42 



BULLETIN" 871, U. S. DEPARTMENT OF AGRICULTUEE. 



PREVALENCE OF INJURIES. 



Most incense cedars do not attain any great age or size without 

 suffering some injury. Many old trees, and more rarely young ones, 

 show numerous injuries, often fire, frost, and lightning having com- 

 bined in the wounding of a single tree. Of the 509 trees on the 

 intermediate area only 116, or 22.8 per cent, escaped without injury, 

 while on the optimum area 38.9 per cent, or 220 of the total 566, were 

 free from wounds. This difference is explained by the fact that the 

 risk from injury has been greater on the intermediate area than on 

 the optimum area, while a greater number of young trees were cut 

 on the last-named area than on the first. The risk of injury is 

 cumulative, increasing with the age of the tree. 



This cumulative risk of wounding is shown clearly in Table IX, 

 in which the trees from all areas are combined and arranged by 40- 

 year age classes. Only those trees the ages of which it was possible 

 to determine exactly are included in this table, while the data on 

 wounds previously presented include all the trees. This accounts 

 for the apparent slight discrepancy between the figures on the total 

 number of trees involved. 



Table IX. — Incense-cedar trees showing cumulative wounding in the combined areas. 



Age class. 



Number 

 of trees 

 (basis). 



Total 



with 



wounds 



(per 



cent). 



Trees 

 with 

 severe 

 wounds 

 (percent- 

 age of 

 total 

 wounds). 



Age class. 



Number 

 of trees 

 (basis). 



Total 

 with 

 wounds 

 (per 

 cent). 



Trees 

 with 

 severe 

 wounds 

 (percent- 

 age of 

 total 

 wounds). 



1 



2 



3 



4 1 



! 



2 



3 4 





1 

 51 

 185 

 284 

 233 

 118 

 94 





 29.4 

 48.6 

 59.2 

 74.3 

 82.2 

 92.6 







6.7 

 14.4 

 28.6 

 35.2 

 44.4 

 43.6 



281 to 320 years . 

 321 to 360 years 

 361 to 400 years 

 401 to 440 years. . . 



Combined 



49 

 19 

 4 

 2 



98 

 100 

 100 



100 



62.5 





68.5 



81 to 120 years 



121 to 160 years 



101 to200vears 



201 to 240 vears 



241 to 280 years 



100 

 50 



1,040 



67.6 



36.3 



In considering the figures in Table IX the reader should keep in 

 mind the fact that since branch stubs are not treated as wounds, 

 wounded trees practically mean fire-scarred trees, as the number of 

 wounds from causes other than fire have been shown to be insig- 

 nificant. 



Considering column 3, which expresses the ratio of the wounded 

 trees to the total trees, it is seen that the trees are subject to con- 

 siderable wounding at a very early age and that this percentage 

 increases very rapidly, until in the older age classes every tree has 

 been wounded and consequently at some time exposed to infection. 



