92 FOREST commissioner's REPORT. 



•ever, the rate of growth falls below 3 per cent, so that further 

 growth of the stand is hardly profitable in spite of the increased 

 value of the wood. With very old trees averaging about 20 

 inches in diameter the annual rate of growth is only i per cent. 

 Paper birch is a comparatively short-lived tree. It matures 

 ■early and goes to pieces rapidly thereafter. This characteristic 

 varies according to local conditions, and trees on good soils live 

 longer than those unfavorably situated. In general, seedling 

 paper birch is mature at from 70 to 85 years, and seldom lives 

 more than 150 or 200 years. Sprouts are much shorter-lived 

 and usually mature at from 50 to 60 years, and live only from 

 70 to 90 years. In pure stands birch matures and decays earlier 

 than when in mixture with other hardwoods or with conifers. 



SUSCEPTIBILITY TO INJURY. 



By far the worst enemy of paper birch is fire. Young seed- 

 lings and saplings are destroyed by light ground fires, while 

 even older trees with their thin and highly inflammable bark 

 may be seriously injured or even killed. Severe fires are almost 

 sure to destroy the entire stand. 



Although paper birch is not deep-rooted it is nevertheless 

 comparatively free from windfall. This is probably due chiefly 

 to two causes : The root system, although relatively shallow, 

 has a considerable lateral extent and affords good support to the 

 tree ; and the slender twigs and branches bend readily before 

 the wind, and therefore oft'er but little risistance to it. 



Living trees are sometimes attacked by a flat-headed bark 

 borer {Agrilus an.vius), and by the birch bark beetle (Xylo- 

 coccns bctulcu), and are defoliated by several kinds of cater- 

 pillars. A number of species of beetles destroy the wood of 

 felled trees. 



Fungi are a secondary enemy. Live trees are occasionally 

 attacked by Pomes ignlarins and one or two other shelf fungi, 

 but these attacks are not serious. Dead trees, on the other 

 hand, are attacked at once by Polyponis bctitlinus and other 

 fungi, and decay very rapidly. 



In many places deer play havoc with young birch. They are 

 fond of browsing on the winter buds and tender new shoots and 

 uot infrequently destroy enough young sprouts and seedlings to 

 interfere seriously with reproduction. 



Among the minor enemies of the birch arc the sapsuckers, 



