170 



PLANT SOCIOLOGY 



and of Carex curvula, C. firrna, and Elyna myosuroides from the Alps 

 (Fig. 95). 



This curve is based upon 125 soil samples from the Swiss, Italian, 

 and Tirolean Alps, taken from the root region of C. curvula (5 to 10 

 cm. deep). It illustrates the pH tolerance of the species but depends to 

 some extent upon the number of the samples examined. The pH opti- 

 mum of the species lies between pH 5.4 and 4.6. C. curvula is thus a 

 distinctly acidophilous plant. Within the optimum boundaries of the 

 species the acidity seems unimportant for the welfare of C. curvula; 

 but in the minimum region (below pH 5.4) acidity becomes the decid- 



er 



<5.5 Atf <AJ 'i.OpH 



Fig. 95. — Theoretical and experimental curve for the pH values for Carex curvula: 

 tests in the Central and Eastern Alps. {Braun-Blanquet and Jenny.) 



125 



ing factor. So also in the equally injurious maximum. Here even a 

 small excess or shortage determines the survival and competitive power 

 of the species. 



Plant Communities and pH Values. — Each plant community has its 

 own definite tolerance of pH values, with a more or less distinct opti- 

 mum. This, of course, does not exclude the possibility of other associa- 

 tions having about the same pH optimum. Very extensive 

 communities, for example, the Quercus pedunculata and the beech 

 woods of southern England, examined by Salisbury (1925) may show 

 two pH optima. This, no doubt, is due to the fact that two different 

 associations, or at least two different societies, are involved. The 

 researches of Wlodek and Strzemienski (1924), Christophersen (1925), 

 and Braun-Blanquet and Jenny (1926) on more narrowly dehmited 

 communities of one or several layers give harmonious unimodal 

 curves, with often a very narrow pH optimum. 



For example, the pH range of the Corex firma association in the 

 Central Alps (from 100 samples) lies within a range of 0.4 pH unit. 



