Southeastern Washington and Adjacent Idaho. 83 
Where the growth is rather pure, the trees, a foot or more in 
diameter, are pruned high and the dense shade permits of a very 
_ sparse ground cover. In such situations, the light intensity may 
fall to 0.01 or even lower. Except fora few specimens of Coptis 
occidentale, Anemone piperi, Moehringia macrophylla, Thalictrum 
occidentalis, or Vaccinium macrophyllum the ground may be 
quite bare. Douglas fir grows only poorly at light values of 0.025. 
Such forests may completely surround bare rock-topped moun- 
tain peaks, but more often give way to shrubs or small areas of 
grass land. Except in such situations I have never seen Douglas 
fir invading prairie. 
Douglas fir is far more common in mixture with Larix and 
white fir than in forests of which it is the dominant. The per- 
centage of Douglas fir is usually determined by the slope and ex- 
posure and consequent soil moisture. While Larix and white fir 
do not grow in as dry soil as Douglas fir, the latter is less domi- 
nant in moister situations because of its greater light requirements. 
Larix is slightly, and white fir much more tolerant than Douglas 
fir. 
The Larix-Abies Associes 
This associes makes up by far the most important forests of 
the region. Great stretches of moist mountain slopes are covered 
with forests of which either Larix or Abies are dominant or more 
often the two form a mictium. The Larix consocies, dominated 
by Larix occidentalis, occupies the higher slopes of lower water 
content intergrading from the Douglas fir above to the more meso- 
phytic white fir-cedar mixture of the moister slopes, ravines, and 
valleys. 
It has already been pointed out that the larch is only slightly 
less xerophytic than the Douglas fir. Its place in the succession 
is intermediate between the Douglas fir and the white fir. Its 
ability to withstand the wind-storms that uproot white pine and 
white fir is an index of a well developed root system, and this 
probably accounts for its ability to grow in soils where the surface 
layers are quite dry. 
The absence of Larix seedlings in a mature forest shows that 
83 
