g2 MISC. PUBLICATION 257, U. S. DEPT. OP AGRICULTURE 



While some forest trees store water and are thus drought-resisting, 

 and others, e. g., deciduous trees, drop their leaves and thus show 

 drought-enduring characteristics, most forest trees come in the class 

 of drought evaders. The methods by which they evade drought are 

 extremely complex and may be tabulated as follows: 



1. Preventing heavy losses of water: 



a. Reducing transpiring area: 



1. Reducing size and number of leaves. 



2. Increasing volume in relation to the surface — thick leaves. 



b. Reducing transpiration rate; 



1. Changing position and curling. 



2. Changing surface structure: 



a. Thickening of cuticle and outer epidermal walls — sun leaves. 



b. Wax and resin. 



c. Hairs. 



3. Changing cell sap concentration. 



4. Forming buds in the petioles. 



5. Changes in stomata: 



a. In number. 



6. In structure and location. 



c. In function or movement. 



2. Assuring adequate supply: 



a. By increasing size and depth of root system. 



b. By displacing water from one organ to another or from stored regions. 



c. Absorption by aerial organs. 



3. Using water efficiently — Low water requirement. 



The methods outlined above will now be discussed in some detail. 

 Any or all of them may be combined in the same plant. It is, there- 

 fore, not possible to say that any one factor is responsible for the 

 development of a tree in any particular dry area. Also different 

 trees in the same area may have different methods of protecting 

 themselves against dry weather, i. e., evading drought, as cited 

 by Baker (6 y p. 68). In the central Kocky Mountains, white fir and 

 aspen grow in dense stands on the best sites and, when mature, have 

 about the same height. On dry sites in the same region the aspen is 

 reduced to dense thickets of trees only 8 to 10 feet tall, while the 

 white fir maintains its height of 60 to 70 feet, although the trees in 

 such circumstances are much more widely spaced. The aspen has 

 maintained itself by reducing the transpiring area, while the fir has 

 increased the absorbing area per tree. 



METHODS OF CHECKING TRANSPIRATION 



Of the trees which reduce the number of leaves, Parkinsonia in 

 the southwestern United States is one of the best examples. In this 

 genus practically all of the photosynthesis is carried on by the green 

 stems, and the leaves here are truly conspicuous by their absence; 

 from the striking green stems and branches, the tree has received its 

 common name "palo verde" (or "green pole"). 



The autumnal fall of leaves in broad-leaved trees is also a phase of 

 reduction in transpiring surface to meet the reduced water supply. 

 It is well known that in periods of excessive drought many leaves are 

 dropped. That this decrease in transpiring area is associated more 

 with moisture than with temperature is shown also by the fact that 

 trees in warm climates with a rainy and dry season lose their leaves 

 during the dry period. Holtermann (102) studied this question in 

 Ceylon, and came to the conclusion that the fall of leaves depends 

 entirely upon rainfall, as shown by the fact that the leaves of 



