TREES 



369 



top each other. The one that succeeds will get most light, 

 and therefore grow the best. In the forest the lower branches 

 of the trees are killed by the shade of other trees, and in time 

 they fall off. The wound left by the branch, the knot hole, 

 generally heals over with a new layer of wood and bark. 

 In the forest the lower branches may not be within thirty 



FIG. 158. An Open Grove of Hard Maples. 

 (The trees have rounded crowns, low branches.) 



or forty feet of the ground, while in the open the same tree 

 would have had a short thick trunk with branches growing 

 low, and rounded crown. 



For the first ten years, the rate of growth of trees is slow. 

 It is most rapid for several decades after this, and slower 

 again later. How long do trees live? That depends upon 

 the species. Some, like the poplars, are short lived, a hun- 

 dred to a hundred and fifty years. Maples and oaks live 



