CLIMATE AND WATER RELATIONS i6i 



July (Fig. 2). This higher intensity of radiation in high altitudes causes 

 plants to be heated above the air temperature in winter much more than 

 in lower regions. So we have found that the temperature of the needles 

 of pine trees is surprisingly often above the freezing point; on 83% of all 

 the days, during a research period from September to May, the temperature 

 of the needles exceeded o°C at least for a short time (Tranquillini, 1957). 

 Especially at the end of winter when the snow had not yet melted and when 

 radiation was very intensive there were high temperatures and liigh vapour 

 pressure gradients from plants to the cold air poor in water vapour. By 

 March the temperature of pine needles rose to 4-6° on the average, in the 

 extreme to 18-4° at midday, in April to 14-4° on the average, in the extreme 

 to 297°; therefore sometimes the temperature is as high as in summer 



(Fig- !)• 



It is very important for the water relations of such parts of plants pro- 

 truding from the snow and being occasionally heated very much, whether 

 the water loss, which at times is as high as in summer, can be made good by 

 water absorption from the soil. This question can be answered from records 

 of soil temperature, carried out by Auhtzky (i 96 1 ) at a snowless and a snowy 

 site at different depths of the soil at our station (Fig. 6). In the snow-covered 

 place the temperature of the soil sinks below 0° only in the upper 20 cm 

 of soil but does not fall below — i°C. This is the result of the good thermal 

 insulation of the snow-cover. On the other hand the ground not covered 

 by snow or with only a thin snow-cover freezes completely to a depth of 

 more than i m at the beginning of December and thaws only near the 

 surface at the end of April, and to a greater depth in the middle of May 

 (Fig. 6). 



Whether the plants can absorb water from the soil in winter depends upon 

 the temperature at which the water freezes in the soil. This has been showTi 

 by measurements of the soil moisture by means of electrical resistance 

 blocks. Water does not freeze at a certain temperature at once but succes- 

 sively. With decreasing temperature ice is formed in the larger pores first, 

 later in the smaller and smallest ones. The water turning to ice last, is held 

 so firmly in the soil that it cannot be absorbed by the plants, although it is 

 not yet frozen. The water available for plants is frozen at — i°C (Larcher, 



1957)- 



It can be concluded that plants in snowless locations certainly cannot 

 absorb water from the soil during a period of 5 months ; this is a long time 

 during which they permanently lose water. At the end of the winter this 

 water loss occasionally reaches an amount similar to that in the summer. 



On the other hand the water absorption of plants that grow in snow- 



