308 



PLANT SOCIOLOGY 



species — "ecesis" — their arrangement in the new habitat, and finally 

 their "competition." 



The colonization of new unoccupied land by plant disseminules may 

 be either by centrifugal or by marginal invasion. In the latter 

 instance there is a closing in of surrounding vegetation, such as may be 

 seen in mountains and elsewhere, where the first stage is determined by 

 the neighboring vegetation (Figs. 153, 154). In case of centrifugal inva- 

 sion where the distribution is by wind, water, or animals there appears 



at first a motley, but by no means 

 accidental, mixture (Fig. 155). 



Assuming equal ecological 

 fitness, the most favored plant 

 colonists are the first arrivals, 

 that is, species from neighboring 

 areas and those with special facil- 

 ities for distribution. 



The conditions of the habitat 

 determine whether or not there 

 shall be at first a definite selec- 

 tion of species that are allowed 

 to become established in the first 

 stages of the community. The 

 more extreme the habitat condi- 

 tions the more relentlessly will 

 seeds and early seedlings be 

 weeded out. On exposed rocks 

 and talus slopes of the Alps, on the 

 tin soils of Germany, on strongly 

 saline bottoms of lagoons only a 

 limited number of species that are highly specialized can germinate and 

 grow. They often form extensive, pure populations such as those of 

 Spartina, Salicornia, and Suaeda on salt-marsh soil. On good soil in 

 regions with numerous species the selection of species comes much 

 later. Hence the motley communities of recent clearings, freshly 

 exposed soil, abandoned fields, etc. 



Usually the succession begins with the lowest organisms. But it is 

 hazardous to attempt broad generalizations. Even trees may appear 

 as first settlers, if the soil is good, and especially if some protection for 

 the seedlings is available. Salix caprea and Betula verrucosa in central 

 Europe follow directly after a forest fire. Pinus mugho establishes 

 itself in the Alps directly upon naked, wholly plantless talus. The 

 physicochemical factors of the habitats are selective in these first stages. 



Fig. 155. — Initial phase of the Oxy- 

 rietum caused by scattering of seeds upon 

 morainic soil on the Sesvenna Glacier at 

 2,700 m., 40 m. from the edge of the ice 

 (1 sq. m.). P. Z., Poa laxa; P. a., Poa alpina; 

 Ox., Oxyria digyna; Pol., Pohlia commutata; 

 Cer., Cerastium cerastoides ; S., Sesleria 

 disticha (sterile) ; Chr., Chrysanthemum al- 

 pinum; Ba., Bartram,ia ityphylla. 



