CORRELATION OF VEGETATION AND CLIMATE. 103 



plant of higher range than Pinus cembroides we may take Quercus 

 hypoleuca, which ranges through 3,000 feet, with the usual alternation 

 at the top and bottom of its range. The ratios for its habitats are as 

 follows: North slope at 6,000 feet 16.0, average of north and south 

 slopes at 7,000 feet 16.6, south slope at 8,000 feet 3.9. Here is close 

 agreement of the ratios for the lower and central portions of the range, 

 with a much lower value for the top, indicating that in spite of the 

 ability of Quercus hypoleuca to withstand the conditions expressed by 

 the ratio of 16, it is likewise capable of withstanding the more favorable 

 water conditions indicated by the ratio of 3.9. Here, in other words, 

 is a typical Encinal plant, accompanied throughout its range by many 

 others, which is able to extend up to an elevation at which the water 

 conditions are much more favorable than they are in the lower part 

 of its range. This is a thing which the Desert plants do not do, and 

 the reason is undoubtedly that the plants of the Desert encounter 

 unfavorable temperature conditions at the same elevations at which 

 they begin to encounter more favorable water conditions, while such 

 a plant as Quercus hypoleuca is capable of withstanding the rigorous 

 temperatures of 8,000 feet and is thereby enabled to range upward 

 into a region of more favorable water conditions. 



Allusion has been made to the more pronounced character of the 

 effects of slope exposure at higher elevations. It is of interest in that 

 connection to contrast the ratios of evaporation to soil moisture for 

 similarly located pairs of habitats at low and at high altitudes. For 

 example, the north slope at 4,000 feet has a ratio of 33.1, the south 

 slope at 6,000 has a value of 33.0. To carry the comparison up 2,000 

 feet: the north slope at 6,000 feet has a ratio of 16.0, the south slope 

 at 8,000 feet has one of 3.9. The greater similarity of the radios for 

 the two lower habitats is in accord with the evidences from the vege- 

 tation (see p. 98), which indicate an altitudinal increase in the potency 

 of slope exposure in the determining of the vegetation. 



The fundamental causes differentiating the conditions on opposed 

 slopes are only partly comprised in the evaporation soil-moisture ratios. 

 The differences of evaporation rate on north and south slopes are 

 largely due to the dry, warm winds which ascend the mountain during 

 the day and partly to the differences of air temperature. The humidity 

 of the air shows only slight differences on opposed slopes. The soil 

 moisture on north slopes is higher than on south ones because of the 

 more direct insolation on south slopes, and because of the higher soil 

 temperature and increased soil evaporation which are due to this. In 

 addition to the differentiating features which are expressed in the 

 ratios, we have the differences of soil temperature, due to the direction 

 of slope, and the differences of air temperature, which are only partially 

 registered in their effect upon the evaporation rate. The increased 

 insolation on slopes as compared with level ground has been worked 



