Bergman and Stallard: climax formations 349 



(2) Cladonia-l'olytrichum Assolies 



The formation of soil in slight amounts takes place by the weath- 

 ering and erosion of the rock surface. The weathering process is 

 aided by the action of the crustose lichens which tend to erode the 

 rock surface and to loosen particles of the rock. The disintegration 

 of the rock and the addition of humus from the decay of lichens 

 increases the water-content of the soil so formed. This increase in 

 the water-content makes possible the establishment of larger lichens, 

 such as Cladonias, and mosses. 



Consocies : The plants typically represented in this associes are 

 Cladonia gracilis, C. rangiferina and Polytrichum commune. One 

 of the first lichens to appear after the crustose lichens is Pannaria 

 microphylla, which may sometimes grow on quite firm rock but usu- 

 ally appears in more or less disintegrated areas. The large foliose 

 Peltigeras and the fruticose Cladonias are the next to appear, com- 

 monly occurring with Polytrichum. 



Among the Cladonias, C. gracilis, C. fimbriata, and such forms 

 are most abundant in open and more exposed places. C. rangiferina 

 may occur also in open places, but does not show the luxuriance of 

 growth which is characteristic of it in more sheltered spots. The 

 appearance of a few shrubs in crevices and pockets furnishes some 

 protection from the sun and drying action of the wind, enabling C. 

 rangifernia to make much better growth after the appearance of the 

 shrubs. 



These sometimes occur only in depressions in the rocks, since 

 disintegrated particles of rock are carried into the depressions by 

 rain, and such places are accordingly the only ones where conditions 

 are suitable for the development of Cladonia and Polytrichum. In 

 other places these plants form very extensive patches over the rock 

 surface, often to a depth of several inches. 



Secondary species : Other species of Cladonia, Peltigera, Par- 

 melia, Umhilicaria and a few small mosses occur with the consocies. 



(3) Herbaceous Associes 



The further disintegration of rock surface by weathering, aided 

 by the action of lichens and mosses and the addition of humus by 

 their decay, forms a soil layer of greater or lesser thickness over 

 the surface of the rock. The increase in the depth of soil and the 

 increase in water-holding capacity by the addition of humus both 



