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 ratios 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 
