50 CONTRTBUTIOXS FROM THE NATIONAL HERBARIUM, 



THE YELLOW PIXE FORESTS. 



The yellow pine belt in eastern AVasliinoton lies between the alti- 

 tudes of 550 and 1,000 meters (1.800 and 3.300 feet). In places the 

 characteristic tree, the yellow or liiill pine {Pin us poiu/erosa). de- 

 scends nearly to sea level, as along the Columbia River, and specimens 

 of the trees are occasionally found at 1,800 meters (0,000 feet) alti- 

 tude. This tree exhibits a marked predilection for soils of granitic 

 origin, and whenever such soil is found, even if comj^letely 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- 

 l)ered with this tree. Likewise narrow tongues of the Columbia 

 basalt of the usual elevation, 000 to 750 meters (2.000 to 2,500 feet), 

 extending into Idaho between the flanks of granitic mountains, are 

 covered with yellow pine forests. Nevertheless, 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 i3rovide in western Idaho, it is evident that the soil 

 factor is not the only one that has j^revented the spread of the pine 

 forest. 



Yellow pine forests (Pis. 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 {Opidasfer pavcifion/s. buckbrush [Ceanothus sanc/uineus)^ 

 and rose (Rosa gymnocarpa) . At a somewhat higher altitude where 

 the yellow pine is at its best, tlie commonest unclershrub is the huckle- 

 berr}' {Vaecinium macro pliylhini). Where such forests are more 

 open the most abundant plant is often the pinegrass {Calamagrostis 

 suksdorfii) . 



Yellow i^ine forests are. however, seldom pure, except at low alti- 

 tudes in rather dry soil. In the moister situations atl'orded by higher 

 altitude, shaded slopes, or valleys, the yelloAv pine is usually mixed 

 with red fir {Pseadotsiiga miicronata) in varying proportions. In- 

 deed, as the moisture becomes greater the j)roportion 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- 



