4 BULLETIN 317, U. S. DEPARTMENT OF AGRICULTURE. 



dominates as the principal tree on all the drier slopes and bench 

 lands. This gives an open character to the forest and is of some 

 significance as regards the growth of mistletoe on the larch wherever 

 this tree is associated with the yellow pine. On the lower eleva- 

 tions the yellow pine often occurs in pure parklike stands, with a 

 ground cover quite characteristic of the typical yellow-pine forma- 

 tion, usually indicated by the absence of any great amount of forest 

 litter and by the uniformity and the small number of species of her- 

 baceous and shrublike plants. On the south slopes and low, dry 

 ridges, where the stand is very open, the yellow pine is quite gener- 

 ally infected with its particular mistletoe, working great injury to 

 the tree. At higher elevations in more moist situations, or even at 

 the same level on protected parts of the typical stand, the yellow 

 pine becomes mixed with larch, Douglas fir, white fir, and lodge- 

 pole pine. The yellow pine gives way to greater percentages of 

 larch, Douglas fir, and lodgepole pine on the north and east slopes. 

 The two last-named species support large quantities of their respec- 

 tive mistletoes wherever the conditions are favorable for the devel- 

 opment of these parasites. The larch predominates in many north- 

 slope stands, especially in the more open situations. Other forest 

 types in which the larch occurs above 6,500 feet or more are of little 

 importance in this connection, since the species of the types at this 

 elevation are not as seriously infected by mistletoe as those on north 

 slopes of 5,000 feet altitude and less. 



The influence of drainage, slope, and the general moisture condi- 

 tions of the soil on the distribution and vigor of the western larch 

 is well shown in the region studied and is likewise reflected in the 

 prevalence and distribution of its principal parasite. Owing to the 

 general prevailing dryness of the region, the maximum development 

 of the larch is attained in moist draws or in fertile valleys not par- 

 allel with the direction of the prevailing winds. In such situations 

 the tree is usually quite free from mistletoe, and uninfected trees 

 often attain a diameter, breast high, of 60 inches or more. A full 

 crown composed of the original branches is retained until late in life, 

 the tree showing few defects except an occasional root-rot or a dead 

 top occasioned by agents other than mistletoe. These situations are 

 more favorable to the development of the host than to the mistletoe 

 occasionally found upon it and must be considered the best sites for 

 growing larch in these regions. On the drier slopes and benches, 

 where the larch is associated more with yellow pine, the influence 

 of the site on the vigor of the mistletoe is at once expressed by its 

 greater abundance and its effects on its host, causing smaller diame- 

 ters and thinner crowns on the infected trees. Occasionally trees in 

 exactly similar situations for some reason escape the ravages of the 

 mistletoe and attain a size of considerable proportions. The full 



