CORRELATION OF VEGETATION AND CLIMATE, 93 



wise increased in the former season to an amount that would greatly 

 reduce the values of the ratio if determined for the humid mid-summer. 

 The ratio of evaporation to soil moisture is not in itself a full index 

 of the comparative aridity of Desert, Encinal, and Forest, for the con- 

 ditions expressed by the ratio are of much longer duration at 3,000 

 feet than at 8,000 feet. The shortening of the arid fore-summer from 

 16 weeks at 3,000 feet to 7 weeks at 8,000 feet (see fig. 2) signifies that 

 the most severe drought conditions of the year are more than twice 

 as prolonged at the lowest elevation as compared with the uppermost. 

 It is necessary here to bear in mind that the effects of drought on plants 

 are cumulative, and that, for example, a period with a given set of 

 conditions of increasing aridity which endures for 16 weeks may be 

 twice as fatal or deleterious as a period that lasts for 14 weeks. For 

 purposes of general climatic description, however, the values of the 

 ratio of evaporation to soil moisture multiplied by the duration of the 

 arid fore-summer may be taken as an index of the aridity of the several 

 elevations (see table 20) . 



Table 20. — Average daily evaporation (E) and the moisture of the soil (SM), together with 

 the ratio of evaporation to soil moisture [ -^-j-^ I for north and south exposures at six eleva- 

 tions in the Santa Catalina Mountains for the arid fore-summer of 1911. 



The ratio of evaporation to soil moisture comprises a measurement 

 of all the external factors which affect the water relations of plants, 

 except the influence of radiant energy on transpiration and the possible 

 effects of soil temperature on this function. It is accordingly unneces- 

 sary to give further consideration to rainfall, which is not in itself a 

 factor for vegetation, at least in such a region as Arizona. If any 

 differences existed between the seasonal distribution of rainfall at 

 different elevations in the Santa Catalinas the fact would be of great 

 importance to the vegetation, but only in the effect it would have on 

 the annual march of the soil moisture conditions. The evidences of 

 observation and instrumentation have shown that the major drought 



