GRAZING TYPES. 279 



original Stipa bunch-grass prairie has been almost wholly replaced by the 

 wild-oats, Avena fatua, and the latter determines a relatively lower value for 

 the community. The sagebrush savannah so characteristic of northeastern 

 Wyoming lies in the edge of the mixed prairie, and the sagebrush is chiefly 

 associated with Stipa, though Agropyrum and Bouteloua are also present to a 

 large degree. The relative abundance of grass and sagebrush varies widely, 

 and the indicator value of the mixture in accordance. Since the sagebrush 

 is eaten to a much less degree during the summer, the carrying capacity is 

 somewhat reduced, though this is partly compensated by its availability 

 during the winter. 



Savannah in relation to fire and grazing. The general view in the Southwest 

 is that mesquite and oak savannah are limited or destroyed by fire and that 

 they have spread rapidly in recent years, since the annual burning has ceased 

 (Cook, 1908). In the absence of definite measurements, many of the state- 

 ments can be accepted only in part, though the general relation to fire seems 

 evident enough. Tree savannah appears to be affected Uttle by burning, 

 except that this must have been a powerful factor in spreading the annual 

 Avena in California at the expense of the perennial Stipa. The effect of fire 

 upon scrub savannah depends upon a number of factors, chief among which 

 are density and height of both shrubs and grasses, the ability of the shrubs 

 to form root-sprouts, and the frequency of fires. It seems certain that an- 

 nual fires in scrub savannah that is densely covered with tall-grasses would 

 destroy the shrubs completely in a few years, no matter how great their abiUty 

 to form root-sprouts. Less frequent burning of open savannah, in short- 

 grass especially, would damage the shrubs much less and might well increase 

 their control by promoting root-sprouting. Moreover, in the more xerophy- 

 tic grasslands, frequent burning during dry seasons injures the grass and 

 would tend to favor the shrubs in consequence (plate 66). 



The general effect of grazing is to increase the shrubs at the expense of the 

 grass. As has been seen, savannah owes its character to a dry cUmate in 

 which the ecesis of shrubs is regarded as usually possible only during the wet 

 phase of the cycle. This means that shrubs and grasses live constantly under 

 keen competition for water, and that anything which reduces the amount of 

 grass will be to the advantage of the shrubs. Since grasses and herbs are 

 usually eaten to a much larger degree, intensive grazing, and especially over- 

 grazing, will reduce their hold upon the soil and correspondingly improve 

 conditions for the spread of shrubs. The seeds of the mesquite and other 

 shrubs are widely scattered as a consequence of being eaten or through unin- 

 tentional carriage, and the seedHngs are more readily estabUshed in areas 

 where the hold of the grasses has been weakened. The local spread of the 

 scrub clumps is chiefly by means of root-sprouts and is promoted by light 

 browsing, but restricted by heavy browsing. Thus, while savannah is pri- 

 marily an indicator of cUmate, its secondary indication is one of grazing and 

 absence of fires, upon which its practical utiUzation must be based. As sug- 

 gested in a later section, this can be done readily only after quadrat measures 

 have made clear the exact behavior of savannah under different methods of 

 burning and grazing. 



Significance of serai types. While serai communities are temporary in 

 comparison with climax ones, many of them persist for tens or even hundreds 



