310 



PLANT SOCIOLOGY 



change of level there were 32 Hchens, 43 mosses, and 202 phanerogams, 

 of which 14 were trees. The accompanying diagram (Fig. 156) gives 

 the development of pioneer vegetation on newly exposed glacial soil 

 in the Alps. 



Development of Vegetation and Soil Formation. — There is an 

 ultimate and intimate causal relation between the processes of soil 

 formation and the development of vegetation. The forces operative in 

 the early stages of soil formation causing the disintegration of the soHd 

 rock and the removal and deposition of the fine material by gravitation, 

 wind, and water produce virgin soil. On this substratum is enacted the 

 struggle of the plant communities. 



'"""'^^"'""" ' '" (SoA/fraga exarafa 



-•JSieyersiti re plans. 

 ^Polyinchurri a/pinum 



■3m ACerasfium cerasfioides 

 '^Ranunculus glaaalis 



I2r 



[Poa alpina 



. Yc%?fa'J'n"fhTmum-Tir^um [Sahx reikulaia 



^^-Hpomia communis '^ \Carclamine.resedifolia 



\Bnumsp. , XArabisalpinq 



\pliilonofis foniana XVeronica alpma , 



,^ ^Gnaphahumsupinurp 



' I5m. ^RhoGomi^rium caneiscens 



I Bryom perrdulum 

 I Bryum venfricosum 



: ZOm- 



-lOmr ■ 



\Poqonaium urnigerum VAnffl rosace alpina 



\Poiyfrichun7 seKangulare_\r^rcxacum / 



Poalaxa, „ . 

 Festuca halleri 

 Phleum alpinum 

 Onyria . J 



Saqina saginciaes 

 Cerastium pedunculaia 

 Saxifraga Stellans 

 Lpilobium alpmum 



'H Dicranoweisia cnspula 

 I Pohlia carinafa 

 yPohlia gracilis 



Fig. 



5 10 15 20 m. 



156,^Appearance of pioneer species upon soil newly exposed by the Sesvenna 

 Glacier, 2,700 m.. Lower Engadine. {.Ajter Braun-Blatiquet and Jenny.) 



Living organisms then begin to play their parts, and the soil devel- 

 opment is directed into definite channels. Penetrating roots and 

 holdfasts disintegrate rock and earth; animals, too, cooperate in 

 reducing the soil particles. Soil bacteria, fungi, and algae enter at an 

 early period and by their activity assist the other vegetation in the 

 formation of humus, effecting both chemical and physical changes in 

 the soil. 



Climatic factors influence the chemical composition of the soil 

 more and more as it approaches maturity. Mineral salts are removed 

 from certain layers and deposited in others (c/. p. 164). Since climatic 

 processes proceed according to law, both soil formation and develop- 

 ment of vegetation must ultimately be due to the prevailing climate. 



The general climate of a region determines whether an accumulation 

 of organic matter and nitrogen is possible and whether the upper layer 

 of soil (root layer) is subject to increasing acidification or to an accumu- 

 lation of salts. On this depends the whole course of the development 

 of vegetation. 



