22 FORESTRY 



or is destroyed the neighboring trees no longer spread out 

 eagerly to close the gap as before. Sunlight gets in and fil- 

 ters through to the ground, which begins to dry out. Shrubs 

 and grass come in again and with them, groups and patch- 

 es of young tree seedlings. If the old timber is not cut, it 

 will disappear in time, either suddenly by being blown 

 down, or gradually by loss from decay. The older the trees 

 get, the less able they are to resist the attacks of fungi, 

 and the structure of the trunk becomes finally so weakened 

 by rot, both in the bole and in the stump and upper roots, 

 that it falls or is blown down. It is in this decadent half 

 of the life of the stand that the forest will reproduce itself 

 naturally. 



These laws of development best illustrated in the case 

 chosen, where stands are of a single species and even-aged, 

 apply to almost all forms of forests, but are not always so 

 evident. Under natural conditions, large clearings, on which 

 stands of even age should spring up, occur only through 

 fire, or extensive windfalls. If a stand of any species were 

 left entirely undisturbed by such agencies, the old trees 

 would drop out one by one or in small groups, and their 

 places be taken by groups of either the same or different 

 species. Finally the forest would be broken up into small 

 groups of trees of different ages and sizes. The conditions 

 are further modified by the large number of species which 

 may be competing for space in the same stand. Yet the 

 two laws, that of the reduction of numbers with growth 

 in height and that of the survival of the most vigorous trees, 

 will" always apply, no matter what the conditions. 



Silvical Characteristics of Tree Species. — Both the 

 form and the composition of a forest will depend, first, 

 upon the quality of the site and its fitness for tree growth ; 

 secondly, upon the tree species in the region and their 

 needs and, thirdly, upon the history of the forest in that 

 locality, such as the occurrence of fires, insect ravages or 

 other destructive incidents. 



