84 



SOIL TEMPERATURES. 



Before the initiation of this project it had been observed that the 

 surface temperature of sandy soils in Nebraska might be high enough 

 aidday to injure young planted trees of yellow pine and jack pine 

 and that they apparently had the effect of drying the lightly protected 

 stems just at the root collar. It was also thought, after observation of 

 the vegetational difference between north and south exposures in the 

 Rocky Mountain region, that soil temperatures might express more 

 fully than any other measure the cause of this difference, which 

 results primarily from the amounts of insolation received. In the 

 preliminary report {3) on this project, soil temperatures were con- 

 sidered only as climatic elements expressing the possible length of the 

 growing seasons. There is still much doubt among ecologists as to 

 whether these temperatures have the greater significance in connec- 

 tion with climatic or with soil conditions ; but it is now evident that a 

 complete separation of the two sets of conditions is out of the ques- 

 tion, and that soil temperatures form an important link between 

 them. That they have been looked upon as results of rather than 

 features of the weather conditions is indicated by the fact that the 

 Weather Bureau has never recorded soil temperatures on an extensive 

 scale except in connection with a few special studies, such as the 

 study of streamflow now being conducted at Wagon Wheel Gap, Colo. 

 The factors influencing soil temperatures, at a given time and place, 

 have been investigated, notably by Bouyoucos (7). The Wyoming 

 Agricultural Experiment Station lias for a few years recorded soil 

 l emperatures at Laramie. A great many other records have no doubt 

 been obtained, but nothing comparable to the great organized mass of 

 air temperature records. During the last four years the Ecological 

 Society of America has lent its influence to the organization of a more 

 or less complete soil-temperature survey of the United States; and as 

 a great in any competent observers have thus become interested in the 

 subject it is to be hoped that comparable data will soon be available 

 for different regions. 



Pearson (17) , in his study of the yellow-pine forest and parks, made 

 comparative observations for only four months, July to October, 

 during which time, at a depth of 2 feet, he found the forest soil to be 

 :>.l degrees cooler than the open park. In February, 1913, he also 

 made some examinations which are of interest here*. The day was 

 warm, and all snow had been melted by the direct and reflected light 

 on the south side of a pine \vvv, while on the north side, and in an 

 opening, the >no\v cover was from (i to S inches dee]). In the first 

 position there was no trace of frost in the soil; in the second it ex- 

 tended down l:; 1 , inches, and in the open park 23 inches. This single 

 rvation. whose genera] import is indicated by data to be pre- 

 ented, is of principal interest in showing the conditions which young 

 Uings musl contend with before the forest is established; for 

 Pearson ( /' ; > has also shown that seedlings in tin 4 pine type are com- 

 pelled by summer beat and drought to adhere very closely to the 

 ded spots, In a later report Pearson {19) has given more com- 

 plete data for all of the forest types, and these will later be compared 

 with the data of the present investigation. 



Shreve '//), in bis study of mountain vegetation, has given only 



slight attention to soil temperatures. The few records of minima 

 which were obtained seem to have no bearing on the present study. 



