50 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



THE YELLOW PINE FORESTS/ 



The. yellow pine belt in eastern Washington 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 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 (G.OOO 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 narroAV tongues of the Columbia 

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

 extending into Idaho betAveen the flanks of granitic mountains, are 

 covered with yellow pine forests. Nevertheless, it is ai)})iu-('nt 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 (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 {Opidaster pauci-florus, buckbrush {Cennothi(s srmr/vineu.^) , 

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

 the yellow pine is at its best, the commonest undershrub is the huckle- 

 berry {Vaccininm inacrophylhim). Where such forests are more 

 open the most abundant plant is often the pinegrass {C alamagrostis 

 suksdorfii) . 



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 valleys, the yellow pine is usually mixed 

 Avith red fir {Psevdots}iga mncronata) 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- 



