FOREST TYPES IX CENTRAL ROCKY MOUNTAINS. 133 



cover is secured earlier; and the low winter air temperatures are 

 much less conducive to evaporation. To the north, in the lodgepole 

 pine type, precipitation is insufficient to balance losses from May to 

 August, and a pronounced drought develops toward the end of sum- 

 mer. However, as early snows are heavy, there is no acute problem 

 of soil moisture, nor are there menacing evaporation stresses in winter. 

 No doubt the evaporation problem is present at low elevations in 

 Wyoming, South Dakota, and Montana; but in the Black Hills, 

 where yellow pine develops excellently both as to reproduction and 

 growth rate, it is safe to say that both soil and atmospheric dryness 

 are considerably modified by an abundance of snow and by low air 

 temperatures. 



TEMPERATURES AND TEMPERATURE GRADIENTS. 



The study in the Pikes Peak region has shown that a fairly regular 

 change in air temperatures occurs with change in elevation, amount- 

 ing to 2.8° F. per 1,000 feet for the year as a whole, 2.9° for the 

 growing season, and 3.3° at its maximum in April. This applies to 

 what may be* called " neutral" sites at all elevations except the 

 highest one. Early study showed the air temperatures 20 feet above 

 the ground to be not essentially different for opposing slopes at the 

 same elevation; but near the ground southerly exposures nave been 

 shown to be warmer than the mean for the elevation, and northerly 

 exposures cooler, to an extent compatible with the types which they 

 respectively encourage. These contrasts are more clearly seen in the 

 soil temperatures. 



The Wagon Wheel Gap locality in southern Colorado shows a 

 temperature gradient of about 2.3° F. per 1,000 feet for the growing 

 season. During the winter months this temperature gradient is not 

 infrequently inverted, and in exceptionally cold weather there may 

 be an inverted gradient of 4° per 1,000 feet, the lowest station here 

 being in a valley affected by cold-air drainage. At a middle eleva- 

 tion this locality is always colder than the Pikes Peak region. 



The relation of soil temperatures to air temperatures is by no 

 means constant for the different forest types. During the growing 

 season such differences as exist between air and soil temperatures 

 are largely the results of local insolation, which does not affect local 

 air temperatures so markedly as it does the temperature of the 

 ground near its surface. If tne year is taken as a whole, soil tem- 



Eeratures may not reflect the air temperatures of different localities, 

 ecause of the marked influence on the former of a light or heavy 

 snow blanket. 



Because summer soil temperatures agree more closely with the 

 tvpe differences produced by different exposures, no less important 

 tnan those produced by differences in elevation; because soil tem- 

 peratures may be seen to have a more direct influence on type com- 

 position at the time of initiation of seedlings; and because winter 

 soil temperatures appear to be indirectly related to survival, through 

 soil moisture, as fully as are air temperatures through evaporation, 

 the detailed consideration of soil temperatures has proved more 

 valuable in the present study than the consideration of air tempera- 

 tures, even though surface temperatures of the soil, which should by 

 all means be recorded, are largely lacking. 



