MOORLAND AND ALPINE PLANTS 



399 



older ones. Such changes may easily be studied in the 

 vegetation of waste-heaps, quarry-tips, or on the scree- 

 covered slopes of our mountains. Bare ground becomes 

 invaded by microscopic plants, Mosses, Ferns, and annual 

 flowering plants (see p. 220). These are succeeded by open 

 associations of perennials, which at first are all herbaceous 

 species, but later, closed associations, with shrubby peren- 

 nials, are developed, and finally trees appear, with a ground 

 flora characteristic of the forest. 



[FOREST) ; 

 ^ STABLE ASSOCIATIONS 



CLOSED 

 I . 

 PROGRESSIVE 

 ASSOCIATIONS 



CLOSED 

 I 

 RETROGRESSIVE 

 ASSOCIATIONS 



OPEN 



OPEN 



invasion < 



[bare ground] 



Fig. 258. Life-cycle of Vegetation. 



The woodland or forest type is the highest and most 

 stable phase in the development of vegetation, and persists 

 for a long period. Eventually changes occur, due to natural 

 or artificial causes, e. g. earth movements, fires, or the 

 cutting of gaps in the forest for roads or railways. In con- 

 sequence of the increased exposure, degenerative or retro- 

 gressive changes set in, and finally the ground becomes 

 denuded of plants. Re-invasion then occurs, and the life- 

 cycle of the vegetation is completed by the advent of pro- 

 gressive associations, which increase in complexity and 

 stability until the forest is once more developed. In this 

 way the vegetation of the earth is ever changing. 



