All three habitat types showed evidence of repeated infestations. Eighty-six 

 percent of the stands on all the habitat types showed evidence of one or more infesta- 

 tions but 47 percent have survived both the earlier and the more recent infestations. 

 The occurrence of repeated attacks was about the same in all three habitat types. 



The effects of mountain pine beetle infestations are very important in the dynam- 

 ics of lodgepole pine stands. These effects vary from the abrupt stand depletion of 

 single infestations to the long range genetic selection caused by repeated infestations. 



Lodgepole pine stands depleted by mountain pine beetle infestations usually 

 are replaced in one of two ways, Tlie decimated stands may be succeeded by other 

 species in the absence of fire or they may be replaced by lodgepole pine seedlings 

 following a fire. 



Studies on three stands of lodgepole pine in the Targhee and Teton National 

 Forests have provided some information concerning the effect of mountain pine beetle 

 infestations. Specifically, three facts of interest were developed: namely, (1) beetle 

 infestations do in some instances occur at varying intervals within the same stand 

 until the lodgepole pine is largely eliminated; (2) residual trees accelerate their 

 growth when the beetle-infested trees die; and (3) growth of succeeding tree species 

 is stimulated either by the release of existing reproduction or the establishment of 

 new trees in the stand openings created by the death of beetle-infested trees. The 

 stand data were collected on 1/10-acre plots systematically located within the stands. 

 Sample trees on each plot were bored to determine age and past diameters. 



Dell Creek . --The most interesting stand studied grows in Dell Creek on the Teton 

 National Forest. Many large lodgepole pine windfalls attest to past beetle infesta- 

 tions. Lodgepole pine trees killed in the most recent infestation, with a few 

 exceptions, still remain standing; but the trees on the ground were killed by beetles 

 in earlier infestations (figures 4 and 5). Although we were unable to date the fallen 

 trees, they obviously had been on^ the ground for various lengths of time. Some were 

 decayed to such an extent that only remnants of recognizable material were left. 

 Despite an advanced state of decay in some of the older windfalls, beetle engravings 

 were visible on small sound remnants of the decayed boles. 



Evidence obtained from the increment cores taken in this stand suggests that at 

 least four mountain pine beetle infestations have occurred since 1892. The subalpine 

 fir in the present stand developed from an understory that has been released by the 

 periodic death of lodgepole pine overwood to become the dominant stand presently on 

 the area (figure 6 and table 4). The sampling errors for the total values of Table 4 

 generally did not exceed 10 percent at the 95 percent probability level. Significant 

 periods of release found in the subalpine fir are shown in the following tabulation: 



EFFECTS OF BEETLE INFESTATIONS 



Stand Depletion and Replacement 



Succession 



Period 



Percent of sample trees 

 showing significant release 



1892-1907 

 1919-1927 

 1937-1947 

 1956-1964 



95 

 60 

 45 

 40 



9 



