WATER UTILIZATION BY TREES 



33 



Temperature 



Temperature is another important factor influencing transpiration . 

 Since temperature affects the diffusion of the water molecules from 

 the cell as well as the general life processes of the plant, it is not sur- 

 prising to learn that an increase in temperature is generally associated 

 with an increase in transpiration. Wiesner (287) found that a leafy 

 twig of Celtis australis L. in diffuse light gave off 0.22 cc of water at an 

 average temperature of 4.3° C; at 10° C, 1.2 cc; at 13° C, 1.8 cc; and 

 at 16° C, 3.0 cc. 



Roeser (177) studied the transpiration of various conifer seedlings 

 in temperatures somewhat above the average. The purpose of the 

 tests was not to get a measure of the maximum transpiration rate but 

 to study the increase in transpiration rate produced by gradually 

 increasing the heat to which the seedlings were exposed. Seedlings 

 of western yellow pine, lodgepole pine, Douglas fir, and Englemann 

 spruce were placed in narrow-mouthed bottles containing about 

 150 cc of water, with the roots entirely submerged. Readings at six 

 different temperatures w r ere obtained with a mercury thermometer 

 whose bulb was placed at the level of the leaves. In all the tests the 

 seedlings were illuminated by a 60-watt lamp, so that the light factor 

 was constant. Results are given in table 12. 



Table 12. — Losses of water per minute by con , 



high temperatures 



■ifer seedlings at average and moderately 

 itu.res 





Ponderosa pine 



Lodgepole pine 



Douglas fir 



Engelmann spruce 



Temperature 

 (° F.) 



Loss per 



minute 



per gram 



of dry 



weight 



Relative 

 increase • 



Loss per 



minute 



per gram 



of dry 



weight 



Relative 

 increase ' 



Loss per 



minute 



per gram 



of dry 



weight 



Relative 

 increase ' 



Loss per 



minute 



per gram 



of dry 



weight 



Relative 

 increase i 



61.2 



Milligrams 

 4.7 

 8.6 

 9.6 

 16.5 

 23.6 

 62.4 



1.00 

 1.83 

 2.04 

 3.51 

 5.02 

 13.28 



Milligrams 

 7.0 

 14.6 

 27.0 

 35.0 

 47.5 

 86.6 



1.00 

 2.09 

 3. 76 

 5.00 

 6.79 

 12.37 



Milligra ms 

 2.1 

 7.4 

 10.0 

 17.2 

 27.8 

 38.4 



1.00 

 3.52 

 4. 70 

 8.19 

 13.24 

 18. 37 



Milligrams 

 2.3 

 11.7 

 15.6 

 23.2 

 43.6 

 65.0 



1.00 



70.8 . 



5.08 



85.0 



6.78 



103.1 



116.2 



124.1 



10.08 

 18.96 

 28.26 



i Transpiration at 61.2° F. is considered as unity in giving relative increases at the higher temperatures . 



It will be noted that lodgepole pine transpired much more than the 

 other species. At the higher temperatures this was followed by spruce, 

 Douglas fir, and ponderosa pine, the last two being very near each 

 other. At the lowest temperature the ponderosa pine ranked second. 

 Roeser interprets these data as showing that both Engelmann spruce 

 and Douglas fir are apparently more capable of increasing their tran- 

 spiration rate under severe conditions of heat than are the pines. He 

 also noted that the amount and rate of transpiration dropped off as 

 the seedlings grew older, the change being least rapid and most 

 uniform in the ponderosa pine, which seems to be more consistent in its 

 normal transpiration through the first year than any of the other three 

 species. It thus seems possible to relate to some extent the tran- 

 spiration ability of these seedlings to their natural site conditions. 



Also, as a general rule, the transpiration rate decreased at any given 

 temperature as the period of exposure increased for all ages up to 1 



121727°— 37 3 



