WATER UTILIZATION BY TREES 79 



WATER REQUIREMENT 



As to actual water requirement, i. e., number of grams of water 

 utilized by the plant per gram of dry weight increment, very few 

 figures are available for trees. Vater (219), in reworking the data of 

 Von Hohnel and Ebermayer, arrived at a figure of 398 for beech, 361 

 for spruce, and 166 for pine, as characteristic of these three types of 

 forests. Oelkers (164) i n his recent summary of this subject calculates 

 the water requirements of the common economic species as follows: 



Birch ii 375 



Ash 244 



Alder 227 



Oak 220 



Larch 220 



Plains pine 213 



Beech 209 



Mountain pine 208 



Spruce 193 



Fir 145 



In North America the most extensive work in this field has been 

 by Bates and Pearson. Bates (11), from potted plants 5 to 6 years old 

 in Colorado, arrived at the following figures as a result of studies over a 

 period of 2 years: 



Pinus flexilis 952 



Pinus contorla 842 



Pinus ponderosa 768 



Pinus aristata 743 



Pseudotsuga taxifolia 634 



Picea engelmanni 456 



Pinus sylvestris 436 



Larix sibirica 295 



Unfortunately, however, only two trees were used in each case. These 

 figures are probably much too large, and it is noteworthy that in 1920 

 some of his results are less than one-half as large as in 1917. The 

 figures for the last two species mentioned are probably more nearly 

 correct than the others because of the fact that these forms were not 

 studied in 1917. 



Furthermore, Pearson (167), working with a few of the same species, 

 using potted plants and keeping the soil moisture constant at 20 

 percent, obtained very much lower results, as indicated in table 30. 



Table 30. 



-Average amounts of water transpired 

 2 growing seasons 



potted conifers during 



Species and number of plants 



Bristlecone pine, 6_ 



Yellow pine, 9 



Douglas fir, 6. 



Engelmann spruce, 



Age when 

 potted 



Dry weight 



Transpired, 



1919 and 



1920 



Initial 



Accretion, 

 2 years 



Years 

 5 

 4 

 4 

 4 



Grams 

 6.54 

 3.82 

 9.92 

 7.44 



Grams 

 16.84 

 23.25 

 28.34 

 23.42 



Grams 

 7,325 

 9, 153 

 10, 784 

 8,462 



Water used 



per gram of 



accretion 



Grams 

 440. 

 395.7 

 381.0 

 S59.0 



It will be noted that the order, for the most part, is the same as 

 that found by Bates. Of the three genera studied by Pearson, Pinus 

 occurs at the lowest elevations and Picea at the highest. The annual 

 precipitation increases upward from about 22 inches, where Pinus 



