CLIMATE AND WATER RELATIONS 159 



In the continental research area dew (63 days) and fog (14 days) are not 

 important for the water relations of plants. 



The even distribution and quantity of precipitation in the summer keep 

 the soil wet in the root zone of plants. 



Because of the equal amount of evaporation and the higher precipitation 

 in summer even continental high regions in the Eastern Alps (i.e. the area 

 near Obergurgl) are more humid than lower ones. 



Therefore it is not surprising that all field investigations of transpiration 

 and water balance carried out in the sub-alpine and alpine region have 

 demonstrated that water relations are only seldom stressed severely. This 

 has been shown by the osmotic values of about 100 species of plant com- 

 munities of this region round Innsbruck measured by Pisek, Sohm and 

 CarteUieri (1935). The osmotic values do not exceed 18 atm; they only 

 rise to 20 atm in the evergreen dwarf shrubs and conifers. Another reason 

 for a favourable water balance is that most of the tested plants are very- 

 sensitive to the smallest water deficits and start reducing transpiration very 

 quickly. This has been shown convincingly by Berger-Landefeldt (1936) 

 and CarteUieri (1941). Therefore even at the end of an extreme dry period 

 in August extending over 3 weeks (which very seldom occurs in high 

 regions) the osmotic values of some dwarf shrubs grown on southern 

 slopes did not exceed 25 atm; only Erica carnea reached 27 atm (Pisek and 

 CarteUieri, 1934). 



CLIMATE AND WATERRELATIONS IN WINTER 



In winter the conditions are quite different. The decrease in temperature 

 with increasing altitude causes a longer cold season at 2000 m and a shorter 

 growing season than in lower regions (Fig. i). 



About half the amount of precipitation faUs as snow. The period with 

 continuous snow-cover lasts 6 months on the average. If the ground is 

 covered with forest the distribution of snow on the ground is even, 

 because the forest raises and whirls the streamlines of the wind. Above the 

 timber-hne the wind acts down to the ground. It carries away the snow 

 from aU wind-exposed elevations and accumulates it at wind-screened 

 locations because the wind mainly comes from one direction only. There- 

 fore in the sub-alpine and alpine region which are strongly undulated the 

 snow cover may be quite different within the smaUest distances. EspeciaUy 

 windblown places hke crests and ridges are covered with snow only a few 

 weeks ; they may be snowless at any time during the winter. On the other 

 hand in wind-protected hollows of ground and troughs the snow stays for 



