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. 1, 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 
