WATER UTILIZATION BY TREES fi\ 



he ranked plants in the following order, it being understood that the 

 larger is this fraction, the more water-demanding are the plants: 



Ratio 



Fagus sylvatica (shade) 153 



Larix europaea DC 133 



Fagus sylvatica (sun) 123 



Quercus pedunculata (shade) 109 



Q. cerris L. (young leaves) 88 



Q. pedunculata (sun) 84 



Betula verrucosa 84 



Quercus ilex (greenhouse) 70 



Abies pectinata (shade) 70 



Pinus austriaca 44 



P. cembra 44 



P. montana 42 



Picea excelsa (shade) 40 



Ilex aquifolium 40 



Abies pectinata (light) 39 



Picea excelsa (light) 30 



It is considered, therefore, that the problem at the present time 

 is to learn what the climatic conditions are in the various regions on 

 the surface of the earth and to correlate these with the vegetation 

 growing there. It can then be determined where these species may 

 be expected to grow outside of their natural range. 



Thus Whitfield (238), studying the vegetation of the Pike's Peak 

 section, found that Pinus ponder osa and Pseudotsuga taxi-folia domi- 

 nate the montane region while Picea engelmanni, Populus tremuloides, 

 and Pinus aristata are most important in the subaipine. At pro- 

 gressively higher elevations, both the air and soil temperature de- 

 crease, while the rainfall and relative humidity increase. In the 

 montane zone the transpiration is accordingly higher • than in the 

 subaipine, although lower than in the plain below. These results 

 agree with those found in general in mountains. 



Boysen-Jensen (19) took as his criterion the value of the relative 

 transpiration, it being understood that the smaller is this value the 

 more control the plant exercises over its transpiration and the drier 

 the natural habitat in which it might be expected to survive. 



On this basis Stocker (210) has assembled the transpiration data 

 from different parts of the earth and from various ecological regions 

 and has found that in general the relative transpiration of plant for- 

 mations is about as follows (in percent): Water plants, 27; swamp 

 and marsh plants, 16; plants of tropical rain forests, 13; mesophytes 

 of the Temperate Zone, 12; desert plants, 10; heath and bog plants, 5. 



Apparently, drought resistance may be attained in many different 

 ways. According to Kearney and Shantz (120), plants may be 

 classified as drought-escaping, drought-evading, drought-enduring, 

 and drought-resisting. 



Drought-escaping plants are those temporary annuals which 

 develop and mature very rapidly in periods when soil moisture is 

 abundant. They have, therefore, no methods of conserving moisture. 

 The drought-enduring plants are desert shrubs which, in times of 

 drought, shed their leaves, and go into a sort of dormancy when the 

 soil moisture reaches the wilting coefficient. Drought-resisting 

 plants are the succulents which have provisions for storing water, 

 united with arrangements for its conservation. Drought-evading 

 plants are those which delay the ultimate exhaustion of a limited 

 water supply by their economic utilization of the supply. 



