THE DEVELOPMENT OF COMMUNITIES 311 



At the extremes, two principal developmental series may be dis- 

 tinguished: an acidophilous, with increasing acidity; and a basic, with 

 the accumulation of soil salts. 



In arid climates with scant precipitation and high evaporation the 

 latter influence predominates and leads invariably to alkaline soils at 

 maturity wherever the rain water cannot readily drain off. The high 

 evaporation causes concentration of the soil solution in the upper parts 

 of the soil. This concentration causes a speedy weeding out of the 

 acidophilous species which are found in the first stages on raw silicious 

 soils and sand. With increasing concentration of the soil salts all 

 neutrophilous species give way, and even those basophiles which cannot 

 endure high concentration of salts. The final stage is usually a halo- 

 phytic shrub steppe or, less frequently, a scrub of deciduous small- 

 leaved, broomlike, often spiny shrubs, such as Acacia and Zizyphus in 

 north and central Africa, South America, and Australia; or succulents 

 in South Africa and North America. 



The opposite extreme of the developmental process is seen in the 

 podsol regions of western and northern Europe, in the mountains of 

 central Europe, and in northeastern North America. The heavy- 

 precipitation, coupled with high humidity (high N.S. quotient), 

 greatly favors the accumulation of humus and the leaching of mineral 

 salts in the upper layers of the soU (cf. p. 164 and Fig. 94). 



In the first stages of rock weathering the soil solution is richer in 

 OH ions and therefore less acid than in the later stages. This is true 

 even in the case of non-calcareous silicate rocks such as granites, 

 gneisses, and amphiboles. Basic raw soils in the podsol region can 

 remain basic only where there is some interruption in the natural course 

 of soil formation such as that due to soil movement through natural 

 causes or human intervention or the addition or removal of soil particles 

 by erosion. In the absence of such hindrances the development of 

 vegetation, following in a general way the soil formation, proceeds in a 

 manner to be described. 



Alpine Regions. — The humus-forming power of the plant covering 

 increases quite rapidly with decreasing temperature and increasing 

 humidity. At alpine altitudes accumulation exceeds decay. Although 

 we do not know the chemical composition of the decaying plant 

 materials, we know that they are rich in organic acids and usually 

 give an acid reaction. On calcareous substrata wind-blown dust and 

 residual soil suffice at first to neutralize the acids. The result is a 

 mild humus of deep-black color, of soft, sticky, and somewhat greasy 

 consistency, with basic or neutral reaction. The Carex firma associa- 

 tion or C. niucronata or Sesleria coerulea turf takes possession of such 



