38 ROOT DEVELOPMENT IN THE GRASSLAND FORMATION. 



more so than the constantly higher evaporating power of the air at the 

 upland station. Greater wind movement at the upper station is an 

 important factor in causing the difference in evaporation, although 

 humidity also requires consideration. The greater evaporating power 

 of the air at the upland station, especially during periods of stress, is 

 alone sufficient to cause considerable difference in the plant popula- 

 tions and, when coupled with water-content, satisfactorily accounts 

 for the variations in the grassland vegetation. From July 7 to August 

 18, the average daily evaporation was 64 per cent greater at the upland 

 station. During this period the average daily evaporation rates at 

 the two stations were 20.8 and 34.1 c. c. respectively. 



Thermograph records of soil temperatures at the two stations at a 

 depth of 6 inches show no marked differences. The daily range at the 

 lowland station was usually 4° to 7° F. ; in the drier soil of the upland 

 station 4° to 12° F. The minimum soil temperatures were 2° or 3° F. 

 lower at the former station and the maximum 3° to 9° F. higher on the 

 upland. Soil temperatures varied from a minimum of 53° F. (average 

 60° to 65° F.) late in May, when the instruments were installed, to a 

 maximum of 85° F. (average 70° to 78° F.) in August. Except in 

 early spring, when low temperatures on the lower, wetter soil may 

 retard growth, while higher temperatures of the soil at all depths on 

 the upland may facilitate absorption, it is not probable that the effects 

 of temperature in the development of vegetation at the two stations is 

 very different. However, as previously pointed out, our knowledge 

 of soil temperature as an ecological factor is far from comprehensive. 



A comparison of the two sets of thermograph records shows that differ- 

 ences of air temperature greater than 5° F. are rare at the two stations. 

 It seems certain that such small differences in temperature variations in 

 the range of the growth conditions (minimum 65° F. late in May, maxi- 

 mum 105° F. in July and August) would be almost negligible in the 

 development of natural grassland. Selecting the week ending July 14 

 as illustrative, the maximum temperatures of 93° to 105° were reached 

 at about 4 p. m., the minimum temperatures of 68° to 72° F. at 6 h 30 m 

 a. m. Not only are the amplitudes of the graphs on the record sheets 

 at the two stations similar, but their courses are almost identical. 

 Greater wind-movement on the upland frequently causes greater minor 

 fluctuations. The night temperatures at the lower station are some- 

 times a few degrees lower (probably due to cold-air drainage on still 

 nights), but quite as frequently they are slightly higher. The average 

 day and night temperatures throughout the growing season at the high- 

 prairie station may be found in figure 9. 



These differences in habitat factors, and especially in air and soil 

 moisture, reflect themselves variously in the structure and root develop- 

 ment of the vegetation. The structure of the vegetation at the upland 

 station has already been discussed. The low-prairie area is dominated 



