PURE AND MIXED WOODS 39 



9. Spruce, Douglas fir (Colorado variety). 



10. Silver fir. 



Some species like larch, Douglas fir, Weymouth pine, 

 Scotch pine, and birch continue their fast growth until they 

 have completed their principal height growth ; others, like 

 hornbeam, remain slow growers through life ; but after the 

 first twenty years, spruce, silver fir, beech, and oak increase 

 their rate of growth, and .soon catch up and pass ash, Norway 

 maple, and sycamore, which do not then grow so fast. 



Trees usually grow in height all through their life, but 

 a time comes when the rate of growth decidedly diminishes, 

 and it is this period at which we say that the principal height- 

 growth ceases. This occurs with Scotch pine at about the 

 sixtieth year, with spruce at about the seventieth, and with 

 oak and beech between the eightieth and ninetieth years. 

 The crowns of broad-leaved trees become rounded off at this 

 period. 



The rate of height-growth must be well considered when 

 planting a mixture of two or more species, as upon it depends 

 the management of the mixture. 



There are three usual forms of mixture: by single trees 

 when each tree is surrounded by another species ; by alternate 

 lines where one line consists of one species, and the next line 

 of another ; and by groups where each species is planted 

 pure in a small group surrounded by groups of other species. 



A mixture by single trees is difficult to manage, and is 

 only possible where all the species grow more or less at the 

 same rate. This is, however, very seldom found to be the case 

 in practice, as usually one species grows faster than the other 

 and the wood tends to become a pure one as the slow growers 

 are overshadowed by the fast growers. Whenever such a 

 mixture has been formed the woodman must very carefully 

 watch the growth of the various species and by the judicious 

 use of the axe must prevent the extinction of one species by 

 another. 



