ABSTRACT 



The Ponderosa Pine Risk Rating System developed in California 

 was studied in western Montana to determine whether it could effec- 

 tively identify individual mature trees most frequently killed by the 

 western pine beetle, Dendroctonus brevicomis LeConte, or the moun- 

 tain pine beetle, D. ponderosae Hopkins (Coleoptera: Scolytidae), 



Risk 3 and Risk 4 trees- -the high risk trees of the four -rating 

 system- -comprised 20 percent of the board-foot volume of 12,000 

 merchantable, risk rated ponderosa pine trees at 35 localities. On 

 study plots in 22 of these localities, ponderosa pine stands that re- 

 mained undisturbed throughout the study initially contained 17 percent 

 of their total pine volume in Risk 3 and Risk 4 trees. Risk 3 and Risk 

 4 trees, however, made up 76 percent of the volume of all ponderosa 

 pine trees killed by populations of the two pine beetles on these plots 

 during the study . 



Ponderosa pine mortality from the two pine beetle species was 

 consistently low during the study, amounting to a mean of only 15,5 

 board feet per acre per year on the 22 undisturbed study plots- -an 

 amount considerably less than the estimated gross ponderosa pine 

 increment on the same plots. 



Subsidiary information obtained from the study indicated that 

 (1) external crown characteristics used by the risk rating system in 

 California to delineate the risk of mature ponderosa pine trees to 

 attack by the western pine beetle were equally effective for this pur- 

 pose in western Montana; (2) Risk 3 and Risk 4 ponderosa pine trees, 

 together, grew an average of 0.18 inch radially during one 10-year 

 period of the study, and Risk 1 and Risk 2 pine trees grew an average 

 of 0.43 and 0.31 inch, respectively; (3) in 15 mature ponderosa pine 

 stands where soil characteristics were measured. Risk 3 and Risk 4 

 trees were progressively more abundant as the fertility, productivity, 

 and water-holding capacity of the soils declined; and (4) the mountain 

 pine beetle was not an important primary killer of mature ponderosa 

 pine trees during the study, and it exerted little influence in predis- 

 posing low risk trees to attack by the western pine beetle. 



It was concluded from the study that managers of mature pon- 

 derosa pine forests in western Montana can use the risk rating system 

 to assess the susceptibility of these forests, or of individual ponder- 

 osa pine trees in them, to lethal attacks of the western pine beetle for 

 10 years or more during periods of endemic beetle populations. 



