18 



BULLETIN 722, U. S. DEPARTMENT OF AGEICULTURE. 



that the average infection age for the slope type is higher than that 

 for the river-bottom type. This is undoubtedly due to the fact that 

 the environmental factors in the river-bottom type are more favor- 

 able to early attack by the fungus. Crowded stands, much shade 

 and suppression, and a higher percentage of atmospheric and soil 

 moisture all presumably contribute to the earher infection in the 

 stands situated along the river bottoms. 



It is to be understood that the figures as given in the tables that 

 follow are not to be taken as absolute. The small numerical basis 

 for some of the data, especially in Table I and part of Tables II and 

 III, and other sources of unavoidable error, make it plain that the 

 figures given are to be interpreted as indicative of the true conditions 

 and not as an absolute analysis. The general tendencies must first 

 be ascertained and the methods for determining theni developed 

 before absolute figures are presented. 



Table II. — Average annual increase in the volume of heart-rot in infected ivestern hem- 

 lock trees of the several age classes, on plats of the river-bottom and southwestern-slope 

 types. 



Age class. 



River-bottom tj-pe : 

 46 to 56 j^ears . . . 

 57 to 62 years.. - 

 63 to 70 years . . . 

 71 to 80 years . . . 

 81 to 90 years . . . 

 91 to 102 vears.. 

 104 to 126 years. 



All age classes . 



Slope type: 



54 to 70 years . . . 

 72 to 75 years . . . 

 90 to 99 years... 

 101 to 120 vears. 

 134 to 160 years. 

 162 to 179 years. 

 184 to 200 years. 

 207 to 225 years. 

 226 to 306 years. 



All a?e classes . 



Average 

 age. 



Years. 

 52 

 60 

 66 

 75 

 86 

 95 

 114 



Interval 

 between 



classes. 



Years. 



Average volume of 

 rot (cubic feet). 



As meas- 

 ured. 



0.27 

 .55 

 .82 

 2.23 

 3.62 

 4.04 

 4.21 



2.25 



62 



74 

 94 

 110 

 149 

 172 

 193 

 216 

 254 



.12 

 .31 

 1.42 

 14.31 

 16.89 

 19.16 

 46.62 

 46.56 

 73.43 



24.30 



Annual 

 increase 

 between 



age 

 classes. 







.035 



.045 



.16 



.13 



.05 



.01 



.07 



.02 

 .06 

 .81 

 .07 

 .10 

 1.31 

 a. 62 

 .71 



.40 



Number 

 of in- 

 fected 

 trees 

 (basis). 



a Interpolated. 



RELATION OF DECAY TO SITE AND TO AGE. 



Western hemlock, which ranges in decay from 50 to 100 per cent 

 of trees infected in some localities and quite generally so over lai^ge 

 areas where the species occurs in nearly pure stands, may be entirely 

 free from its principal disease (heart-rot, caused by EcMnodontium 

 tinctorium) in some regions. 



