beetle effects over the next two decades for all stands 

 within analysis areas, which would not allow scheduling 

 earUer harvest of stands with a high probabiHty of in- 

 festation and mortality. 



Another approach is recommended when the location 

 of stands within analysis areas is identified through 

 timber surveys or stand examinations. Beetle attack 

 may then be simulated by a "prescription" that shows 

 the effects of an epidemic in the absence of timber 

 management. If other management practices were not 

 implemented, it would be necessary to constrain the bee- 

 tle "prescription" by assignment to a certain acreage. 

 Thus there would be two prescriptions— one for some 

 stands in parts of the analysis area with infestation, and 

 one for other parts with no effects of infestation. 



Stands in the Helena National Forest were analyzed in 

 a FORPLAN run by grouping habitat types so mortality 

 factors could be directly applied to yield tables. A pro- 

 cedure was adopted and used to adjust yield tables based 

 on the coefficients developed for the Helena National 

 Forest plan (Brohman and others 1982). Coefficients 

 were based on the assumption that a 50 percent loss of 

 lodgepole pine would occur over a 5-year period. The 

 estimated loss as a percentage of volume by age classes 

 was determined as shown: 



Yi' = Yi (1 - V. L) 

 Yi' = Y2 (1 - 3/4 L) 

 Y- = Yj (1 - L), j >3 



where 



L = proportion of volume lost to beetles (50 percent = 0.50), 

 Y, = tabular volume for decade j of the plan, and 

 Yj' = adjusted volume expected to exist in decade j. 



Such coefficients must be derived for each habitat 

 type or habitat type group to be applicable to the model. 

 Decade 1, 2, or 3 of the Forest Plan may correspond to 

 different decades in the yield table for different stands 



or habitat type groups within analysis areas. For exam- 

 ple, if groups of stands are 105 years old, then Yj is the 

 tabular yield shown at 110 years (25 percent loss by 

 year 5). If the current age is 165 years, then Yj is the 

 tabular yield shown for 170 years (25 percent loss by 

 year 5, and 75 percent loss by year 10 at 175 years). The 

 graphs in figure 7 were developed using this approach 

 and the INDIDS/Rate of Loss Model for the Helena Na- 

 tional Forest in the absence of beetle attack. The factor 

 or proportionality is (1-L), the proportion of stand 

 volume not killed. 



That the predicted results graphed in figure 7 wiU ac- 

 tually happen is questionable. Beetle-induced mortality 

 will reduce competition for trees that are not attacked. 

 However, trees not susceptible to bark beetle attack are 

 usually smaller and less vigorous. These trees will pro- 

 bably respond to a decrease in competition. But amount 

 of response will depend on tree age and various site fac- 

 tors. We do not know at what rate the remaining live 

 stand will grow compared to what it would have done 

 without attack. 



The final step in the FORPLAN run for the Helena 

 National Forest plan was to adjust existing yield tables 

 by the appropriate coefficient for each habitat type 

 group. Regenerated stand tables were not adjusted, 

 because management should prevent mountain pine bee- 

 tle outbreaks over a rotation. The assumption that the 

 beetle will infest susceptible stands throughout the 

 National Forest in the next 20 years may not be totally 

 correct, but it seems probable based on available infor- 

 mation. By including coefficients in the yield tables, the . 

 FORPLAN model should show which highly susceptible 

 lodgepole stands need immediate harvesting, and which 

 stands should be harvested before becoming highly 

 susceptible. By using assessments from FORPLAN and 

 harvesting in high-hazard, susceptible stands before an 

 epidemic develops, land managers should be able to 

 minimize tree mortality caused by the beetle. 



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