150 TENDING OF YOUNG HIGH FOREST, ETC. 



after that age any undue struggle for existence must be 

 avoided or the whole crop will be a failure. After the first 

 thinning has been made, other very slight thinnings should 

 be made every 5 to 8 years until the principal height growth is 

 attained ; and directly the humus disappears and weeds begin 

 to grow, underplanting or undersowing should take place. 



For this purpose, Spanish Chestnut or Beech, or a 

 mixture of them, are the best trees to plant. 



It must always be remembered that the execution of 

 thinnings and the density of the canopy will greatly affect 

 the amount of moisture available for the crop. 



As far as the question of moisture alone is concerned, a 

 serious interruption of the canopy will not be so harmful on 

 fertile soils as it will be on poor soils, or wherever the condi- 

 tions for the growth of timber are unfavourable. 



The soil will probably contain a maximum amount of 

 moisture when the canopy is neither too thick nor too thin. 



With this object in view, the canopy should be as thin as 

 is compatible with the retention of humus and dead leaves. 



For if the canopy is unduly thick, transpiration through 

 the leaves is at its maximum ; so also is the interception of 

 atmospheric precipitations, rain and dew ; whilst evaporation 

 of moisture from the surface of the soil is reduced to a 

 minimum. 



Now, if this canopy be somewhat broken, transpiration 

 and interception are lessened, and, provided that humus does 

 not disappear, evaporation of soil moisture will not be very 

 much increased. 



But, if the canopy be still more broken, humus will 

 disappear, and evaporation of soil moisture will be increased ; 

 later on, grass and a rank growth of weeds will appear, and 

 transpiration and interception of moisture will again take 

 place, though this time, chiefly from the soil covering of 

 grass and weeds, instead of only from the trees as in the 

 former case. 



Having regard to this, it is obvious that the greatest care 

 is necessary on dry soils or on hot southern aspects, where 

 usually the soil is lacking in sufficient moisture. 



