FOREST TYPES IN CENTRAL ROCKY MOUNTAINS. 85 



Larsen (IS), in his companion report on the types of Idaho and 

 vicinity, gives the results of systematic soil-temperature observations, 

 at depths of -J, 1, and 2 feet, extending over a period of several years. 

 One of the most interesting features of these data is that the soil of t be 

 white pine type, in the vicinity of the Priest River Forest Experiment 

 Station, which occupies a northeast slope that is almost without insola- 

 tion for several months, barely freezes to a depth of one foot. This is 

 apparently due to the fact that early snows are retained, the winter 

 blanket becomes very heavy, and radiation is so checked that much 

 of the summer heat is retained. (See Bouyoucos (8) .) 



It will be shown that lodgepole pine, in its optimum environment, 

 is similarly protected. It appears to be of great significance that 

 western white pine, perhaps the nearest relative of any of the eastern 

 conifers attaining a good development in the Rocky Mountain 

 region, does so under very low atmospheric temperatures, but witli 

 the resultant protection of a high relative humidity, and with its 

 moisture supply probably never completely cut off by the freezing 

 of the soil. A southwest slope, also in the vicinity of the Priest 

 River Forest Experiment Station, shows about the same soil minimum 

 at 12 inches; but its mean temperature for the whole "rest period" 

 is 3.3° F. higher than that of the northeast slope, and for the entire 

 year 4.6° higher. In neither place does the soil freeze to a depth 

 of 24 inches. This southwest slope, occupied by western yellow pine 

 and Douglas fir, shows a mean annual temperature at 12 inches depth 

 of 47.9° F., and a January temperature of 34.1°, both of which are 

 3.1° higher than the corresponding temperatures at Fremont (Sta- 

 tion F-2). The midwinter air temperatures, however, are lower 

 in the Idaho region. 



Special conditions affecting soil temperatures in this study. — Soil 

 temperatures have been observed at each of the special stations for 

 this study, including MonumenUand Foxpark, practically during the 

 entire period of the operation of the stations. In addition they nave 

 been recorded at each of the meteorological stations at Wagon 

 Wheel Gap, three of which are mentioned in this phase of the study. 

 A few scattered observations taken in connection with other pro- 

 jects will also be called into use. For the most part these were made 

 without any special preparation, that is, a thermometer was simply 

 inserted into a hole of the desired depth and protected from direct 

 air circulation. These observations are mainly for a depth of 12 

 inches. 



Prior to 1914 the three stations under observation at Fremont 

 were equipped with 1 and 2 foot soil thermometers. In 1914 it 

 was decided that the 2-foot depth had no individual significance, 

 and with the installation of several new stations the new ones and 

 the older ones were equipped for readings at 1 and 4 feet. This 

 arrangement has been standard since 1914. 



The stations at Wagon Wheel Gap were at first equipped with 

 1-foot thermometers only. During 1913, 4-foot thermometers were 

 added at all stations then in operation and the standard equipment 

 was placed at stations established in the fall of that year. The 

 1-foot thermometers have been in wooden tubes a greater part of 

 the time. The Foxpark station had 1 and 2 foot thermometers 

 from 1914 to April, 1916. The 4-foot thermometer was placed 

 when more extensive observations were begun in 1916. Local £ 

 tion F-13 (limber pine) had only 1 and 2 foot thermometers 



