30 A Study of the Vegetation of 



At the stations on Kamiak Mountain where evaporation data 

 were obtained, water content samples were taken at the same 

 intervals of time. These data have been plotted in graphic form 

 in Fig. 7. The water content of the soils of the pine" consocies 

 was obtained from the station at the north base of the mountain, 

 that of the shrub from the Opulaster consocies on the northeast 

 slope. The gradual depletion of the water content of these soils 

 to the end of the growing season is characteristic. To fully ap- 

 preciate the conditions represented on the chart, a description of 

 the soils is necessary. 



A mechanical analysis of the soils shows that those of the 

 prairie are somewhat coarser textured than those of the fir- 

 tamarack community (Table II). Likewise, a chemical analysis 

 of these two types of soil shows that among other differences 

 there is a markedly lower humus and total nitrogen content in the 

 soils of the forest (Table III). This is due to fires burning out 

 the organic matter. 



The greatest difference in the soils, however, and the one which 

 most profoundly affects the vegetation in this semi-arid region is 

 their water holding capacity. An average of 6 tests showed the 

 prairie soils on Kamiak had a water holding capacity of 48 per 

 cent., while the finer soils of the north slope which support a 

 forest of fir and tamarack hold 78 per cent, of their dry weight 

 of water. The water holding capacity of the soil of the pine con- 

 socies was intermediate between the preceding. We may com- 

 pare the soils of this region to a gigantic reservoir replenished 

 mostly during the non-growing season and rather thoroughly 

 emptied of its water during the summer. 



The actual amount of water available to the forest- and shrub- 

 covered soils of the protected slopes and ravines is often consid- 

 erably greater than that of the prairie. Snow is blown from the 

 wind-swept prairie and deposited on the sheltered slopes. I have 

 seen such drifts 20-25 feet deep along the brow of Kamiak Moun- 

 tain. The late melting of the snow makes a locally late spring 

 and consequently shortens the period of drought in late summer. 



Soil samples taken on Kamiak Mountain in May and in July, 

 1914, bear out the conditions found in the previous year, but 



30 



