GRAZING TYPES. 277 



indicator of the wet phase of the cycle. The control of the grasses is so com- 

 plete that the additional water-content necessary for the germination and 

 establishment of the trees or shrubs is present only during the maximum of the 

 wet phase, often only a single year. Once established, and with their roots 

 at greater depths than those of the grasses, the trees or shrubs persist indefi- 

 nitely. During succeeding wet phases they tend to increase in number, while 

 in critical drought periods the number may be reduced, as is regularly the 

 case where fires are frequent. Counts of the annual rings of a number of 

 shrubs in different savannah areas confirm the view that ecesis is normally 

 confined to wet phases of the climatic cycle (plate 65). 



The indicator significance of savannah or park naturally depends upon the 

 kind and the region, as well as upon the dominants. The best examples of 

 tree savannah are to be found along the line of contact of forest or woodland 

 with grassland. Oak savannah is the most common, occurring typically in 

 central Texas, in Arizona, New Mexico, and Mexico, and in California and 

 Lower California. Savannah in which yellow pine is the tree is frequent 

 along the lower edge of the montane forest, where it extends out upon plateaus 

 or plains. It is well-developed in northern Arizona and New Mexico, but is 

 most extensive on the low ranges and high plains east of the central Rockies 

 and around the Black Hills. Both pifion and cedar form savannah, but the 

 latter is much more frequent and extensive. Typical scrub savannah is 

 largely confined to the Southwest, ranging from Texas through southern New 

 Mexico and Arizona, and northern Mexico. Its most characteristic shrub is 

 mesquite, Prosopis juliflora, but Yucca, Acacia, Ephedra, and other domi- 

 nants of the desert scrub occur frequently. Owing to its habit of growing in 

 clumps or groups, chaparral tends to form grassy parks rather than typical 

 savannah, especially along the edge of the Petran association. Sagebrush 

 extends into several of the grassland associations to form what is essentially 

 sagebrush savannah, though its low stature tends to obscure the exact re- 

 lation. This is especially true where it meets the tall-grasses, as in Wyoming 

 and Oregon, but the savannah nature is obvious where tall sagebrush is 

 scattered through short-grass, as in southeastern Utah. 



Parks differ from savannah chiefly in that the two communities concerned 

 mix by alternating groups or areas rather than by scattered individuals. 

 Excellent examples of grass parks occur in the subalpine forests of Colorado, 

 where spruce and balsam inclose extensive meadows of Festuca, dotted with 

 groups of young conifers or aspens. Somewhat similar parks occur at timber- 

 line, where the forest breaks into groups which extend well up into the alpine 

 meadows. Sagebrush parks occur most commonly in the lower subclimax 

 portion of the woodland zone, while sagebrush areas dotted with groups of 

 lodgepole pine or aspen are frequent on the western slope of the Rocky 

 Mountains in Colorado and Wyoming. Chaparral parks are best developed 

 in California, especially in the case of subclimax chaparral in the pine forest 

 and where the climax type meets the pine-oak woodland. In the Rocky 

 Mountain region they occur chiefly as scrub openings in the pinon-cedar or 

 oak-cedar woodland. 



Savannah and park are alike as indicators in that they denote a transition 

 from one community to another. They differ for the most part in that savan- 

 nah is an indicator of climate, and park usually of local or edaphic conditions. 



