MESA AND FOOTHILL VEGETATION 



2 7 



Other factors are, however, of importance. The mesas are not uniformly 

 covered with trees even where the soil is coarse. North slopes have 

 much more timber on them than south slopes. Here, evidently, soil 

 moisture, evaporation and direct heat of the sun play an important 

 part. 



Thermograph Records. — To compare climatic conditions on the 

 mesa and adjacent plain two thermographs were kept running during 



TABLE VII 



Mean Temperatures* for the Spring Months of 1908 at University Campus and 

 Horse Mesa (Mean = Maximum + Minimum -7-2) 



* The mean temperatures for the two stations except for May are nearly the same, although, as shown 

 in Table VI, the maxima and minima differ considerably. 



TABLE VIII 

 Comparison of Temperatures at Campus and Mesa, May, 1908 



Monthly mean* 



Mean maximum 



Mean minimum 



Mean daily range 



Number of days on which the temperature 



went to 32 degrees or lower 



Date of latest frost 



Mesa 



54-1 

 61.0 



41-5 



19-5 



2 

 May s 



* Calculated by the formula 7 a. m. +2 p. m. + 9 p. m. +9 p. m.h-4 =mean. 



the spring of 1908. The months of March, April and May are critical 

 ones for most plants in this region. Plants which get through the winter 

 successfully are often killed by late frosts following warm weather in 

 March and April. The instruments used in the present study were 

 placed in suitable shelters of identical pattern, one on the campus of the 

 University at an altitude of 5,420 feet, the other on Horse Mesa, a mile 

 to the south, at an altitude of 5,835 feet. The campus station, although 

 so close to the mesas and foothills, may be considered as at the upper 



