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



and that the mesophytic hardwoods use up 8 to 11 inches of water per 

 year, the less xero phytic conifers 7 to 10 inches, and the more xero- 

 phytic conifers 2 to 4 inches. 



Minckler (153) calculated that an American elm with a diameter 

 breast high of 13 inches transpired 8.7 liters of water a day. With 

 a growing season of 150 days and 375 trees to the acre, this is equiva- 

 lent to 4.8 inches of rainfall. A red maple, however, growing in a 

 very moist habitat transpired 51.7 liters daily. This is 7,775 liters 

 for the growing season, and again assuming 375 trees to the acre it 

 equals 28.3 inches of rainfall per acre. It must be recalled, however, 

 that this latter figure is a maximum, under favorable conditions, and 

 does not indicate the minimum requirement; such semihydrophytes 

 would probably need nearer 30 than 20 inches of annual rainfall as a 

 minimum. 



This is in agreement with the computations of Horton (103) who, 

 basing his calculations on the transpiration data of Von Hohnel, 

 computed the weight of leaf tissue in stands of various species, ages, 

 sites, and densities. He thus calculated that 60-year-old even-aged 

 full-stocked stands would have the following maximum seasonal 

 transpiration: 



Maximum trans- 

 piration, inches 

 per acre 



Douglas fir 7. 67 



White pine 8. 06 



Red spruce 22. 95 



Hickory 9. 90 



Beech 25. 68 



These figures are considerably higher than the previous ones because 

 they represent maximum and not minimum demands. 



The precipitation water consumed by trees, however, does not 

 include all of the precipitation. A certain proportion of the precipita- 

 tion remains suspended in the crown and a certain other proportion 

 evaporates from the soil and surface vegetation or runs off and is 

 unavailable to the forest stand. . According to forest meteorological 

 measurements, probably not more than 40 to 50 percent of the total 

 annual precipitation is available to forests. Assuming that only 

 one-half of the total annual precipitation is available to a forest stand, 

 it is seen that pines cannot be expected to grow where there is an annual 

 precipitation less than 6 inches; oak, 10 inches; spruce, 17 inches; and 

 the more mesophytic hardwoods, 20 inches. 



If 50 percent were added to these figures such trees might be 

 expected with a reasonable degree of certainty not merely to survive 

 but also to thrive, other site factors being favorable: 



Annual precipi- 

 tation, inches 



The more xerophy tic conifers 9 



The more xerophytic hardwoods 15 



The more mesophytic conifers 25 



The more mesophytic hardwoods 30 



The conifers may consequently be expected to survive with 9 to 25 

 inches of annual rainfall, depending on the species, while commercial 

 hardwoods cannot be grown easily on less than 15 to, 30 inches. 

 These figures agree well with the known distribution of forests in the 

 North Temperate Zone and elsewhere, and, on this basis seem fairly 

 reliable. Even in sections such as the southwestern United States 

 where temperature and humidity conditions are somewhat extreme, 



