318 



Relations within the Species 



fected by competition. The growth form of a sohtary tree is com- 

 pletely different from that of a tree of the same species that has de- 

 veloped in a stand closely surrounded by other individuals. In the 

 latter situation the lower branches are killed by the reduction of light, 

 or the growing buds and branchlets are knocked off by the branches 

 of trees near by, as they whip about in the wind. The smaller, 

 neighboring trees that bring about this pruning action are referred 

 to as "trainers" by foresters, since they cause the tree to develop 

 without lateral branches in a form that will later be suitable for saw 

 timber. 



Growth ring for 

 the season of 1944 



nner bark 



Outer bark 



Land cleared 

 spring 1937 

 Growth 

 released 



Progressive 

 overcrowding 

 Competition 

 retards growth 



Fig. 9.3. Cross section of locust tree (Robinia Pseudo- Acacia) , 17 cm in diam- 

 eter, grown in Belmont, Mass., showing effects of early overcrowding and sub- 

 sequent "release." 



Many examples of the harmful effects of overcrowding could be 

 cited for other types of plants and for animals. We find the same 

 principles of competition at work among populations of microbes as 

 among forest trees. Bacteria deplete their supply of nutrients as they 

 multiply and produce an accumulation of metabolites, until the fur- 



