16 BULLETIN 722, U. S. DEPARTMENT OF AGRICULTURE. 
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 0 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 explained 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 z or xa) 
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 (Abies 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 
defines it as the age at which ‘‘infection rarely leads to more than 
negligible decay unless the tree is handicapped by quite unusually 
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 oe to first 
infection by the fungus. 
1 Hanzlik, E. J., and Oakleaf, H. B. Western hemlock; its forest characteristics, properties, and uses. 
In Timberman, v. 15, no. 12, 1914, p. 25-33, tab. 3. 
2 Meinecke, E. P. Forest pathology in forest regulation. U.S. Dept. Agr. Bul. 275, p. 47-48. 1916. 
3 Meinecke, E. P. Op. cit., p. 48. 
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