WATER UTILIZATION BY TREES 75 



WATER CONSUMPTION BY TREES 



All of the preceding work provides a basis for a discussion of the 

 crux of the problem: How much water do trees really need? What 

 are their actual minimum and optimum water demands? Methods 

 of measuring water consumption, as found in the literature are, in 

 general, three, which may be outlined as follows: 



1. Transpiration per unit leaf area from specimens (a) in pots, or 

 (b) cut from trees and applied to large trees in the open; 



2. Transpiration per unit mass of leaves based on (a) fresh weight 

 or (b) dry weight ; these also may be taken from potted plants or from 

 cut twigs and then applied to entire stands or individual trees; 



3. Water consumption (mostly transpiration) per unit of total dry 

 matter — water requirement. 



The term "water requirement", introduced by Briggs and Shantz 

 {22), as used here does not mean the water consumption referred to 

 leaf area or mass but to the total dry matter produced by the plant. 

 Forests are crops, and the silviculturist is interested in knowing how 

 many grams of water are required to produce a gram of crop material. 



These various methods of calculating water consumption have their 

 advantages and disadvantages, which should be considered in determin- 

 ing the value of a given piece of work. In calculating the needs of a 

 stand or of a crop grown under field conditions from pot experiments, 

 certain deviations are to be expected. Briggs and Shantz (22) found 

 that the water requirement of field crops was a bit higher than the 

 figures as determined from potted plants, while Leather (184) found, 

 in India, that the water requirement of crops grown under field con- 

 ditions was generally lower than that of the same crops as determined 

 from pot experiments. Since anything which affects the growth rate 

 will affect the water requirement, these differences are not surprising. 

 On poor soils and under poor nutrient conditions, plants will require 

 more water to produce a unit of increment in a given time. It is, 

 therefore, difficult to establish comparable water requirements unless 

 the growth conditions are accurately defined. Also the fact that 

 young plants have juvenile leaves less able to control water losses 

 should be taken into consideration. 



Transpiration, as has been seen, is a vital process, which means 

 that the plant exercises some control over it. If water is abundant, 

 more will be transpired under a given set of physical conditions than 

 if little water is available. Even Von Hohnel (100) showed that ash 

 transpired about 100 kg per 100 g of leaf mass in a unit of time when 

 it was well watered and only 56 kg when lightly watered. 



It is also difficult to determine the water requirement of forest trees, 

 since the dry weight at the beginning of the test can be determined 

 only indirectly. In annual crops the weight of seed is taken as the 

 initial weight; but this is not practical in dealing with forest trees, 

 where the early conditions of growth (e. g., in nursery beds) are very 

 different from the later growth conditions. In small potted trees, 

 however, it is possible to estimate the initial dry weight from the 

 green weight. 



It has been pointed out above that cut shoots and twigs transpire 

 differently from twigs growing in their normal position. Ivanov (115) 

 studied the effect of transpiration by cutting off the shoots of rooted 

 plants and measuring with a stop watch the amount of time it took 5- 



