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 o.oi or even lower. Except for a few specimens of Coptis 

 occidentale, Anemone piperi, Mo [ ehringia 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 



