12 



PLANT SOCIOLOGY 



about 2,000 young beeches to come to maturity. According to 

 Cajander (1925), a fifty-year-old pine requires an area of 2 sq, m.; at 

 the age of seventy-five years it requires 3.8 sq. m.; at one hundred 

 and twenty-five years, 11 sq. m.; at one hundred and fifty years, 15 

 sq. m. On a given area, therefore, only a definite maximum number of 

 well-developed individuals of one species can find a place in the sun. 

 Competition between Individuals of Different Species. — Within the 

 bounds of a social unit that is in equilibrium, very rarely does one single 



Fig. 5. — Tuii- ; (i iu1ma \Mth periplieral spreading (Soc. 3) as pioneers upon 



snow soil lu the Norwegian mountains near Yoss at 1,300 m. {Photo by Lid.) 



species gain control to the exclusion of all others. Usually a plant 

 community exhibits a motley mixture of the most varied plant forms. 

 The reason for this lies in the fact that the locality includes many 

 ecologically different habitats due to layering in air and in soil, rather 

 than to a great variety of edaphic conditions within a very limited 

 space. Within a homogeneous plant community the edaphic condi- 

 tions are often extremely uniform, although the opposite has been 

 claimed by many investigators. The example of the beech forest with 

 its successive waves of flowers and assimilation in the ground and herb 

 layers is well known {cf. p. 47). Many such examples could be cited. 

 Many communities of the subtropics begin their annual development 



