FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 129 



between the forest types, essentially the same relations. All tend to 

 show that the differences between sites in availability of the moisture 

 are less than are indicated by the bare moisture contents. 



Rather for the purpose of illustration than because of the value of 

 the data, calculations of the probable natural soil moisture and of the 

 availability of this moisture are presented in Table 44. These calcu- 

 lations can not be extended to the surface-moisture values, because 

 the native surface soils have not been physically' examined. More- 

 over, such calculations would be of little value, because the surface 

 soils of the wells are so directly influenced by atmospheric conditions. 



Probably the most valuable feature of Table 44 is the clear way in 

 which it shows that the high moisture percentages obtained with 

 the heavy soil at Station D, Wagon Wheel Gap, are really on a level 

 with those for the local spruce stations. The slightly higher avail- 

 ability at Station D is easily explained by the almost complete 

 absence of forest. Similarly, although the calculations can not be 

 carried out for the two Douglas fir stations, the high wilting coeffi- 

 cients indicate that Station W-Al (north exposure) must be just 

 about on a par with similar local stations, but that the strongly 

 exposed Station W-A2 is certainly much more arid. 



As has been pointed out, these calculations show the lodgepole 

 pine site to be very moist. This may be due in part to the fact that 

 only a few trees surrounded the soil well, but is probably the result 

 of low evaporation stresses. The limber -pine site is seen to be in a 

 class with the average yellow-pine site. Among the local stations 

 subject to practically the same atmospheric conditions it is evident 

 that direct insolation tends to keep the more insolated pine sites in 

 a drier state than the others; but the very slight difference between 

 fir slopes and spruce bottoms after a prolonged period of drying 

 indicates that each soil, through direct drying and the drying accom- 

 plished by the trees, tends to come into vapor-pressure equihbrium 

 with the atmosphere. The probabilities are, therefore, that in the 

 event of more prolonged drought, sufficient to permit an equihbrium 

 to be reached, equally insolated soils will attain the same status quite 

 regardless of the character and amount of vegetation thereon. This 

 is of the utmost importance, indicating, in the present case, that the 

 moisture supply is effective only in controlling the number of in- 

 dividuals and can not differentiate between species, at least as be- 

 tween spruce and fir sites. 



It is fairly evident, then, that soil moisture below the surface 

 layer is only an incidental factor in the distribution of the several 

 species. Strongly insolated sites and those exposed to the higher 

 temperatures and to the greater atmospheric stresses of low eleva- 

 tions, as might be expected, are always the driest, but their most 

 important feature, owing to the close connection between insolation, 

 surface temperatures, and surface moisture, is doubtless the very 

 rapid drying of the surface layer after every rain, rather than any 

 decided lack of moisture at greater depths. 

 73045°— 24 9 



