DENSITY OF CANOPY 151 



Often under such conditions, especially towards the end 

 of a rotation, it is found that only a limited number of trees 

 can thrive, but though a heavy thinning will probably be 

 indicated, the greatest care must be taken not to destroy the 

 humus. 



It must be evident that the aspect upon which trees are 

 growing is a matter of great importance in determining the 

 degree of thinning that should take place. 



As already indicated, it will usually be necessary on a 

 southern aspect to have the canopy as thin as is compatible 

 with the retention of humus ; but on northern aspects the 

 relative density of the canopy may usually be either greater 

 or less. 



A denser canopy is admissible because, owing to the 

 greater amount of moisture in the soil and air on northern 

 aspects, tree-growth is more vigorous in spite of the fact that 

 the light is less intense. This, however, would not be the 

 case if there was plenty of moisture on any given southern 

 aspect. 



Then again, on the other hand, the canopy may be 

 thinner on a northern than on a southern aspect, because the 

 humus will not disappear so quickly, and any rank growth 

 of grass and weeds will be slower to appear. 



It will nearly always be preferable to have the thinner 

 canopy, provided that the principal height growth of the crop 

 has been attained, since the individual trees will put on a 

 greatly increased increment ; and the money realised for that 

 portion of the crop which is removed, should earn 3^ or 4 

 per cent, interest, and perhaps more. 



And moreover, whenever the conditions for the growth 

 of timber are favourable, though a somewhat thicker canopy is 

 admissible than when the reverse is the case, it will usually 

 be advisable to have a somewhat thinner canopy than is the 

 case on poor soil, provided always that the principal height 

 growth has been obtained, though not until then, for the 

 evil effects of wind and insolation will not be so serious, and 

 the trees will be quicker to form close canopy again. 



When thinning mixed woods, the requirements of 



