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 (EF) and the moisture of the soil (SM), together with 
the ratio of evaporation to soil moisture em) for north and south exposures at six eleva- 
tions in the Santa Catalina Mountains for the arid fore-summer of 1911. 
: Rupe- . E Duration of 
Elevation. mara Vegetation. E SM _— arid 
F S M_ | fore-summer. 
3,000 feet.......| §& Desert. ...... 101.1 2.0 50.5 16 
4,000 feet.......] 8S Desert....... 80.4 2.0 40.2 
4,000 feet.......] N Encinal...... 82.7 2.5 33.1 14 
5,000 feet.......) S Encinal...... 61.7 3.1 19.9 
5,000 feet.......] N Encinal...... 74.4 3.5 21.3 13 
6,000 feet.......} 8 Encinal...... 59.4 1.8 33.0 
6,000 feet.......) N Forest....... 56.1 3.5 16.0 11 
7,000 feet.......] 5S Forest....... 62.8 2.6 24.1 
7,000 feet.......) N Forest....... 49.9 5.5 9.1 9 
8,000 feet.......] 8S Forest....... 29.3 7.4 3.9 
8,000 feet.......] N Forest....... 29.4 11.3 2.6 7 
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 
