50 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
THE YELLOW PINE FORESTS. 
The yellow pine belt in eastern Washington hes between the alti- 
tudes of 550 and 1,000 meters (1,800 and 3,300 feet). In places the 
characteristic tree, the yellow or bull pine (Pinus ponderosa), de- 
scends nearly to sea level, as along the Columbia River, and specimens 
of the trees are occasionally found at 1,800 meters (6,000 feet) alti- 
tude. This tree exhibits a marked predilection for soils of granitic 
origin, and whenever such soil is found, even if completely isolated, 
the yellow pine is quite sure to occur. The zonal distribution of the 
tree is not primarily due, however, to a soil factor. The raised dome 
of the northern portion of the Blue Mountains, wholly basaltic, is tim- 
bered with this tree. Likewise narrow tongues of the Columbia 
basalt of the usual elevation, 600 to 750 meters (2,000 to 2,500 feet), 
extending into Idaho between the flanks of granitic mountains, are 
covered with yellow pine forests. Nevertheiess, it is apparent that 
this tree encroaches on the clayey basaltic soils with difficulty. 
Whether this is owing to the inability of the seedlings to struggle 
with the herbaceous vegetation or to a lack of adaptation to the soil 
itself, or to some other factor, remains to be determined. From the 
fact that the yellow pine establishes itself on basaltic clay soils under 
favorable conditions of moisture and temperature, as in the Blue 
Mountains, or of the shading and abundant seeding that the sur- 
rounding forests provide in western Idaho, it is evident that the soil 
factor is not the only one that has prevented the spread of the pine 
forest. 
Yellow pine forests (Pls. XIV, XV), where pure, are open in char- 
acter, and marked by the relatively small amount of forest litter. 
There is a rather scattered growth of various shrubs, consisting of 
ninebark (Opulaster pauciflorus, buckbrush (Ceanothus sanguineus), 
and rose (/?osa gymnocarpa). Ata somewhat higher altitude where 
the yellow pine is at its best, the commonest undershrub is the huckle- 
berry (Vaccinium macrophyllum). Where such forests are more 
open the most abundant plant is often the pinegrass (Calamagqrostis 
suksdorfit). 
Yellow pine forests are, however, seldom pure, except at low alti- 
tudes in rather dry soil. In the moister situations afforded by higher 
altitude, shaded slopes, or vallevs, the yellow pine is ustially mixed 
with red fir (Pseudotsuga mucronata) in varying proportions. In- 
deed, as the moisture becomes greater the proportion of the red fir 
increases until it becomes the predominating tree. The increasing 
proportion of red fir is usually accompanied by a proportional in-~ 
crease in the density of the forest and the amount of litter. Shrubs, 
too, become more abundant both in species and individuals, and 
under favorable circumstances, as in old burns, some of them, espe- 
