with lodgepole pine seedlings (figure 11). Four years after the burn, 10,000 acres 

 were stocked so heavily with seedlings that thinning would be required to place the 

 stand in good growing condition. In addition to providing conditions for area re- 

 stocking, the fire cleaned up accumulated fuel that resulted from the beetle attack. 

 Eighty to 90 years from now these newly established lodgepole pine trees will reach 

 sizes attractive to the beetles; then these trees probably will be ready for another 

 mountain pine beetle infestation. 



All of the stands that originated during past years of high fire occurrence in the 

 Rockies have reached simultaneously a stage of increased insect susceptibility. This 

 means that the increased susceptibility is present over extensive areas. When these 

 forests reached the proper stage of growth (i.e., diameter and phloem thickness) they 

 provided the habitat in which the beetle populations could build up and sustain infes- 

 tations. Furthermore, the outbreaks spread over wide areas because trees of susceptible 

 diameter and age occurred extensively. However, repeated beetle infestations, dwarf- 

 mistletoe infection, fire, and logging all have contributed to stand changes resulting 

 in the variability of present lodgepole pine stands as well as conversion to other 

 forest types. Tackle (1954) recognized at least six different stand types including 

 both pure and mixed stands. He pointed out most of the above-mentioned factors in 

 stand formation, but he failed to recognize insects, particularly the mountain pine 

 beetle, among them. From our observations we conclude that the mountain pine beetle has 

 exerted widespread, and in some instances rather dramatic, influence upon stand forma- 

 tion in Rocky Mountain forests. 



■I 



The absence of fire in lodgepole pine stands, whether caused by organized fire 

 protection or natural controls, combined with stand depletion by the mountain pine 

 beetle, favors the displacement of lodgepole pine. The establishment and growth of 

 succeeding trees, especially of Douglas-fir at the lower elevations and subalpine fir 

 and spruce at the higher elevations, are encouraged by the environment in the beetle- 

 decimated stands. Unless wildfire runs through these stands before repeated beetle 

 infestations and other agents of mortality remove most of the residual seed-bearing 

 lodgepole pine, the stand eventually will convert to climax species. The historical 

 role of fire in stand formation and in the sustaining of lodgepole pine was stressed 

 by Horton (1956) in Alberta. Fire or logging may intervene to reverse the successional 

 trend and reestablish lodgepole pine as happened in the Sleeping Child fire. 



Growth Potential 



Mountain pine beetle infestations remove the most vigorous element of the stand 

 because they prefer the largest trees, usually with the thickest phloem. The residual 

 trees are usually of the intermediate and suppressed crown classes with some slow 

 growing dominants and codominants. Occasionally the smaller residual trees are older 

 than the larger trees in the stand. The stand structure becomes less favorable for 

 rapid tree growth with each repeated infestation. 



Even though the residual trees are released they rarely grow as large, within the 

 same time, as those which had been killed by the mountain pine beetle. The limited 

 number of residual lodgepole pines in heavily depleted stands is made up of old (some- 

 times nearly 300 years), extremely thin barked trees. These trees often grow extremely 

 slowly with 10-year diameter increments of as little as 1/10 inch or less. 



Genetic Selection 



Genetic selection, a more subtle effect, probably is accomplished through the 

 selective killing of lodgepole pine trees by the beetle. Because each beetle infes- 

 tation removes the most vigorous element (i.e., the largest trees) of the stand, it is 



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