68 CARNEGIE INSTITUTION OF WASHINGTON. 



and of individuals, is found on the playa soil, where the average of two 

 areas showed 223 plants of only 4 species. Each plant on this soil 

 has an average space of slightly less than 50 square meters. The 

 richest vegetation, in both species and individuals, is found on the 

 coarse outwash, where there is a total of 1,494 individuals of 16 species. 

 Each plant in this habitat has slightly more than 6 square meters of 

 space. Other areas enumerated showed populations intermediate in 

 size and variety between the playa and coarse-outwash soils. In an 

 outwash area dominated by covillea, this plant was found to form 87 

 per cent of the population, whereas in the coarse-outwash area three 

 of the commonest species formed over 15 per cent each and two others 

 over 10 per cent. These statistics have been confined to the perennial 

 woody plants. 



Relation of Slope Exposure to Soil Temperature, by Forrest Shreve. 



The universality of the influence of slope exposure in modifying 

 the character of the vegetation, outside of tropical latitudes, indicates 

 that the angle of incidence of the sun's rays is the fundamental deter- 

 minant of the environmental differe.ices between opposed slopes 

 which face north and south. It would appear that the temperature 

 of the soil is the immediate factor by which differences of insolation 

 affect the other aspects of the environment, such as the rate of water- 

 loss, the warming of the lowest layers of the atmosphere, and the length- 

 ening or shortening of the growing season. During April and May 

 1921 a series of readings at a depth of 3 inches was taken by the soil 

 thermograph on north and south slopes at 3,000, 4,000, and 5,000 

 feet in the foothills of the Santa Catalina Mountains, near Tucson. 

 The soil was a closely similar granitic loam in all cases, and the slopes 

 were approximately 15° from the horizontal in each mse. Toward 

 the end of the period of observation the maximum soil-temperatures 

 increased with altitude, which confirms similar observations made in 

 former years with soil thermometers. Throughout the period the 

 maximum readings for south-facing slopes were slightly lower than 

 those for north-facing slopes. This result appears to have its only 

 possible explanation in the fact that the late afternoon sun falls on 

 the north-facing slop»e more directly than on the south-facing slope 

 a ad, in the absence of a plant covering, causes the already warmed 

 soil to attain a slightly higher temperature in the late afternoon, an 

 explanation which is confirmed on some of the records by the later 

 occurrence of the maximum on the north-facing slopes. 



In order to compare the influence of slope exposure on soil-tempera- 

 ture under dissimilar climatic conditions, two soil thermographs were 

 installed on north-facing and south-facing slopes near the sea at a 

 point about 5 miles south of Carmel, California. In order to compare 

 the soil-temperature conditions outside of the fog belt with those 



