210 



PLANT SOCIOLOGY 



A profile in the Alnus incana forest showed the following soil 

 granulations : 



Table 19. — Soil Particles in an Alnus Forest (I), in a Recently Deposited 

 Sand Bank (II), and in the Aar Dunes (III) 



Depth, centimeters 



Coarse 

 sand 2 

 to 0.2 

 mm., 

 per cent 



Fine 



sand 0.2 



to 0.1 



mm., 



per cent 



Very fine 



sand 0.1 



to 0.05 



mm., 



percent 



Silt 0.05 



to 0.02 



mm., 



per cent 



Raw clay 



less than 



0.02 



mm., 



per cent 



I: 



to 10 sand and humus 



1 5 to 25 sand 



30 to 45 sand 



II: 



Sand bank 



Ill: 



Aar dunes 



2.6 



2.6 



40.5 



57.5 



12.8 



10.1 

 20.4 

 54.0 



32.7 



82.1 



19.8 



19.3 



2.0 



3.5 



0.9 



29.0 



14.5 



0.8 



1.0 



0.9 



38.3 



41.4 



2.8 



5.7 



3.8 



The predominance of the coarse and fine sand in the lowest, and of 

 silt and raw clay in the topmost strata shows the process of soil forma- 

 tion and the accompanying development of vegetation. The lowest 

 stratum represents an old sand bank upon which, after a gradually 

 progressive process of soil formation, the recent A. incana forest has 

 established itself, with rich herbaceous undergrowth. Newly deposited 

 sand banks, without any vegetation at all, showed a composition very 

 similar to the lowest stratum (Table 19, II). It is noteworthy that 

 these river deposits contain mostly coarse sand, while the derived 

 dunes, built up by wind, consist (like loess) of fine sand as seen in the 

 Aar dunes (Table 19, III). 



These river sand dunes in the Rhine, Aar, and Limmat valleys are 

 stabilized at an early stage of their development by Calamagrostis 

 epigeios colonies (geophyte), an association replaced later by a mixed 

 deciduous forest. Coarse gravel banks are first covered with a loose 

 Mesobrometum sod in which, as a rule, Pinus silvestris soon gains a 

 foothold. 



Basic gravel soils, poor in clay, near the large alpine rivers are 

 exceedingly porous and poor in nutritive substances.^ These soils are 

 dry, and therefore in the pioneer stages of the development of their 

 vegetation they are invaded by unspecialized pioneers, among which 

 the plain turf moss, Tortella incUnata, and species of Sedum play 

 an especially important, constructive role. These Sedum-Tortella 



^ The most important bearer of nutritive substances is raw clay. 



