FOREST INSECTS ON THE CULBIN SANDS. 14 1 



mind that the small extent of our forests, more than insular and 

 climatic factors, has given us our comparative immunity from 

 serious insect outbreaks. The losses caused in forestry by 

 insect pests in Britain are only now being realised ; Pine Weevil, 

 Cockchafer, Oak Tortrix, and Silver Fir Chermes cause much 

 more loss than appears at first sight. As our forests extend, 

 these and other pests if unchecked will become more serious in 

 proportion, and it is important that we should earnestly study 

 our forest insect conditions now, so that when our forests are 

 more extensive we may know some of the principles on which 

 successful forest protection is based. It is gratifying to be 

 able to say that the Commissioners' work on these lines is 

 already promising results in the control of Hylobius, and it is 

 hoped that as finances allow this work will be continued and 

 extended. 



19. Form-Factor as an Expression of the Degree of 

 Taper in Coniferous Trees. 



( With Figures.) 



By Mark L. Anderson. 



Form-factors are notoriously variable, and this variation 

 depends on several different elements which are themselves not 

 simply related. It is, therefore, no easy matter to show con- 

 clusively how and why form-factors vary, but a very good idea 

 can be obtained from an examination into the variation of the 

 form-factors of a series of trees of the same mathematical form 

 or degree of taper, but of different sizes. This method is useful 

 because it brings cut clearly the fact that, as expressions of the 

 degree of taper of stems, form-factors are unreliable. Their 

 chief use is as reducing factors for ascertaining the volumes of 

 stands, of which the basal area and mean height are known. 



There are, unfortunately, several types of form-factor, few of 

 which are in practical use. Investigation into the variation 

 of some of these, in addition to that of the common type, is 

 useful. 



