246 



PLANT SOCIOLOGY 



plant material and the rapidity of the processes of decay. The decay 

 curve (Fig. 122) is an expression for this relation. For each climatic 

 region there seems to be a specific coefiicient of humus formation. In 

 warm, dry regions a rapid and almost complete decay is accompanied 

 by a small production of organic matter. Any accumulation of humus 

 is therefore very small. According to Hilgard (1914), the mean humus 

 content of arid soils amounts to only 1 per cent. In the moist tropics 

 little humus is usually accumulated, since the temperature conditions 

 favor complete decomposition. 



In a cool, moist cHmate the conditions for humus formation are 

 much more favorable. Decomposition here is much retarded by cU- 



/96 



5% 



30% 



y^^'.y^'.'^^^^yyKKKKK^KKy 



^^^^^^^^^^^ 



Curve of production 



Aria 

 (fooj 



ffumid 

 (500) 



Perhurrjitf 

 (tooo) 



Fig. 122. — Schematic representation of the relation of climate to humus formation; 

 vegetation and other factors unchanging. Numbers in brackets are N. S. quotient 

 (see p. 143). {After Braun-Blanquet and Jenny.) 



matic and unfavorable microbiological conditions. Large quantities 

 of humus collect, which upon soils poor in bases readily take on the 

 form of dry peat. The kind and quantity of humus formed are deter- 

 mined, aside from chmatic effects, by the dominant vegetation. But 

 within one and the same plant community great fluctuations are often 

 observed, so that reliable average values must rest upon very numerous 

 humus determinations. These conditions have been investigated for 

 some high-alpine plant communities. 



The percentage of humus in the root layer of the soil of alpine 

 meadow communities varies from 20 to 40 per cent. Dwarf-shrub 

 communities give a somewhat higher mean. 



The basophilous, lime-constant association of the Firmetum stores 

 an average of 15 to 20 per cent; the weakly acidophilous Elynetum, 30 

 to 40 per cent; the pronouncedly acidophilous Curvuletum, 25 to 35 



