380 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



partial cutting, leaving a sufficient reserve of timber for future growth, so as 

 to enable a second cutting in from 30 to 40 years. Bark beetle losses in these 

 stands reserved for future grovpth have in certain areas not only offset all incre- 

 ment, but have reduced the original forest capital from 1 to 15 percent. Recent 

 experiments have indicated the possibility of avoiding this loss by reinovina; 

 insect-susceptible trees in the initial cutting. These susceptible trees are those 

 of slower growths which can be detected at the time of marking the timber for 

 sale. Recent sales have been marked on this plan. 



RISK CLASSES FOR PONDEROSA PINE 





M^ 



Low Risk 



Hiqh Risk 



FiGUBE 1. — Touched-up photographs to illustrate types of ponderosa pine sus- 

 ceptible to bark beetle attack. Lowered vigor and resistance and greater 

 susceptibility indicated by higher numbers. (Salmon and Bongberg.) 



Keen (1936) was the first definitely to classify ponderosa pine on 

 the basis of susceptibility to bark beetle attacks. He proposed a 

 classification based on age and vigor (crown size, shape, and density) 

 that has served very well for this purpose and has been widely 

 adopted by foresters for other purposes as well (fig. 2). He says: 



Once the type of tree most likely to be killed can be recognized with a fair 

 degree of certainty, it is possible to make partial cuttings of beetle-susceptible 

 trees, either for the purpose of salvaging valuable high-risk trees before they 

 are damaged by beetle attack or for the silvicultural objective of reducing mor- 

 tality and Increasing net growth. 



