DEPARTMENT OF BOTANICAL RESEARCH. 67 



season ; a closer correlation of the rainfall of nearly all month groups with the 

 stump data than with the average data from the 10 sections. 



These results indicate a positive correlation between the growth of pine 

 and redwood and seasonal rainfall. Also that the growth in thickness of the 

 basal portion of the trunk is influenced more by rainfall than is the growth of 

 the trunk as a whole, and that the total annual growth performance of these 

 trees, in the California coastal climate, is dependent on the precipitation of the 

 two or three months which precede the initial spring growth. 



Relation of Altitude and Slope Exposure to the Temperature of the Soil, 



by Forrest Shreve. 



During the summer of 1922 a series of six soil thermographs was maintained 

 on the Santa Catalina Mountains, near Tucson. The instruments were in 

 pairs at 7,000, 8,000, and 9,000 feet, one of each pair being installed on a 

 north-facing slope and the other on a south-facing slope. The gradient of 

 the slope in each case was close to 15° from the horizontal. At 7,000 feet 

 the soil was loam of granitic origin, very poor in organic matter, at the other 

 elevations a clay loam overlying diorite and rich in humus. The bulbs of the 

 thermographs were placed at a depth of 3 inches in situations with natural 

 vegetation, typical of the several elevations. The surface above the bulbs 

 was covered with a litter of pine or oak leaves at the two upper stations and 

 was relatively bare at the 7,000-foot station. The shading of the soil varied 

 at the different stations, in keeping with the difference in the character of the 

 vegetation, from an almost continuous shade at the station on the north- 

 facing slope at 9,000 feet to almost complete insolation on the south 'slope at 

 7,000 feet. This circumstance is calculated to augment the differences to be 

 expected at such stations, but the results indicate the conditions actually 

 encountered by the vegetation much more nearly than would be the case if 

 each instrument was installed so as to secure continuous insolation. 



In a similar series of observations taken in 1921 at 3,000, 4,000, and 5,000 

 feet (see Annual Report, 1921) it was found that during April and May the 

 maximum soil-temperature increased with altitude, and that the maximum 

 readings for north slopes were slightly in excess of those for south slopes. As 

 the general validity of these results at other elevations and other seasons 

 seemed highly questionable, the 1922 observations were planned to cover a 

 longer period at elevations exhibiting a more pronounced differentiation of 

 vegetation and on slopes of slightly greater gradient than those employed in 

 1921. The data for the first 14 weeks have been analyzed on the basis of the 

 calibrations of the instruments at the time of installation. The first seven 

 weeks were dry, the second seven intermittently rainy. For the stations- at 

 8,000 and 9,000 feet there is a consistently higher soil-temperature on the 

 south slopes than on the north slopes, the average difference at the former 

 elevation being 15.2° for the dry weeks and 11.0° for the wet ones, and at the 

 latter elevation 13.6° for the dry weeks and 11.7° for the wet ones. For the 

 station at 7,000 feet the average temperature, derived from the daily maxima 

 and minima, was higher on the south slope by 8.9° in the dry weeks and 8.3° 

 in the wet ones. There were also four weeks on which the average weekly 

 maximum for the north slope approached that for the south slope within 3° to 

 5°, these being the hottest weeks of the entire period. These results approach 

 the character of those secured at the lower elevations in 1921. 



