FOREST PATHOLOGY IjST FOREST REGULATION. 11 



shrunk but little, differences in internal tension will cause the outer 

 layers to split vertically. With rising temperature the frost crack 

 closes. Not always is the bark able to stretch sufficiently over a 

 frost crack. Often the bark tears open, and if low temperature 

 occurs again and agam, the cracks will not be able to heal over and 

 will remain open for many years, giving the air access to the heartwood 

 and incidentally allowing spores of wood-destroying fungi to ger- 

 minate and infect it. Even if no infection takes place, these frost 

 cracks very seriously impair the value of the timber. The older and 

 bulkier the tree the greater is the danger of frost-crack formation. 

 The risk naturally increases with the length of the cutting cycle. 

 Infection, of course, can take place only through open frost cracks; 

 internal frost cracks, besides impairing the value of the timber, can 

 not be without influence on the chemistry and physics of the wood. 



Although lightning occasionally strikes the smaller trees, even 

 poles and saplings, it is to be expected in the nature of things that 

 taller trees will be more exposed to injury from this cause. Very 

 little is known, so far, as to the actual damage done by lightning in 

 our forests. Destruction of individual trees has been frequently 

 reported, and Plummer 1 gives a series of illustrations of injury to forest 

 trees from lightning. He treats, however, only of those very gross 

 cases in which even the least educated eye will recognize the cause 

 of the injury. 



We know through Robert Hartig's 2 classical investigations, which 

 were continued by Von Tubeuf, that destructive lightning is rare 

 in comparison to the overwhelmingly greater number of cases of 

 lighter injury from lightning, varying from more or less large wounds 

 visible on the outside of the tree to the small and insignificant local 

 killing of parts of the cambium and of the living bark which can 

 only be detected by careful dissecting. 



The symptoms of lightning injury in our forest trees are easily 

 recognized from Hartig's excellent descriptions. They are particu- 

 larly common and conspicuous in white fir. 



For practical purposes, we have to consider here only those forms 

 of lightning injury which in some way endanger the life, the health, or 

 the commercial value of the tree: this will include not only actually 

 destructive cases, but also very large numbers of lesser injuries. 

 The accumulation of risk during a long-time cutting cycle becomes 

 self-evident, particularly in view of the fact that the danger from 

 lightning increases quite out of proportion to the increase in height 

 of the tree and the development of the root system. 



' Plnmnur, P. 0. Lightning in relation to forest fires. U. s. Dopt. A^r.. Kon-st. Sitv. iiui. in, :t'.> pp., 



I'll 2. 



(Hartig, Robert. (Tntenaohaagen (iij'T BlltzsohlAge in ffiddbttumen, In Porstl. Naturw. Ztsohr., 

 • 1897, Beft 8, pp.OT 120; Hefl i. pp 148 168; Hefl 6, pp. 108 206. 83 figs. 



Robert. ' Mum-mi Qber Blitzboschfldlgung der Bftuxne. /» Centrbl, Qesam, 



:,: .17 71; Heft 12, pp 823 M, Bgl 81 110. 



