30 FORESTRY 



Under-planting, or otherwise introducing species for the 

 purpose of protecting the soil, comes into use in the case of 

 older woods. When, either from natural causes or past 

 mismanagement, woods have become too open to shield the 

 soil from wind and sun, an underwood is frequently planted 

 in order to restore or maintain the productivity of the ground. 

 The desirability for this lies in the fact that under an in- 

 sufficient leaf-canopy, unfavourable changes occur in the con- 

 dition of the upper layers of the soil, resulting in retarding 

 the older wood's further development, and threatening to add 

 difficulties to the regeneration of the area. To avoid this 

 deterioration, shade-bearing species usually Beech or Silver 

 Fir, less often Spruce are introduced below the existing 

 crop. 



The conspicuous merit of mixed woods consists in the 

 additional value and quantity of the material raised. Es- 

 pecially is this so when light-loving and shade-bearing species 

 are together in the mixture. The productive capacky of the 

 soil is in such cases more fully utilised than in a wood com- 

 posed only of light-demanders. The action of the shade- 

 bearing species lies in the early and complete removal of the 

 light-demanders' side branches, consequent on these being 

 killed from want of light. Cleaner and better-shaped boles 

 result, and the timber is enhanced in value. 



Further, there is the effect upon the soil. Under the thin 

 canopy provided by light-demanders in their later periods, 

 heavy rains wash away valuable chemical substances from 

 light soils, while heavy clay is rendered more impervious. 

 Strong light upon forest soil encourages the growth of 

 heather, blaeberry, and so forth, which gives rise to ac- 

 cumulations of fibrous organic matter ; grass also springs up, 

 impoverishing and drying the soil. All this is prejudicial 

 to forest growth, and can be prevented by maintaining a 

 close canopy such as may be had from the association of 

 shade-bearing with light-demanding species. 



Mixed woods are less exposed to certain dangers than 

 woods consisting of a single species. Against wind the 



