THE THEORY. 301 



On the other hand, the early isolation of a portion of the 

 trees has weighty drawhacks. In the case of many species it 

 affects injuriously the height growth of the trees. Then, 

 isolated trees maintain their side branches low^ down, and even 

 develop fresh ones, which for many purposes seriously reduce 

 the value of the stems. In the case of conifers the quality 

 of timber also may be lower, owung to the formation of 

 exceptionally broad concentric rings. Isolated trees are 

 further liable to form more tapering boles than those grown in 

 crowded woods. Above all, in the majority of cases a suffi- 

 cient layer of humus cannot be preserved, its place being taken 

 by weeds. Moreover, a suital^ie degree of moisture cannot be 

 maintained. It follows that, except on really fertile soils, 

 other means must be devised to preserve the continued 

 activity of the soil. This is done by the introduction of an 

 underwood, or soil protection wood. But even then it is 

 found that the extra diameter increment, laid on after isolating 

 the trees, will hold out only on soils of some quality, while on 

 indifferent soils it will, after a few years, sink back to its 

 previous amount. 



The general theory of the method of treatment in the case 

 under consideration may be shortly described as follows : — 



Commencing with the first thinnings the most promising 

 trees are singled out, and these are isolated sufficiently by 

 increasingly heavy thinnings, so as to permit the introduction 

 of an underwood. As soon as the latter has established itself, 

 and is capable of protecting the soil, a further heavy thinning 

 is made, by which the remaining trees are completely isolated. 

 Subsequently more thinnings follow, as required by the 

 extension of the crowns of the trees. 



The underwood can be established in a variety of ways, 

 such as by sowing or planting, by natural seeding, or by 

 coppicing a portion of the overwood. In some cases the 

 underwood itself is allowed to grow into timber trees (two- 

 storied high forest) ; in others it remains a soil protection 

 wood. 



