314 PLANT SOCIOLOGY 



found the highest pH values on young soils with calciphilous plant 

 societies. On the English coasts Salisbury (1925) has shown clearly 

 the gradual acidifying of old dune soils and has expressed the belief 

 that natural undisturbed soils "in this latitude tend to become more 

 and more acid" (Fig. 158). 



In the first stages of these dunes we find in abundance basophilous 

 species such as Carlina vulgaris, Euphorbia paralias, Senecio jacobaea, 

 Gentiana campestris, G. amarella, Chlora perfoliata, etc. In the later 

 stages the basophiles disappear and Calluna becomes dominant. 

 Similar changes occur on the dunes of Lake Michigan. The young 

 dunes give a neutral or alkaline reaction ; the older ones become some- 

 what acid with an abundance of Vaccinium. 



Tiixen (1930) studied the climatic tendency toward podsohzation in 

 northwestern Germany and its relation to the development of vegeta- 

 tion. He concludes that, assuming the climate to remain constant, all 

 the forest associations will become transformed into the Querceto- 

 Betuletum, a decidedly acidophilous association. The forest suc- 

 cession proceeds as follows: Fagetum-hercynicum, calcicolous; 

 Querceto-Carpinetum (two stages), basophilous; the chmax Querceto- 

 Betuletum. 



In cool, humid chmates the development of vegetation becomes 

 essentially a struggle of the acidophilous against the neutrophilous and 

 basophilous species, so that on originally similar substrata the acido- 

 philous communities always indicate a more advanced maturity of soil 

 than the basophilous. The distribution of the associations on the 

 glacial gravel terraces of the Alpine foothills is similar: on the younger, 

 lower terraces (Wlirm glaciation) basophilous and neutrophilous 

 associations prevail; on the older, decalcified, high terraces (Riss 

 glaciation) the acidophilous are found. 



Deep-rooted trees are less dependent upon the changes due to soil 

 formation than are other plants. In the struggle for supremacy 

 among trees in the climax forest the decisive factors are climatic: 

 temperature, precipitation, humidity, hght.^ 



It is interesting to notice how rapidly the basophilous species are 

 crowded out by the acidophilous under certain conditions on raw 

 calcareous soils in the high Alps. In the initial phase of the snow-slide 

 association of the Arabidetum coeruleae acidophiles are lacking and 

 basophiles predominate over neutrophiles. In the terminal phase of 

 the association the two groups of species are about balanced, or the 



1 The seedlings of trees, however, are dependent upon the upper layer of soil. 

 This layer is therefore a determining factor in the development of natural vege- 

 tation under all conditions. 



