WATER UTILIZATION BY TREES 39 



appreciated this sufficiently in calculating the annual losses of water 

 from forests. 



t Thus, Ivanov found that 1 -year-old twigs of deciduous trees in the 

 vicinity of Leningrad lost more water per surface area in the winter 

 than did the conifers, the figures for which have been given above. 

 While Larix transpires 1 (the standard used by Ivanov), 1 -year-old 

 twigs of Acer pseudoplatanus transpired 1.6, of Populus nigra L. 1.1, 

 and of A. tataricum L. 2.7. 



Weaver and Mogensen {229) found also, it will be recalled, that in 

 winter the transpiration of conifers in Nebraska was only slightly 

 greater than that from defoliated deciduous trees; and Thren (213) 

 has recently reported that during a short cold spell in Germany if the 

 transpiration of Pinus was 1, that of Picea was 0.64 to 2.09, of Fagus 

 0.34 to 1.07, and of Quercus 0.52 to 1.36, indicating that winter tran- 

 spiration in hardwoods is not far behind that in conifers. 



The fact that winter buds also transpire has been noted. Wiegand 

 (234) exposed buds of Pinus laricio and Aesculus hippocastanum for 3 

 days in the open at a temperature of —7° to —18° C. Some (a) 

 remained continuously exposed to the cold, while others (b) were 

 brought 10 times for a few moments during the period of observation 

 into a warm laboratory. The percentage loss of the original water 

 content as shown by weighing is as follows: Pinus (a) 3.4, • (b) 5.0; 

 Aesculus (a) 0.4, (b) 0.6. Buds in general transpire more in winter 

 than branches, and it was upon this basis that Ivanov explained his 

 results with long and short twigs; the fact that twigs with long inter- 

 nodes transpired relatively less than those with short internodes, he 

 explained as due to the fact that the shorter ones had a higher pro- 

 portion of buds. A sample of his results, giving relative transpira- 

 tion (based on Larix as 1), is as follows: Acer pseudoplatanus: long 

 internode twigs, 1.6, short internode twigs, 4.0; Populus nigra: 1.1 

 and 6.3; and P. tremula L.: 1.9 and 5.8, respectively. 



Transpiration in 1 -year-old twigs of trees hi winter was found by 

 Ivanov (114) to vary with the latitude and, according to him, species 

 of southern origin transpired more than those of northern in the genera 

 Acer, Salix, Betula, Tilia, and TJlmus. As a sample of his results is 

 here recorded the Acer series: 



Transpiration rate 

 (Larii*=l) 



Acer platanoides 1. 45 



A. pseudoplatanus 1. 60 



A. campeslre L 2. 53 



A. tataricum 2. 68 



On the basis of these findings, Ivanov concluded that in general 

 the weak transpiration enabled the trees to spread farther northward, 

 because as one proceeds northward, the amount of available water in 

 the soil in winter becomes less and less. The northern distribution is 

 thus connected with the physiological dryness of the site. In many 

 cases the transpiration figures support the thesis of Ivanov; Abies 

 sibirica and Picea obovata, for example, showed a transpiration rate of 

 0.3 to 0.8, and these species are among those found farthest north. 

 One should be warned, however, against assuming that this is the 

 only factor determining the northern distribution of species, as 

 pointed out by Rubner (178, p. 72). 



