88 A Study of the Vegetation of 



the deeper strata are very moist. Outside of meso-hydrophytic 

 situations it does not flourish in the region and even here it does 

 not reach the usual size of the species. It sometimes forms a 

 very broken and fragmentary zone between the alder-willow com- 

 munity and the climax cedar forest. Its ability to tolerate shade 

 to a greater degree than Douglas fir or tamarack probably ac- 

 counts for the presence of its seedlings in drier situations, for I 

 have shown that conditions under typical stands of white fir and 

 tamarack are, as regards the seedling stage, fairly mesophytic. 

 Likewise, its power to germinate in the open, providing there is 

 sufficient soil moisture, accounts for its presence, often in dense 

 stands, in situations where drifts of melting snow keep the soil 

 well moistened in otherwise apparently rather dry, rocky situ- 

 ations. Like other shade-enduring species, it makes a more rapid 

 growth in these situations than in the shade. 



The Thuja Consociation 



The climax forest community of the region and the one in the 

 xerosere with the lowest light intensity, greatest soil and air 

 humidity as well as lowest temperature, is that dominated by the 

 cedar, Thuja plicata. The best protected and most mesophytic 

 mountain slopes as well as the well watered ravines are clothed 

 with a growth of this cedar. These often form 90 per cent, or 

 more pure stands and the trees may reach diameters of 3-5 feet. 

 In such well developed forests, the light intensity ranges from 

 0.015 to 0.005 an d even to 0.003, an d except for the seedlings of 

 the cedar the ground may be quite bare. But usually the forest 

 canopy is sufficiently open to permit a growth of a few shade- 

 enduring shrubs and numerous herbs. Of these the following are 

 typical of the consociation : 



Ribes lacustre Rubus parviflorus 



Aconitum columbianum Circaea pacific a 



Anemone piperi Claytonia asarifolia 



Asarum caudatum Clintonia uniflora 



Athyrium cyclosorum Coptis occidentalis 



88 



