are frequent but have their greatest abundance 

 where the soil is relatively shallow. 



Shrubs occur sporadically, usually in restricted localities. Where 

 the desert grassland borders on the xeric evergreen forest there is no 

 line of demarcation between the two. Throughout northern Arizona 

 there are hundreds of square miles on which a very open stand of low 

 junipers is found in typical desert-grassland country, and in south- 

 eastern Arizona the same conditions are found on passing into the 

 evergreen oak forest. Higher rainfall, or better conditions for retention 

 of soil moisture, favor the occurrence of a few trees without permitting 

 the growth of a stand sufficiently dense to break the continuity of the 

 desert grassland. 



