i6o WALTER TRANQUILLINI 



7^ months ; such places are constantly and. reliably covered with deep snow. 



The composition of the vegetation reacts very sharply to these variations 

 in snow cover. This means that they are of great importance to the vegeta- 

 tion and they influence basically the water relations of plants in winter as 

 we shall see later on. 



The areas with no or only a thin snow-cover support only a certain com- 

 munity of Lichenes (Alectorietum), or the dwarf azalias (Loiseleurietum) 

 rich in Lichenes ; in troughs very long covered with snow there grows the 

 alpine rose (Rhododendretum ferruginei) or some other community (in 

 our station area, i.e. Festucetum Halleri). Between these extremes of loca- 

 tion there are many transitions which are occupied by defined plant 

 communities (Vaccinietum uhginosi and Vaccinietum myrtilli) (Fig. 5). 



Corresponding to the different snow cover we can separate 3 types of 

 wintering by evergreen plants : 



1. plants growing in locations mostly without any snow protection 



2. plants that grow in snow-covered places, but with parts project- 

 ing from the snow 



3 . plants wliich are completely covered by snow during the winter. 

 First we v/ill consider the evaporation conditions to which plants or 



parts of plants protruding from the snow are exposed. The evaporation 

 conditions firstly depend on the temperature of the plant. Therefore they 

 depend on solar radiation. 



In winter the snow-covered mountain slopes reflect so much radiation, 

 that in this season a horizontal area gets a radiation twice as intensive as one 

 in low regions. The curve of the annual trend of solar radiation is so 

 asymmetrical that the intensity of solar radiation in April is as high as in 



Alectorietum 

 lextr. 'windexposed ) 



Alectorietum 

 Loiseleurietum 

 Vaccinietum uliginosi 

 Vaccinietum nnyrtilli 

 Rhododendretum ferrugine 

 Festucetunn halleri 



Fig. 5. Average duration of continuous snow cover (>5 cm high) for several plant 

 communities at our research station near Obergurgl. (From Turner, 1961b.) 



