1967 



Figure 7. — Diameter trend 

 aurves of residual 

 subalpine fir trees 

 dialing four mountain 

 pine beetle infestations 

 in the lodgepole pine 

 overwood, Dell Creek ^ 

 Teton National Forest. 

 The superimposed 

 orosshatched bars 

 show the periods 

 of infestation. 



YEAR 



Curves showing the trend of past diameters in the subalpine fir understory and the 

 periods of beetle infestations superimposed over them are presented in figure 7. The 

 changing upward trend in these curves reflects the release of the understory following 

 the death of lodgepole pine in the overwood. For example, the rather abrupt upward 

 change in the curves from 1907 to 1927 reflects an improvement in diameter increment 

 during that period. It is noteworthy that all the trees in the stand did not show 

 simultaneous release as would be expected from weather effects. Furthermore, the 

 available weather records from the nearest but somewhat distant stations show generally 

 below average precipitation between 1917 to 1937. 



The greatest release of subalpine fir followed the first suspected infestation 

 that occurred from 1892 to 1907. During that period 95 percent of the cores showed 

 significant release and this is reflected in the upward trend of diameters following 

 1907. This trend continued for two decades into the middle of the moisture deficient 

 period, 1917 to 1937, as well as through the second infestation. During the third ; 

 infestation, 1937 to 1947, the diameter curves steepened again, probably reflecting 

 the release during that period. While the larger trees showed the greatest release \ 

 effect during the earliest infestation, the three smaller classes of trees displayed 

 continued response following the later infestations. The earlier infestations 

 apparently involved the death of greater numbers of lodgepole pine trees than the 

 later ones; consequently, the earlier infestations had a greater release effect upon 

 the subalpine fir stand. Furthermore, the larger subalpine fir trees had attained a 

 more dominant position in the crown canopy by the time of the last infestation; there- 

 fore, they were not as subject to release as the smaller trees. The curves illustrate 



11 



