22 THE STUDY OF PLANT COMMUNITIES • Chapter II 



FIG. 2. A stand of mixed conifers in Idaho— U. S. Forest Service. 



which results whenever several organisms require the same things 

 in the same environment. The intensity of competition is deter- 

 mined by the amount by which the demands exceed the supply. 

 Competition may occur between individuals of the same species. 

 Because they are alike, their demands are identical, and, if the sup- 

 ply of water or nutrients or light is insufficient to satisfy the needs 

 of all, then some will be eliminated. This is particularly notice- 

 able in young, crowded forest stands but is equally true among 

 roadside w r eeds or in a vegetable garden. All plants may survive 

 for a time in a stunted condition; then some individuals are gradu- 

 ally eliminated. Whether in the forest or in the garden, thinning 

 to reduce competition between species usually pays with more 

 lumber or better vegetables. 



Stratification.— Usually there are several species involved in 

 competition within a stand. If plants of several species that start 

 simultaneously make the same demands upon the habitat, they may 

 survive in about equal numbers and occupy the same position in 

 the community. Those whose requirements differ will affect each 



