122 PLANT SOCIOLOGY 



The Androsacion alpiuae; on non-calcareous residual gravel (pH 6.4 to 4.7), an 

 acidophilous community of moderately damp coarse gravel. 



The Salicion herbaceae; on non-calcareous fine gravel, glacial clay, and per- 

 manentlj' saturated humus soils approaching podsol (pH. 6.5 to 4.6), also acido- 

 philous. 



The Salicion herbaceae attains a considerable extension in the upper 

 belt of the alpine levels. It was described by Oswald Heer (1836) 

 under the suggestive name of Schneetdlchen and has ever since been the 

 prototype of alpine snow-lands. 



This alliance is divided into several associations and many facies, 

 which have different adjustments to the length of the snowless growing 

 period (Figs. 67, 68). The Salicetum herbaceae requires an average 

 exposure of 23^^ to 4 months. The Polytrichetum sexangularis, a 



Fig. 68. — Arrangement of associations according to duration of snow cover, Minor 

 Pass, Bernina, Switzerland, 2,450 m.. Northwest exposure, a. Winter snow blanket; 

 b, snow, July 21, 1927; 1, Polytrichum sexangulare association; 2, Normal facies; 3, 

 Gnaphalium supinum facies; and 4, Ligusticum mutellina facies of the Salix herhacca 

 association; 5, Car ex curmda association. 



community of mosses and liverworts, thrives with \}4, to 3 months free 

 from snow but may exceptionally endure more than a year of snow- 

 cover. In the beginning stages of this association on raw soils Anthelia 

 juratzkana, Gymnomitrium varians, Dicranum falcatum, and Pohlia 

 commutata are the pioneers. In their cushions Polytrichum sexangulare 

 likes to colonize and may finally become dominant. 



The pioneers of the snow-pocket associations at the extreme limits 

 of life are adapted in many ways to the short growing period and pre- 

 sent therefore a biotic group that is sharply characterized ecologically. 



The most important adaptations are: 



1. Extensive preparation and development of leaf and flower buds under the 

 snow. All the mosses and most of the flowering plants, such as Arenaria biflora, 

 Cerastium cerastioides, Cardamine alpina, Soldanella sp., and Gnaphalium supinum, 

 overwinter with green shoots beneath the deep snow layer. 



