18 BULLETIN 418, U. S. DEPARTMEN'T OF AGRICULTUEE. 



canyon sides, to draws, and to the north hillsides, the intervening 

 drier and more exposed areas being treeless. Instances are seen here 

 and there where an advance guard of young yellow pines is progressing 

 from the forest into the desert, indicating that the limits of the yellow- 

 pine forest are being extended in places. Bordering the desert it is 

 often in mixture with western juniper (Juniperus occidentalis). At 

 the upper altitudinal limit of its distribution the typical yellow-pine 

 forest gives way rather suddenly to a very different, much denser 

 stand of other species. 



ASSOCIATES. 



The associates of yellow pine in mixed stands are variable, and 

 depend upon the locahty. In the Blue Mountains western larch 

 (Larix occidentalis) is its usual companion and grows with it in an 

 intimate and harmonious mixture. In the moister situations white 

 fir (Abies concolor) is a common associate, as is also Douglas fir 

 (Pseudotsuga taxifolia) in most parts of the State. All of these 

 species occur to a large extent in groups by themselves ; in the Blue 

 Mountains it is common for the south slopes to be covered with a 

 fine stand of yellow pine, while the north slopes are covered almost 

 entirely with larch, white fir, and Douglas fir. Lodgepole pine (Pinus 

 contorta) is another common member of the mixed forests, particu- 

 larly along the eastern slopes of the Cascades. It is a thrifty and 

 militant species, and has the ability to occupy burns to the exclusion 

 of all others. With the help of periodic surface fires, which have 

 encouraged its reproduction and at the same time discouraged the 

 reproduction of yellow pine, it has been able to encroach upon land 

 where yellow pine might be growing. 



On the southern Cascade and Siskiyou Mountains (Klamath, 

 Jackson, and Josephine Counties) the forest is different from that in 

 the drier parts of the State. In these two ranges the yellow pine is 

 intimately mixed with sugar pine, Douglas fir, white fir, and incense 

 cedar, and occurs in the largest proportion on southerly exposures. 

 On the cooler, moister situations it gives way to heavy stands of 

 Douglas fir. Here there is ordinarily a great deal of underbrush and 

 chaparral, and the more open the woods the greater the amount of 

 brush. In this region its usual mature size is larger than in the other 

 parts of the State. 



DISTRIBUTION OF AGE CLASSES. 



Yellow pine grows commonly m many-aged stands; i. e., trees of 

 aU ages from seedlings to 500-year-old veterans, with every age grada- 

 tion between, are found in intimate mixture. In some stands there 

 is a preponderance of very old trees; in fact, in many of the virgin 

 stands of central and eastern Oregon there are more of the very old 

 trees and less of the younger than the ideal forest should contain. 

 Usually two or three or more trees of a certain age are foimd in a 



