54 Forest Club Annual 



up the bulk of the vegetative covering on the open ridges 

 there are many species of less abundance, some of which are 

 however quite conspicuous. The three common western cac- 

 tuses, Opuntia poly cant ha, Opuntia fragilis, and Cactus vivi- 

 parus are more or less conspicuous members of this formation. 



It occasionally happens that the ridges have become so 

 narrowed that practically all surface soil has been moved 

 away taking the grasses with it. A substratum of almost 

 bare rock is then left exposed. In such places one finds an 

 association of mat and rosette plants among which the fol- 

 lowing species are common : Arenaria hookeri, Sedum steno- 

 petalum, Picaradenia acaulis, Phacelia heterophylla, Eri- 

 ogonum flavum, and Homalobus caespitosus. These are all 

 acaulescent or Low-stemmed plants which grow aggregated 

 into dense cushion-like mats, or plants with a basal tuft or 

 rosette of leaves. This peculiar growth-form seems to be 

 related to low water content and infertility of the soil. 



But on the whole the vegetation of the ridges and buttes 

 is grassland fringed with Pinus ponderosa, with now and then 

 individuals or clumps of this species scattered over the lower 

 lying portions of the formation. In some places the soil has 

 become badly broken and has thus enabled the pines to invade 

 and establish themselves in considerable numbers over rather 

 extensive stretches of this formation. In such places the ap- 

 pearance is that of a prairie dotted with pine trees. 



The Pinus ponderosa formation, the second in our series., 

 is a well defined formation both as to topography and com- 

 position. The formation is invariably confined to extremely 

 xerophilous situations of which talus slopes and steep sides 

 of canyons and buttes are the most common. From the fact, 

 as given above, that Pine Ridge is a great complex of steep 

 talus slopes and ridges covered to a varying density with a 

 coniferous forest, one can form some idea of the importance 

 of this formation. Here in northwest Nebraska is found our 

 most extensive woodland or forest area. 



Many stages in the development of this pine formation 

 may be traced within the region in which these studies were 

 made. Starting with the bare young talus slope it is possible 

 to build up a series of plant successions which ultimately 

 culminate in the fully stocked canyon sides characterized by 



