106 VEGETATION OF A DESERT MOUNTAIN RANGE. 



It is possible to say, in brief, that the conditions presented by stream- 

 sides and flood-plains are such as to depress the ranges of very many 

 plants by as much as 1,000 feet, and of a few plants by amounts as 

 great as 2,000 feet. A depression of as much as 2,700 feet, found in 

 the case of Quercus hypoleuca, does not represent the lowest occurrence 

 of established plants, but rather a chance survival at an elevation in 

 which it would doubtless be impossible for the tree to reach maturity. 

 It can at least be said that throughout the entire length of Sabino 

 Canon, from the mouth to the Basin, there are no known occurrences 

 of full-grown trees or even shrubs of Quercus hypoleuca. 



The extent to which the types of vegetation are depressed in their 

 ranges by the influence of streams and flood-plains is about the same 

 as the average depression of the individual species, that is to say about 

 1,000 feet. In the case of the occurrence of a closed Encinal in the 

 Basin of Sabino Canon there has been a depression of 1,500 feet in 

 the limit of this type of vegetation from 5,500 to 4,000 feet. 



Some evidence has already been given leading to the view that the 

 lower limits of all Encinal and Forest plants are determined by those 

 features of the environment which in turn determine the water relation 

 of plants. The facts of the depression of vertical ranges by streams 

 form an additional evidence of this view. So far as concerns atmos- 

 pheric water-demand the plants growing beside streams are subjected 

 to the same conditions as plants of the nearby upland, but the conditions 



of water supply are infinitely better for them. In other words, in the 



v 

 ratio of evaporation to soil moisture, SM , the numerator is the same 



for stream-side and upland plants and the denominator is greatly 

 increased for the latter, thereby lowering the values for the ratio. In 

 the cases alluded to in which the lowest individuals of a species not 

 only grow in a flood-plain but in the shade of larger vegetation, the 

 plants are under ameliorated conditions with respect to the numerator 

 as well as the denominator in the ratio. 



A number of mountain plants are able to survive when taken down 

 to the Desert provided they are placed under conditions in which one 

 or both of the sets of conditions indicated by the above-mentioned 

 ratio are ameliorated. Parthenocissus from 6,000 feet survives with 

 irrigation and partial shade; Echinocereus polyacanthos, from 5,000 to 

 7,000 feet, survives with occasional irrigations during the arid fore- 

 summer; Zauschneria calif ornica, Aquilegia chrysantha, and Sedum slelli- 

 forme, all ranging from 5,500 to 7,500 feet, are capable of survival at 

 Tucson from year to year when grown in complete shade with frequent 

 irrigation during the arid fore-summer. These facts point to the ability 

 of such plants to withstand at least the shade temperatures of the 

 Desert, provided the moisture supply of the soil and the moisture 

 requirement of the air are made more nearly like those conditions in 

 the mountain habitats of the plants. Many introduced plants have 



