154 WALTER TRANQUILLINI 



Besides the above mentioned protections against a negative balance, the 

 abihty to endure extensive drought periods depends on several internal 

 conditions prolonging the period in which the sub-lethal deficit will be 

 reached. Such conditions are the following ones : 



1. rate of cuticular transpiration 



2. the remaining water storage from which plants recover their 

 cuticular water loss after complete stomatal closure 



3. the amount of sub-lethal deficit, i.e. the 'resistance to dehydration'. 



Before describing the water relations of some characteristic plants of the 

 sub-alpine region in the Tyrolean part of the Central Alps it is necessary to 

 discuss the chmatic features of this area. I have made use of the investiga- 

 tions of our extensive ecological research station situated at an altitude of 

 2000 m above sea level, just above the timber line near Obergurgl, the 

 highest village of Austria. The station covers a part of the western slope of 

 the Gurgler valley. The studies were made by a team of botanists and 

 cUmatologists. 



CLIMATE AND WATER RELATIONS IN SUMMER 



From a physical point of view transpiration of plants depends on the 

 vapour-pressure gradient from the transpirating surface to the surrounding 

 air. Therefore transpiration is the greater the less water vapour the air 

 contains and the higher the temperature of the transpiring surface, which 

 is almost saturated with water. All the other chmatic factors affecting 

 evaporation uifluence transpiration over this vapour-pressure gradient. 

 Solar radiation raises the temperature of plants above the temperature of 

 the air and increases thereby the gradient. The wind cools off the plants 

 whereby transpiration is reduced; on the other hand, it removes the air 

 rich in water vapour from the vicinity of leaves and replaces it with drier 

 air thereby increasing the gradient again. 



With rising altitude the vapour pressure of the air decreases so much 

 that the air at an altitude of 2000 m contains on the average only one half 

 of the amount of water vapour as at sea level As Rathschiiler (1949) has 

 pointed out for the Salzburger Alps tliis difference in gradients is especially 

 marked during the night, while there is no difference between the bottom 

 of valley and higher stations in the daytime. Its importance for evaporation 

 loses significance if the temperature differences at different altitudes are 

 considered. 



Figure i shows the annual trend of the air temperatures at Obergurgl in 

 comparison with the values of a station at an altitude of 500 m in the 



