WATER UTILIZATION BY TREES g3 



Pearson (168) reports that pmon-juniper woodland associations occur 

 in regions of 16.5 inches annual precipitation, ponderosa pine in re- 

 gions of 21.35 inches annual precipitation, and Douglas fir where the 

 annual precipitation is 26. 3G inches. 



In connection with this question of the minimum amount of water 

 necessary for the growing of trees, it must be borne in mind that, aside 

 from total amounts, one must consider also the seasonal distribution 

 and the humidity. Thus Mayr (150, p. 26) states that in the Tem- 

 perate Zone there must be at least 2 inches of rain in the 4 months 

 of the growing season (May to August) for forests to exist. This is the 

 absolute minimum and even then forests are established naturally with 

 this amount of rainfall only if the humidity is over 50 percent. Regions 

 with 2 to 4 inches of rainfall during the growing period and a relative 

 humidity under 50 percent, e. g. prairies and steppes, have no natural 

 forests but can be forested if special care is taken during the establish- 

 ment of the stand. Once established they can "make their own cli- 

 mate", in his opinion. If the precipitation is less than 2 inches during 

 these 4 months, no forests can be established, no matter how high 

 the humidity, since the seedlings will always dry out; and with over 

 4 inches of rainfall during this period, trees will grow, no matter 

 how low the humidity. Any species can grow at 70 percent humidity 

 and with 4 inches of rainfall during these 4 months. 



Also the "effectiveness" of the precipitation must be considered, as 

 has been pointed out recently by Bowman (18), who, in considering 

 the shelterbelt project in the western United States, called atten- 

 tion to the fact that it is important to know not only the average 

 annual rainfall but also its distribution over a period of time. Five 

 dry years in succession have a far different effect upon the establish- 

 ment of tree plantings than 5 dry years in regular alternation with 5 

 wet years. Bowman also emphasized the fact that a generalized 

 rainfall map tells very little about the rainfall effectiveness in any 

 given locality. In establishing a plantation in any particular region, 

 it is extremely important to know not only the general precipitation 

 characteristics but the specific ones for the given region. In other 

 words, rainfall must be interpreted in detail in terms of plant physi- 

 ology and the forest-crop potential under a wide variety of conditions. 



SUMMARY AND SUGGESTIONS FOR RESEARCH 



The fundamental problem for the silviculturist — the minimum 

 amount of water necessary for the production of his crop — still remains 

 to be solved. While this probably varies between 6 and 20 inches of 

 rainfall annually, depending upon the species and other factors, a 

 clear-cut answer to this question cannot yet be given. The many 

 factors which must be considered in solving this general problem 

 have been reviewed; some of the methods of approach to the problem 

 and the sources of error have been examined; and also some of the 

 criteria by which critical judgments may be reached concerning 

 solutions have been offered. Even a casual consideration of this 

 subject, however, will leave one convinced that much work still 

 remains to be done. 



Since the effectiveness of a given amount of rainfall depends, as has 

 been pointed out before, upon the months in which it falls, the tem- 

 perature, the relative humidity, the degree of windiness, etc., it is 



