Table --Cause and amovnt of ponderosa pine tree mortality from 1948 

 through 1968 on 22 plots not subjected to sanitation-salvage 

 cuttings ^ 



Cause : 







: Mortality 



of tree : 



Total mortality 



: per acre 



per year 



mortality : 



Trees 



: Board feet 



: Trees 



: Board feet 



Bark beetles 



132 



103,360 



0.020 



15.5 



Lightning 



16 



29,330 



.003 



5.5 



Other insects^ 



18 



9,080 



.002 



1.8 



Unknown 



11 



15,790 



.002 



2.7 



Windstorms 



189 



151,250 



.026 



21.4 



Total 



366 



308,810 



.053 



46.9 



■^See table 9 for number of years of records form each plot. 

 ^Principally, the pine engraver (7ps pini) and the California flat- 

 headed borer (Melanophila calif omica) . 



The next greatest source of pine tree killing was infestation by bark beetles. 

 It also was the most consistently occurring form of mortality in the plot stands year 

 after year (fig. 6). Bark beetles accounted for the death of 132 pine trees (36 percent 

 of all the trees killed during the study). 



Of these 132 trees, 124 were killed by the western pine beetle ( Dendroctonus 

 brevicomis) and eight by the mountain pine beetle (Dendroctonus ponderosae). None of 

 the trees killed by Dendroctonus beetles had been previously top-killed by the pine 

 engraver (Ips pini)j nor was this form of damage to mature ponderosa pine trees observed 

 in the plot stands. 



Systematic bole analyses to determine the insect species responsible for the death 

 of these trees indicated that the trees were killed by low level populations of attack- 

 ing pine beetles. Not only were the attacks per square foot of bole bark surface rela- 

 tively few, but the areas of infested cambium occupied by developing broods of these 

 insects were scattered and likewise small. This provided ample habitat for secondary 

 cambium- feeding insects to subsequently infest these same trees. Consequently, rela- 

 tively large proportions of the available bole cambium of beetle-killed trees were 

 infested by Ips emarginatus (LeConte) , Ips plastographus (LeConte) , and by assorted 

 wood-boring beetles of the families Buprestidae and Cerambycidae (Johnson 1967). 



Lightning, other cambium-feeding iiisects, and unknown causes together made up about 

 12 percent of the trees killed (fig. 7). For the most part, the occurrence of tree 

 killing from these causes was sporadic and unpredictable as to their locale and timing. 



22 



