SECONDARY SUCCESSIONS 119 



vegetation are made up by invaders from the adjacent form- 

 ations. In the most perfect types of succession, this dis- 

 similarity between the new and the old vegetation continues 

 to the last stage, in which the reappearance of the facies 

 precedes that of the subordinate layers. In many forest 

 successions, however, the general physical similarity of the 

 ultimate stages permits the early reappearance of the her- 

 baceous and shrubby species, and the final stages affect the 

 facies alone. Successions in burned areas operate usually 

 within the water content groups. The reconstruction of a 

 mesophytic forest takes place by means of mesophytes ; of 

 the rarer xerophytic and hydrophytic forests, through 

 xerophytes and hydrophytes respectively. This is due to 

 the fact that the alteration of the soil is slight, except 

 where the burning of the vegetation permits the entrance of 

 erosion, as on mountain slopes. 



(b) Succession in lumbered areas. Com- 

 mercial lumbering, especially where practised for wood- 

 pulp as well as for timber, results in complete or nearly 

 complete destruction of the vegetation by removal and the 

 change from diffuse light to sunlight, or by the action of 

 erosion upon the exposed surface. In the first place, short 

 mesophytic successions will result : in the second, the suc- 

 cessions will be long and complex, passing through de- 

 creasingly xerophytic conditions to a stable mesophytic 

 forest. Where a forest is cut over for certain species alone, 

 the undisturbed trees soon lake full possession, though the 

 causes effective in the beginning will ultimately restore the 

 original facies in many instances. Such successions are 

 anomalous, and will be treated under that head. 



(c) Succession by cultivation. The clearing 

 of forests and the "breaking" of grassland for cultivation 

 destroy the original vegetation : the temporary or permanent 

 abandonment of cultivated fields then permits the entrance 

 of ruderal species, which are the pioneers of new suc- 

 cessions. This phenomenon takes place annually in fields 

 after harvest, resulting in the secondary formations of 

 Warming, in which practically the same species reappear 



