120 BULLETIN 1333, U. S. DEPARTMENT OF AGRICULTURE. 



Somewhat similar conditions are noted at the middle elevation at 

 Wagon Wheel Gap (Station W-Al) . Here, however, as farther south, 

 the marked deficit is in May and June. Although the winter precipi- 

 tation as a whole is adequate, the amount in November and Decem- 

 ber is hardly sufficient, in connection with the sudden drop in air 

 temperatures, to prevent deep freezing of the soil. 



The spruce type in the Wagon Wheel Gap locality, the only one 

 which we may compare with Station L-l in the Pikes Peak series, 

 does not show much more precipitation than the local station except 

 during the winter, and is much more liable to drought during the 

 summer. The Wagon Wheel Gap site produces a superior spruce 

 forest, but its conditions are not more conducive to abundant repro- 

 duction. 



The lodgepole t} T pes are all inferior to the control station in total 

 precipitation, and only hah as favorable in growing-season precipita- 

 tion. All, however, secure a better snow blanket in the winter, not 

 only because of larger snowfall, but because the snow is protected 

 by low air temperatures. Frequently in lodgepole sites the ground 

 is not exposed after the first heavy snow in September. It is seen, 

 then, that lodgepole receives and thrives with less precipitation 

 during the growing season than even yellow pine, but that, like 

 yellow pine, it is surrounded by conditions conducive to the main- 

 tenance of an unfrozen or only slightly frozen soil during the winter. 



SOIL MOISTURE. 



When it is observed that losses by evaporation may be two or three 

 times as great from one site as from another receiving the same pre- 

 cipitation, it is at once apparent that the record of precipitation is 

 almost without value in indicating the moisture conditions under 

 which the several species develop. If it is at all true that the amount 

 of moisture available, or its degree of availability, appreciably 

 influences the character of the plant formation, then an attempt 

 should be made to measure the soil moisture as directly as possible. 

 This should be done notwithstanding the fact that precipitation and 

 evaporation rates, when considered together, constitute a valuable 

 basis for comparing the moisture conditions of broad regions and the 

 general types of their flora. 



That the proper use of soil moisture data in a comparison of sites 

 is 1) v no means a simple matter becomes more apparent as the behavior 

 of the soil solution is more exactly studied. In the "Research 

 Methods in the Study of Forest Environment," (24) a serious effort 

 has been made to depict the soil-moisture problem as it actually 

 exists. In order that the viewpoint and treatment in that publica- 

 tion may be understood, it is sufficient to say that the soil solution is 

 regarded as having osmotic properties similar to those of the plant 

 solution, and that the osmotic pressure of (he soil solution is always 

 opposed to that pressure within the plant which gives rise to absorp- 

 tion by the plant (root) cells. If the plant pressure is increased In- 

 ordinary water loss through transpiration, or if the soil solution is 

 diluted, the rate of absorption must increase. If the plant pressure 

 is decreased or the soil pressure increased, the flow into the plant 

 must be decreased, stopped, or even reversed. 



