404 CARNEGIE INSTITUTION OF WASHINGTON. 



in lakes or swamps, and those arising out of the secondary succession 

 in burns. Both of these exhibit the essential feature of grassland or 

 sedgeland surrounded by trees, though they regularly show character- 

 istic differences in the structure of the grassland, as well as of the forest 

 mass about it. Under natural conditions all parks tend to slowly 

 disappear, owing to the gradual invasion of trees or shrubs, but, under 

 widespread grazing in the mountains, the seedlings are destroyed 

 and the park will remain as long as the trees do. In the montane 

 region natural parks usually occur in the pine or cedar-pinyon commu- 

 nities in the Rocky Mountains, and in the pine or Douglas fir forest on 

 the Pacific Coast. In the subalpine region a larger number of trees is 

 concerned, though spruce, fir, and lodgepole pine are the most impor- 

 tant. In the former the grass center is mixed prairie in the Petran 

 region and bunch-grass prairie in the Sierran ; in the latter, it is sub- 

 climax grassland. 



Natural parks are usually associated with savannah at the lower 

 timber-line, and often also at the upper. Between these limits savan- 

 nah is rare, owing to the climax nature of the forests. It is the typical 

 expression of forest, woodland, or scrub in contact with a grassland 

 climate, whether lowland or alpine. In appearance it closely resem- 

 bles the artificial parks of cities with their grassy cover. Savannah 

 is an expression of the cyclic nature of climatic factors, as there seems 

 to be no doubt that it is due to the successful ecesis of woody plants 

 during the favorable phase of a climatic cycle. Moreover, both its 

 persistence and extension are promoted by grazing, with the result 

 that a border of savannah is characteristic of mountain fronts that 

 touch grassland. It is less typical of alpine timber-lines, partly 

 owing to the relative infrequence of alpine plains, and partly to the 

 recent advent of grazing. 



Sagebrush, desert scrub, and chaparral may form natural parks as 

 well as savannahs, but the latter are much more common. They 

 give rise to primarj^ parks rather infrequently, and burn parks are 

 the rule, though these arise in cliaparral usually only after repeated 

 burning has made root-sprouting impossible. The interaction of 

 climatic cycles and grazing has broadened the original savannah 

 border of sagebrush and of desert scrub, until it occupies a larger 

 area than the climax itself. The savannah character, however, is 

 often obscured by grazing, smce overgrazing often dwarfs sagebrush 

 and mesquite so that they are over-topped by the grasses. 



Comparative Studies of Forest, Chaparral, and Grassland, by R. J. Pool. 



A comprehensive investigation of pine forest, chaparral, and mixed 

 prairie is under way in the Pike's Peak region. This deals with the 

 structure and development of each climax, but is directed especially to 

 the interrelations of the three communities and the factors involved in 



