The major causes of tree death in 1978 were categorized as weather, un- 

 known, and insects (see Tables 21 and 22). Weather caused tree mortality by 

 windthrow, snow breakage, and lightning. The category termed "unknown" 

 was used by inventory crews when they could not determine which damaging 

 agent was primarily responsible for killing the tree. (The inventory's mortal- 

 ity tree procedure allowed only one damaging agent to be recorded. Frequent- 

 ly the death of a tree was caused by two or more agents in concert or suc- 

 cession.) Mortality caused by insects was generally due to bark beetles and 

 occasionally defoliators. 



Spruce, Douglas-fir and lodgepole pine were the major victims of weath- 

 er, while most of the trees killed by "unknown" factors were lodgepole pine 

 and Douglas-fir. Because the inventory crews had limited experience at iden- 

 tifying root diseases, it has been suggested that much of the Douglas-fir 

 mortality recorded as caused by weather was probably caused primarily by 

 root rots. Mortality due to insects was highest in lodgepole pine. 



Logging, fire, and disease also contributed significantly to mortality. 

 The disease category includes various rots, rusts, and mistletoe. Logging 

 mortality includes trees killed during the logging operation — it does not in- 

 clude trees cut for harvest. 



At the time of measurement, weather and insects shared dominant roles 

 as the major causes of death in Working Circle 3. However, the causes of 

 mortality in a forest are dynamic and cyclic, and in a future inventory the 

 major causes of death may be different. The mountain pine beetle 

 ( Dendroctonus ponderosae Hopk.) epidemic had just begun in 1978 and has 

 continued in various locations since that time. Lodgepole and ponderosa pine 

 are the species that will be affected most by this epidemic. 



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