CAUSES OF PINE TREE KILLING 



The killing of mature ponderosa pine trees from several causes was measured on 22 

 plots that retained undisturbed virgin stands throughout the study. The timber stands 

 on these plots were not subjected at any time to ground fires, flooding, or other 

 environmental disturbances that would cause the death of ponderosa pine trees within 

 1 year or less. 



Measurements from the 22 plots recorded the death during the study of 366 mature 

 ponderosa pine trees having a combined volume of 308,810 board feet (table 8) . They 

 were killed by attacks of Dendrootonus bark beetles, lightning strikes, other cambium- 

 feeding insects, windstorms, or unknown causes. 



Similar measurements of ponderosa pine mortality from 13 other plots used for 

 portions of the study are not reported here. The stands on these latter plots were 

 lightly cut under several experimental tree selection systems soon after the plots were 

 established. The cuttings were made to determine if such treatments could influence 

 the postcutting rate of ponderosa pine tree killing by bark beetles or other quick- 

 acting lethal agents. 



The uprooting or lethal breaking of trees by windstorms proved to be the greatest 

 source of mortality: 52 percent of the 366 trees killed on the plots were from this 

 source (table 8) . Such killing occurred on almost all the plots each year, but several 

 plots were subjected to catastrophic tree killing from two or three windstorms of hurri- 

 cane force that swept through parts of western Montana during the study. On two plots 

 (Loon Lake and Jake Little Ranch) , the damage was so great that the usefulness of these 

 plots ceased after 4 and 7 years, respectively (table 9). 



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