16 lU'LLETlK 722, U. S. DEPARTMENT OF AGRTCULTUEE. 



secured from a careful checking of all the data and from the table 

 by Hanzlik and Oakleaf ^ giving the average annual diameter growth 

 for western hemlock as obtained under average conditions in western 

 Washington. 



Rating of Vigor. 



o=Thrifty. — Width of average ring in sap 0.20 inches and up; 1 in crown 

 rating; classed as or x under degree of injury. 

 oo=Fair. — Width of average ring in sap 0.12 to 0.19 inches; 2 in crown 

 rating; classed as x or xx under degree of injury. 

 ooo=Poor. — Width of average ring in sap 0.04 to 0.11 inches; 3 in crown 

 rating; classed as xx or xxx under degree of injury. 

 oooo=Low. — Width of average ring in sap 0.03 inches and less; 4 in crown 

 rating; classed as xxx or xxxx under degree of injury. 



Other methods specially adapted to develop certain data will be 

 found explamed under the headings which follow. 



The size of the average ring in the sap is the most important 

 factor in the vigor determinations. The injury ratings (as x or xx) 

 in the oo vigor class are intended to give a certain leeway in so far as 

 the injuries found on the trees are concerned. Many trees have an x 

 rating for injury, yet the vigor as indicated by crown size and by 

 width of average ring in sap indicates a thrifty tree. A similar 

 leeway is given the other vigor classes. 



INFECTION AGE. 



In studying the life history of a particular type, such as the river- 

 bottom type of western hemlock, it becomes evident in the course 

 of the work that certain age classes within that type represent a 

 definite stage in the development of decay. This has been brought 

 out by Meinecke ^ in his work on white fir (Ahies concolor) . The 

 factors governing the entrance and development of a fungus in its 

 host tend to determine a certain average age which indicates the age 

 of first infection, an age at which the stand is most liable to first 

 infection by the fungus and below which the infection rarely occurs. 



Judging from Meinecke's^ discussion of the age of infection, he 

 defuies it as the age at which ''infection rarely leads to more than 

 negligible decay miless the tree is handicapped by quite miusually 

 severe conditions." An attempt has here been made more accu- 

 rately to define this average age. The youngest trees only were 

 used and of these only those which were infected. This age is briefly 

 outlined as the average age of the youngest trees open to first 

 infection by the fungus. 



1 Ilanzllk, E. J., and Oakleaf, EI. B. Western hemlock; Its forest characteristics, properties, and uses. 

 Tn Tlmberman, v. 15, no. 12, 1914, p. 25-33, tab. 3. 



2 Meinecke, E. P. Forest pathology in forest regulation. U. S. Dept. Agr. Bui. 275, p. -17-48. 1916. 



3 Mcincfko, K. I'. Op. cit., p. 48. 



