ECONOMIC VALUE OF FOREST FLORA 199 



able feature with all species, and cannot be considered apart 

 from other factors, especially soil and situation. While 

 certain species, such as larch, may be almost decimated by 

 disease or insects in one locality, they may be perfectly 

 free in another. Species, again, may escape these injuries 

 for years or generations and then become victims on a 

 large scale. The larch sawHy, beech-felted scale, silver fir 

 and Weymouth pine aphides are familiar examples of new 

 insect attacks ; whilst amongst fungi the larch disease and 

 heart-rot among larch, the pine blister on Wej^mouth 

 pine, etc., are terribly on the increase in man}' districts. 

 Other insect and fungoid pests, again, only appear at 

 intervals, while some are only destructive on young trees 

 or seedlings. The oak leaf roller moth, pine sawfly, pine 

 weevil, etc., are instances of these respective types. 



Of all forest trees probably larch is the most subject to 

 both insect attacks and fungoid diseases of a serious 

 nature, and especially so in districts or on soils which do 

 not suit all its requirements. Next to larch, Scots pine 

 suffers badly from the pine beetle at all stages of growth 

 except where precautions are taken in removing breeding 

 material, or fellings are uncommon. Apart from these 

 two species it can scarcely be stated that any British 

 forest tree is seriously troubled with either diseases or 

 insects of great economic importance, except as indivi- 

 duals, or on unfavourable sites which check growth, and 

 increase the relative importance of the parasites. 



4>. Ability to (jrow rapidy on comparatively poor soil. 

 In many respects the value of any forest tree largely 

 depends upon this quality. On good soils practically all 

 trees grow rapidly for a few years, but specific rate of 

 growth varies with the soil and situation, and while some 

 species show a growth directly proportionate to the rich- 

 ness of the soil, others are more influenced by local 

 climate and situation durinaf later staq-es. 



