108 



PLANT SOCIOLOGY 



strated that certain color types have their maximum in certain definite 

 plant communities. The maximum of red pigmentation, correspond- 

 ing to the greatest insolation, was in the dwarf-shrub associations, 

 which also contain the minimum of species that show distinctly green 

 pigmentation. He insists that there is also a marked agreement 

 between the phytochromatic spectrum and the anatomy of the species 

 of each group studied. 



The annual light curve under a cover of evergreens runs rather 

 evenly. In the several-layered deciduous forest communities the 

 ground layer experiences a well-lighted and favorable springtime 

 (vernal-fiora aspect), followed by a period of low light intensity caused 

 by the maximum of the foliage and having a maximum development 



J une 



March April May 



Fig. 59. — Decrease in the light intensity in the lower layers with the advance of 

 the season and the unfolding of leaves (A) in birch forest with Vaccinium undergrowth; 

 (B) in Alnus glutinosa forest; (C) in Fagus silvatica forest; and (D) in Pinus silvestris 

 forest. 



of summer shade plants (Fig. 59). In beech woods and other decidu- 

 ous forest communities with deep shade the annual rhythm of vegeta- 

 tion is most intimately connected with the course of the light curve. 



The relative minimum of light, that is, the light requirement of the 

 most feebly illuminated assimilating leaves, is nearly constant at the 

 same latitude but decreases toward the poles. The minimum for 

 the larch is }^i, for the beech >^o to ^^go of the total light in the open. 

 According to Hesselman (1917), the light requirement of trees on poor 

 soil is greater than that on good soil. 



The relative light minimum may become a deciding factor in the 

 competition between two plant communities with different light 

 requirements. The literature of forestry and of plant sociology 

 furnishes many examples. As a rule, the tree layer that has the 

 lowest relative light requirement wins. In many parts of central 

 Europe Fagus silvatica forms the victorious tree layer; in mountains, 



