14 BULLETIN 1131, U. S. DEPARTMENT OF AGRICULTURE. 



[ Frost-ring formation may occur in the wood from the action of 

 either late or early frost during the course of the growing season 

 or from the freezing of the cambium during the winter when the 

 tree is dormant. The frost rings, therefore, may register at any 

 point within the growth ring, the relative position of the frost ring 

 within the growth ring signifying the time at which the injury 

 occurred. 



Frost rings arise through late frost only when the cambial ac- 

 tivity has already commenced and some new xylem cells have been 

 differentiated. As a rule, there is a definite zone of spring-wood 

 tracheids intervening between the outer limit of the summer wood 

 of the preceding annual ring and the frost ring. In the case of 

 early frosts the frost ring may either register late in the summer 

 wood of the growth ring or not until the immediate beginning of 

 the ensuing growth ring. Frost injury occurring during the dor- 

 mant period likewise is recorded as a frost ring in the immediate 

 beginning of the ensuing growth ring. 



Young trees injured by repeated frosts often develop an abnor- 

 mally compact and bushy form, especially in Abies grandis and 

 other species, which readily form compensatory shoots. Frost in- 

 jury that results in the killing of the young shoots often detracts 

 greatly from the straight axial growth of the trees and, where fre- 

 quently repeated, may render the tree absolutely valueless for com- 

 mercial purposes. In addition, late-frost injury may render young 

 conifers more susceptible to weakly parasitic fungi than they would 

 be otherwise. 



Late-frost injury, when occurring late in the season after any 

 considerable portion of the growth ring has been formed, results 

 in a false or double ring formation, which is often confusing in 

 age determinations. Frost-ring formation from late-frost injury 

 has not been observed by the writer in coniferous stems larger than 

 2 inches in diameter, although it often occurs in larger stems of 

 fruit trees that are subject to various forms of frost injury. 



As may be expected from their structure, frost rings constitute 

 a plane of weakness in the wood, which may not only predispose 

 to the formation of circular shake in the standing tree, but may 

 require the manufactured wood to be discriminated against for use 

 in small pieces where great strength is required. 



