SOCIAL LIFE AMONG PLANTS 17 



or edaphic factors, if it occurs at all, must be a consequence of the 

 change in vegetation but not its cause. We have here a direct effect 

 upon vegetation through the newly immigrating swarms of plants. 



There is also a very rapid succession of communities in the early 

 stages on neglected arable land, ruderal habitats, and burned areas, 

 where often several short-lived plant communities may succeed one 

 another, although no immediate change in the soil can be demonstrated. 



This fleeting conquest and the rapid succession of artificially 

 induced pioneer stages are in contrast with the slow, tense struggle of 

 the later stages of primary succession. The numerical superiority of 

 shoots and disseminules of certain species is certainly important here; 

 but it is not the cause of the changes in vegetation. The vegetation 

 itself affects and gradually changes the physicochemical factors of the 

 habitat and therefore favors the development of certain competitors at 

 the expense of their neighbors. 



In any case one must not expect a common and uniform reaction on 

 the part of all the species of an association. Each species has its own 

 reaction to each factoral change in the habitat. 



If the light factor suffers a change because of the vegetation (devel- 

 opment of the tree layer), certain species disappear earher; others, 

 later. From a mixed stand of deciduous and needle-leaf trees in 

 Finland Alnus incana is the first to be eliminated. In fifty years it is 

 everywhere overshadowed and leads a precarious existence. Later 

 Populus tremula and Betula disappear. After three hundred years 

 there is left only the original old pines together with Picea excelsa and 

 its progeny, since the pine cannot reproduce in the shade of the spruces 

 (Cajander, 1926). 



If the character of the soil is gradually changed by accumula- 

 tion of humus, the individual species, both dominant and subordinate, 

 react to this. Scarcely perceptible changes in the acidity of the soil 

 solution may favor the scattered and apparently weaker competitors 

 in their struggle with the dominant plants. By means of changes in 

 acidity due to vegetation it may happen that a community literally 

 poisons the soil for itself. Thereby it becomes possible for species of 

 another association better adapted to the new conditions of the habitat 

 to win places for themselves and finally to drive the formerly dominant 

 community out of the field. A succession on calcareous talus in the 

 Central Alps due to the influence of soil acidification may make this 

 clear. 



Sesleria coerulea and Fesiuca violacea, tussock plants furnishing large 

 amounts of humus, are highly acidifying. They prepare the soil for the 

 successive penetration of the acidophilous species of the Curvuletum, 



