102 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



Neither the data for soil moisture nor those for evaporation show 

 the exact alternation exhibited by the vegetation itself, by virtue of 

 which a given north slope is similar in vegetation to a south slope 

 about 1,000 feet above it, and a given south slope is similar to a north 

 slope about 1,000 feet below it (with the exception of the highest 

 altitudes). The conditions of evaporation found through the range 

 of Pinus cembroides, which has been used as an example of the effects 

 of slope exposure, are indicated in figure 18, where curves are given 

 showing the seasonal march of evaporation on a north slope at 5,000 

 feet, the average of the evaporation on north and south slopes at 6,000 

 feet, and the amounts on the south slope at 7,000 feet. These curves 

 follow a course which is parallel and indicate evaporation conditions 

 which are remarkably 

 similar for the lower, cen- 

 tral, and upper indi\dd- 

 uals of this pine, except 

 for the higher rate at 

 5,000 feet during the arid 

 fore-summer. 



The ratios of soil mois- 

 ture to evaporation at 

 different altitudes have 

 been worked out sepa- 

 rately for the north and 

 south exposures at the 6 

 stations (see table 20). 

 Since these ratios are an 

 expression of the conditions of the arid fore-summer they must be 

 taken as elucidating only those phases of slope exposure which are 

 themselves due to the climate of that season. Any subsidiary in- 

 fluence of temperature in affecting the slope exposure phenomena of 

 vegetation in the Santa Catalinas still awaits a full investigation. 



The comparative conditions of the lower, central, and upper habitats 

 of Pinus cembroides may be again investigated in the light of the ratios, 

 which are as follows: North slope at 5,000 feet 21.3, average of north 

 and south slopes at 6,000 feet 24.5, south slope at 7,000 feet 24.1. These 

 figures indicate a still more remarkable similarity of the water con- 

 ditions in the three habitats than the evaporation figures do. To 

 compare a plant of lower range we may take Agave schottii, which 

 encounters conditions expressed by the following ratios : north slope at 

 4,000 feet 33.1, average of north and south slopes at 5,000 feet 20.6, 

 south slope at 6,000 feet 33.0. These figures fail to show as close 

 agreement, but indicate a close similarity of the water conditions at 

 the lowest and uppermost habitats, and a more favorable set of con- 

 ditions in the central habitat. To make a similar comparison for a 



Fig. 18. — Graphs showing seasonal march of rate of evap- 

 oration for a north slope at 5,000 feet (dotted line), the 

 average for a north and a south slope at 6,000 feet (solid 

 line), and for a south slope at 7,000 feet (broken line). 



