WATER UTILIZATION BY TREES 



81 



northern white pine has about 19 acres of leaf surface per timbered acre 

 and pitch pine about 7.6 acres. Tiren (214) calculated that Scotch 

 pine has about 8 acres of leaves per acre of woods, and Zon (249) agrees 

 that 8 acres is an average figure. On this basis, an average stand 

 would transpire about ){ to 2% inches of rainfall per month, according 

 to the figures of Bates (11). With a growing season of 5 months and 

 (assuming) a low transpiration for the other 7 months, there would 

 then be need for at least 2 to 12 inches of rainfall a year, a figure which 

 agrees fairly well with those of European workers in forests of equal 

 density. The species used by Bates are characteristic of dry, open 

 sites, especially the pines; and, according to Baker (6, p. 59), there are 

 certainly less than 8 acres of leaf surface per acre of limber pine and 

 bristlecone pine forest. 



Since plants can maintain themselves on less water than they use 

 when it is plentiful, one should also keep in mind that (1) average 

 forests of the Rocky Mountains doubtless use amounts of water nearer 

 the lower values than the higher and (2) favorable site classes produce 

 a given increment more rapidly than poorer ones. 



Age of trees also must be taken into account. The studies of Weber 

 (232, p. 156) indicate that an increment of 1 foot board measure per 

 acre per year means an increment of about 15 pounds, including 

 branches, roots, leaves, etc. On this basis the water consumed 

 annually would vary between wide limits, as computed by Baker 

 (6, p. 61) and indicated in table 31. 



Table 31. — Annual water utilization of western American conifers in stands of vary- 

 ing growth rate, ranging from open arid to dense and very productive, based on 

 average water requirement 



Annual increment 



Mean water 

 requirement 



Water per acre 



Surface pre- 

 cipitation 



Feet, board 

 measure 



Pounds 



50 



100 



200 



500 



1,000 



750 



1,500 



3,000 



7,500 



15, 000 



560 

 560 

 560 

 580 

 560 



Pounds 



420, 000 



840, 000 



1, 680, 000 



4, 200, 000 



8, 400, 000 



Inches 



1.77 



3.54 



7.08 



17.70 



35.40 



For average mesophytic species the annual use of water would be 

 5 to 15 inches, which agrees very well with the figures obtained by the 

 methods previously discussed. If we took the averages of the precipi- 

 tation water as calculated by Von Hohnel, Vater, and Burger, we 

 should probably come to a figure not far wrong. Such a method of 

 calculation would indicate the minimum amount of precipitation 

 water used by the various types of forest species to be somewhat as 

 follows: 



More xerophytic conifers (pines) 75 mm (about 3 inches) 



More xerophytic hardwoods (oaks) 120 mm (about 5 inches) 



More mesophytic conifers (spruces) 215 mm (about 8.5 inches) 



More mesopttytic hardwoods (beech) 260 mm (about 10 inches) 



These figures agree very closely with those of Baker (6, pp. 57-59), 

 who considers that the water requirement of trees averages about 300 



121727°— 37 6 



