ch. i THE STRUGGLE FOR EXISTENCE 231 



describe the constant changes which may take place in 

 such a crop during the various phases of its existence. 



In a crop composed of individuals of the same species 

 but of various ages a certain number of individuals have 

 already established themselves in a dominant position ; 

 the struggle will then go on between those which are 

 striving to fill up the gaps in the upper tier left between 

 these, or in the fresh gaps which may be formed by the 

 removal of some of the dominant trees. 



In a mixed forest of varied ages the struggle is 

 again more complicated by the varied requirements, 

 rates of growth, and vitality of the various species. In 

 one place there may be shade -avoiding trees in the 

 upper tier with shade-enduring trees below them gradu- 

 ally working up their way, or if the converse is the case 

 the shade-avoiding trees are suppressed and gradually 

 killed. In a gap formed by the removal of one of the 

 trees in the upper tier may be found, struggling up to 

 the light, species which like side shelter but must have 

 light overhead. In fact, the composition of the crop in 

 its species, height, and girth classes and its density may 

 vary so much from one place to another that a connected 

 description becomes difficult. 



It is evident that if this struggle for existence is 

 allowed to go on unchecked the forest becomes largely 

 composed of dominated, suppressed, and dying trees, and 

 that even the stronger trees are not able to attain the 

 same development in such a crowded wood as if they had 

 more growing room. It is then that the forester must 

 come in, and by special operations favour the growth of 

 such trees as will be most useful for the owner of the 

 crop. If the forest is a pure one the operations must 

 simply tend to give sufficient growing room to the 

 strongest and finest individuals, to enable them to 

 attain the best dimensions possible for the purposes of 

 the forest owner. In a mixed forest the strongest may 

 not necessarily be the most valuable, and in that case 

 the operations must aim at giving sufficient growing 

 room to a proper proportion of the more valuable species. 



