18 BULLETIN" 418, U. S. DEPARTMENT OF AGRICULTURE. 
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 locality. In the Blue Mountains western larch 
(Larix occidentalis) is its usual companion and grows with it in an 
intimate and harmonious mixture. In the moist er 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 in many-aged stands; i. e., trees of 
all 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 found in a 
