CORRELATION OF VEGETATION AND CLIMATE. 107 



shown themselves incapable of withstanding the atmospheric aridity 

 at Tucson even when grown under the most liberal irrigation. The 

 inability of a plant to pass water on to its transpiring tissues as rapidly 

 as it is withdrawn by a desert atmosphere is undoubtedly a feature 

 common to very many mesophilous plants, and it is apparently the 

 cause which prevents a greater number of palustrine mountain plants 

 from descending the large streamways to the Desert, and it doubtless 

 prevents a lower descent upon the part of many Forest species which 

 reach the flood-plains of the Lower Encinal. 



It might be argued that the low occurrence of Forest along the 

 streams of the Encinal and the descent of the Encinal into the Desert 

 Slopes are due to the influence of cold-air drainage rather than to the 

 effects of soil moisture, or that cold-air drainage is at least an important 

 contributory factor. It is difficult to believe that low temperatures, 

 especially those of the winter months, should be a favoring factor for 

 plants which are subjected during the day to just as high temperatures as 

 are the plants of the upland. During the summer months the low noctur- 

 nal temperatures might be of some slight importance, but such importance 

 would reside solely in aiding the plant to recover from the excessive 

 transpiration of the preceding day and to build up a reserve of water 

 against the transpiration of the following day, as has been shown by 

 Edith B. Shreve to occur in Parkinsonia microphylla.* The facts that 

 it is the highest diurnal temperatures that are apt to be deleterious 

 to low-ranging mountain plants and that their effect can be only 

 indirectly and slightly offset by the lowest nocturnal temperatures 

 make it appear that cold-air drainage has at least a very minor r61e 

 in this connection as compared with the moisture conditions. 



THE ROLE OF TOPOGRAPHIC RELIEF. 



Each of the leading types of vegetation in the Santa Catalinas 

 reaches the uppermost limit of its occurrence on ridges and high south- 

 facing slopes. This carries the Desert upward into the Encinal and 

 carries the Encinal up into the Forest in such a manner that there is 

 an interdigitation of the vegetistic regions throughout the portions of 

 the mountain in which the topography is mature enough for it to be 

 manifest. This appearance of interdigitation is partly brought about by 

 the influence of streams (which has just been discussed) and is some- 

 times merged with the influence of slope exposure. These facts do not 

 in the least obscure the high range of each type of vegetation on the 

 narrow ridges which point due south or north and are therefore free 

 from the influence of slope exposure. 



On the ridges which lie between the tributaries of Soldier Canon 

 have been found the highest individuals of all of the characteristic 



* Shreve, Edith B. The Daily March of Transpiration in a Desert Perennial. Carnegie Inst. 

 Wash., Pub. 194, 1914. 



