Endemic Mortality 



Mortality rates were derived by Lee from yield tables for Alberta and verified 

 by him using data from remeasured sample plots. Judging by the nature of their source, 

 his rates are presumed to apply to the development of stands in the absence of mountain 

 pine beetle. Overall rates are highest in stands composed of small-diameter trees. The 

 rates decline with increasing mean d.b.h., reach a minimum at 10.6 inches, and then 

 begin increasing. The rates are independent of stand density. 



In order to distribute the mortality rates over the range of diameters within the 

 stand. Lee's rates were multiplied by a factor that depends on the percentile of the 

 tree within the basal area distribution. The factor used was: 



[0.25 * 1.5 (1. - K7r/100.)] 



where PCT is the percentile computed by the subroutine PCTILE which is described in 

 Appendix I. 



The effect of this factor is to give the lowest mortality rate to the tree of 

 maximum d.b.h. in the stand, and to give the smaller trees a rate that increases as the 

 percentile declines. The maximum rate for the smallest tree in the stand would be 1.75 

 times the average rate for the stand. The effect of introducing the PCT variable is to 

 distribute the mortality more heavily among the smaller trees in the stand. Lee 

 observed that the mean diameter of mortality trees was 2 inches less than the stand 

 overall mean diameter, and that the distribution of the mortality appeared to follow 

 the normal Gaussian distribution. Accordingly, he calculated the number of trees 

 expected to die by diameter classes from the normal distribution. However, in his 

 procedure there would be no explicit relation to the number of trees actually in the 

 class in a particular sample of the stand. The procedure using PCT provides an 

 explicit estimate of the mortality rate for each tree record, thus overcoming the 

 difficulty in Lee's procedure. 



Beetle- Induced Mortality 



Mountain pine beetle infestations are a major cause of the disintegration of 

 lodgepole pine stands. The severity of losses depends on the ecological habitat type 

 and elevation (Roe and Amman 1970). The probability with which a severe beetle out- 

 break occurs is as yet unknown. However, one of the antecedent conditions of an 

 outbreak is that the stand must contain some trees larger than 12 inches d.b.h. 

 Within a stand, the emerging beetles attack trees with higher probability if the tree 

 is in the upper end of the diameter distribution. In turn, the attack density (entrance 

 holes per square foot of bole area) is higher on the larger, thicker barked trees 

 (Safranyik and Vithayasai 1971) . 



The size of the emerging population depends on two dominating factors: the density 

 of attack, and the thickness of the phloem -within which the larvae feed and pupate 

 (Amman 1969). Phloem thickness is directly related to radial growth. The quantitative 

 relation of phloem to radial growth was established by the work of D. M. Cole^ in the 

 research work unit studying the silviculture of lodgepole pine. 



Relative losses of beetles during flight are highest when the population density 

 is highest and when the population is in the declining stages of an outbreak. 



^Dennis M. Cole. Phloem thickness relationship in lodgepole pine trees. Intermt. 

 For. and Range Exp. Stn. (In preparation.) 



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