164 CLEARANCES AND UNDERPLANTING 



smaller number of trees, which correspondingly 

 benefit thereby. 



(4) Owing to the destruction of humus food supplies are 



temporraily increased. 



(5) An immediate utilisation of reserve food material, 



which was stored up in the stem whilst close canopy 

 prevailed, takes place. (Hartig's theory.) 



In connection with this latter reason, it should be noted 

 that reserve food material is far greater in broad-leaved 

 deciduous trees than in conifers, and greater in Larch than 

 in the other conifers. 



Sometimes this increased increment does not take place 

 for a year or so until after the partial clearance. 



In such cases the reason will often be that the trees were 

 not vigorous enough at the time of the partial clearance. 

 However, in a few years' time, when the foliage and roots 

 have recovered their normal activity, and also after new 

 foliage and new roots have grown, the trees begin to actively 

 respond to the treatment they have received. 



One result of the increased increment is the production of 

 broader annual rings than would otherwise have been the 

 case. 



Now, in the case of conifers, broad rings are mostly made 

 up of spring wood which is of low density, for the tracheids 

 which are formed in the spring have thin walls and large 

 lumina ; the reverse, however, is the case with the autumn 

 wood. 



But in the case of broad-leaved trees, it happens that 

 timber with broad annual rings consists mostly of autumn 

 (i.e. summer) wood, which is, generally speaking, of greater 

 density than the wood formed in the spring. 



Especially is this the case with reference to the so-called 

 " ring pored " trees Oak, Ash, Elm, Spanish Chestnut, and 

 Acacia for the spring wood of these trees consists mostly of 

 large " vessels," which are not nearly so strong or dense as 

 the " sclerenchyma " or hard tissue which forms the greater 

 part of the autumn wood. 



Bearing this in mind, it will not usually be advisable to 



