2 MISC. PUBLICATION 257, U. S. DEPT. OF AGRICULTURE 



need for a summary of present knowledge concerning the amounts of 

 water utilized by our economic forest species and of the various 

 problems associated with this utilization. 



The following brief review of some of the problems involved and of 

 the various contributions to their solution is here presented in the hope 

 that students and workers will thus be able to obtain a general perspec- 

 tive of the field, of the present state of our progress, and of some of the 

 problems still awaiting solution. Because of the size of this problem, 

 however, it has been deemed advisable to limit the present discussion 

 primarily to the tree itself. Problems associated with the relation of 

 the trees to the water supply and with the factors outside the tree which 

 determine the available supply, e. g., the soil complex, are, therefore, 

 omitted in the following pages, in an attempt to focus the attention on 

 (1) the amount of water the various tree species utilize and require 

 during their growth and (2) the factors which determine this use of 

 water. Only when these fundamental problems have been more 

 completely studied will it be possible to take advantage of the known 

 facts of climatology in their relation to optimum land productivity. 



GENERAL USES OF WATER IN TREES 



Water is without doubt one of the most important environmental 

 factors influencing the life of trees. Trees will not grow except in the 

 presence of at least a certain minimum quantity of water, depending 

 upon the species; and, next to temperature, no factor plays so impor- 

 tant a role in determining the distribution of the various tree species 

 over the surface of the earth as does water. While the uses of water 

 within the tree are manifold, they can be grouped into at least four 

 major categories. 



First of all, water enters into the composition of the dry matter of the 

 tree. In the process of photosynthesis, carbon dioxide obtained from 

 the air is combined with water obtained chiefly through the roots to 

 form carbohydrates, according to the equation 6C0 2 +6H 2 = 

 C 6 H 12 6 +60 2 . Although this equation tells no tiling of the interme- 

 diate steps or of the details of the process, it outlines the general scheme 

 of the reactions and states the end products, which are sugars and 

 oxygen. The sugars are built up later, with the elimination of water, 

 into starch, and ultimately into cellulose, which makes up the bulk of 

 the material in a tree, while the oxygen, in the economy of nature, is 

 returned to the air. 



According to this equation, since the molecular weight of water is 

 18 and that of sugar (glucose) is 180, it takes 108 g of water to make- 

 180 g of carbohydrates; or, in other words, about 60 pounds (nearly 

 a cubic foot) of water are required to make 100 pounds of dry matter. 

 But since trees actually absorb about 300 pounds of water for each 

 pound of dry matter manufactured, only about one-fifth of 1 percent 

 of the water absorbed ever becomes incorporated into the substance 

 of the tree. 



While the use of water in the manufacture of food for the frame- 

 work of a plant is its most important use to vegetation in general and 

 to woody plants in particular, this is by no means the only function 

 which water has in the tree. In order to carry on its normal functions, 

 each living cell must contain enough water to keep the protoplasm in 

 optimum condition and in contact with the cell wall. This requires a 



