128 BULLETIN 1233, U. S. DEPARTMENT OF AGRICULTURE. 



Table 43. — Minimum (month) "iveU" moisture during season of 1917 and -per- 

 centage ratio to mean growing-season moisture (in parentheses) . 



Group. 



Depth. 



face. 



1 foot. 



2 feet. 



(49) 



(78) 



(79) 



2.78 



5.91 



6.44 



(66) 



(65) 



(63) 



3. 37 



5.72 



6.40 



(66) 



(76) 



(75) 



5.67 



10.18 



11.08 



(38) 



(71) 



(73) 



3.84 



12.66 



14.53 



(35) 



(63) 



(62) 



3. OS 



6.66 



7.72 



(3S) 



(72) 



(78) 



3.29 



8.46 



9.49 



Control station 



Four yellow pine sites.. 

 Seven Douglas fir sites. 



Four spruce sites 



One lodgepole pine site. 

 One limber pine site.. . 



(78) 



7.15 



(65) 

 7.28 

 (79) 



12. 31 

 (73) 



15.66 

 (53) 

 8.11 

 (S3) 



10. S7 



The data in Table 43 do not appear essentially unlike those of 

 Table 42 in fixing the relative positions of the species, yet the paren- 

 thetical figures showing the relation between these minima and the 

 approximately normal growing-season condition are full of signifi- 

 cance. It is to be noted that the degree of exhaustion as indicated 

 by these percentages is nearly the same for the 1, 2, and 3 foot depths 

 with perhaps a slight tendency toward greater exhaustion at a depth 

 of 2 feet than elsewhere. On the other hand, although the yellow 

 pine sites do not show greater exhaustion at the surface than in the 

 deeper soil and the north-slope fir sites show only slightly greater 

 drying of the surface, the spruce sites, as well as the limber pine and 

 lodgepole pine sites, show very marked drying out. In the spruce 

 sites this can not be ascribed to insolation; it must be charged in 

 part to the loose duff which forms the surface layer and in part to the 

 prevalence of roots near the surface. As the soil wells can not pos- 

 sibly show either of these factors fully, it is evident that the natural 

 surface soil in a dense spruce forest must represent a very great 

 degree of drought. That is, perhaps, the most striking point that 

 has been brought out by all of tne soil-moisture 'determinations, 

 especially when one considers the importance already attached to 

 surface temperatures. Calculations as to the availability of the 

 moisture, made on the basis of moisture equivalents, indicate that 

 one of the two spruce sites for which the surface data are available 

 becomes physiologic ally drier than any of the others, and (he second 

 is drier than any except the warmest yellow pine sites and the wind- 

 exposed limber pine site. 



The attempt to reduce the data of Table 42, first, to terms of 

 moisture percentages for the native soil and then to terms of avail- 

 ability, does not materially alter the evidence of Tables 42 and 43, 

 namely, that the yellow pine sites are the most arid and that the 

 spruce situations are only slightly moister than the north slopes 

 which bear Douglas fir. On trie other hand, these calculations do 

 show that, ov\ing to a very sandy soil used in the soil well, the lodge- 

 pole site is considerably more favorable than the original moisture 

 data would indicate. The approaches, through osmotic equivalents, 

 through the direct ratio of moisture contents to moisture equivalents 

 or through consideration of the willing coefficients, all produce, as 



