HRITISII FOREST TREES 93 



annual increase in height culminating with shoots averaging 

 one and a half feet on soils of the best quality between 

 the twenty-sixth and fortieth year, and later, with of 

 course smaller averages, on those of merely average or 

 inferior quality (vide tables on page 36 38). Throughout 

 this period of energetic development, the natural sup- 

 pression of dominated stems gradually progresses, but 

 without any practical interruption of the canopy taking 

 place, so that the boles are enabled to assume that full- 

 wooded cylindrical shape which renders them so valuable. 

 According to Baur this maximum of approach to the 

 cylindrical is attained when the average height of the crop is 

 from sixty-six to eighty feet, but with advancing age it sinks 

 only gradually. 



One decided drawback of the close canopy and even 

 development of spruce woods at this stage of their 

 life-history is the consequent danger from accumulations 

 of snow, which often occasion serious damage a danger 

 however not so much to be feared in either Scotland or 

 England as on the Continent with its severe winters. 

 Later on the density of the crop also gradually diminishes 

 without the continuity of the canopy being very seriously 

 interrupted, whilst from the time that it has reached the tree- 

 forest stage of development, a rich growth of mosses (Hypnum} 

 covers the soil, which, however, gives place to whortleberry 

 ( Vacciniuni) and similar weeds when self-thinning with 

 consequent interruption of canopy has gone too far. It 

 is at this stage of growth that pure forests of spruce of 

 equal age are most exposed to the danger of windfall and to 

 attacks of bark-beetles (Bostrichini). No species is so little 

 able to resist the force of storms as the spruce, and when 

 once violent winds succeed in breaking up the canopy, 

 they seem to act in a concentrated and cyclonic manner, 

 throwing down everything that offers resistance to their 



