Southeastern Washington and Adjacent Idaho. 17 



On the more mesophytic slopes of the treeless hills of the well 

 developed prairie a poorly developed shrub associes occurs. This 

 is made up of Symphoricarpos racemosus, Rosa nutkana, R. piso- 

 carpa, dwarfed specimens of Prunus demissa-, Crataegus brevi- 

 spina, Amelanchier spp., and sometimes Opulaster pauciflorus, 

 and small trees of Populus tremuloides. These shrubs are better 

 developed in the canyons and are usually forerunners (especially 

 Opulaster pauciflorus) of the yellow pine; along the stream 

 margins they give way to the flood-plain forests of Populus, 

 Crataegus, Amelanchier, Salix, and Alnus (Fig. 35). 



On the outlying buttes as well as in the mountains, the Opu- 

 laster consocies often occupies considerable areas between the 

 pines and the grass land (Fig. 23). It almost invariably occurs 

 on the sheltered sides of lower buttes and mountain ridges, while 

 its presence as a half-dead relict under the pioneer forest indi- 

 cates its former possession of numerous other situations. 



The Prunus consocies is less abundant than the preceding, but 

 it is well represented in dry situations on butte and mountain 

 sides. Like Opulaster, it is replaced by the pines in the normal 

 succession. The chief species is the shrub-like tree, Prunus 

 emarginata. 



On the higher mountains especially, the Ceanothus consocies 

 plays a role similar to the other shrubs of this associes. The two 

 species C. velutinus and C. sanguineus, of which the former is 

 the more important, act as forerunners for the yellow pine and 

 Douglas fir. 



The Pinus consocies is represented by the western yellow pine, 

 Pinus ponderosa, the most xerophytic forest tree in the region. 

 This consocies not only occupies dry slopes and ridges on the 

 buttes and mountains, but has worked down along the Palouse 

 River canyons as well as along Rock Lake and Rock Creek and 

 Union Flat, to an altitude of only 1,500 feet (Figs. I, 24, and 45). 

 Everywhere the pine is the pioneer forest tree, and forms a fring- 

 ing belt along the prairie. While it is usually preceded by some 

 shrub consocies, yet it is able to invade the prairies, and especially 

 those with a more sandy soil, without such an intervening stage. 



The Pseudotsuga consocies, when pure, occupies dry, rocky 



17 



