Legacy Effects of Mountain Pine Beetle Outbreak: Defense, Growth and Survival of Residual Lodgepole Pine Trees in Alberta

• Author / Creator

  Zhao, Shiyang

• Periodic mountain pine beetle outbreaks have affected millions of hectares of lodgepole pine forests in western North America. Within these stands often some pine trees remain alive. In addition to causing direct pine mortality, outbreaks also have short- and long-term legacy effects on both above and below ground communities, resulting in altered stand conditions following outbreak. Currently, the effects of the altered stand conditions on the survival of the remaining residual live pine trees are unknown. Giving the observed low natural pine regeneration in the post-outbreak stands in Alberta, these residual pine trees have become a critical component of recovery of lodgepole pine forests. In my PhD thesis, I studied various aspects of the residual pine trees in post-outbreak stands in Alberta. In particular, I first determined whether the survival of residual trees can be explained by their growth and resin duct-based anatomical defenses. Then, I evaluated the potential risks of several species of bark and woodboring beetles to residual pine trees. For the relationship among resin ducts, growth and tree survival, I first compared the resin duct characteristics and radial growth between residual and beetle-killed trees and characterized the resin ducts and radial growth of residual trees before and after outbreak. The results showed that residual trees had larger resin ducts than beetle-killed trees before outbreak and continued having so in post-outbreak stands. Tree radial growth (ring width) was not associated with tree survival. Next, I examined if outbreaks altered the historical growth-defense relationship of residual trees. I found that after outbreak, residual trees prioritized resin duct defenses over growth by increasing resin duct production and area occupied (per mm2 of radial growth), compared prior to the outbreak. I subsequently determined the impacts of multiple stand factors on the relationship between growth and resin duct characteristics of residual trees before and after outbreak. Results showed increasing resin duct density of healthy (asymptomatic) residual trees with increasing percent lodgepole pine mortality, suggesting inducibility of resin ducts in lodgepole pine trees. Interestingly, the stand density of live overstory trees (including pine and non-pine species) before outbreak was related to the resin duct characteristics of residual trees before outbreak. These results suggest that intra- and inter-specific competition can affect anatomical defenses of lodgepole pine trees.
  To quantify the populations of bark and woodboring beetles and their insect predators in post-outbreak stands, I set up three types of passive insect traps seven years after the last mountain pine beetle outbreak in post-outbreak stands. The results from trap data showed that the percent lodgepole pine mortality were not associated with the abundance of bark and woodboring beetles. In addition, there were no difference among the three different categories of residual trees (healthy, declining, or attacked by mountain pine beetle but survived) in terms of the abundance of bark and woodboring beetles or their predators. These outcomes might be a result of low abundance of suitable host trees for bark and woodboring beetles, intra- and inter-specific competition within and between bark and woodboring beetle species, and predation. Overall these results demonstrate that post-outbreak stands did not promote bark and woodboring beetle populations. Through my thesis, I conclude that residual trees may be more resistant to the future mountain pine beetle attacks and bark and woodboring beetles do not pose threats to these residual trees in post-outbreak stands. Leaving residual trees, instead of salvage harvesting, could be critical for the establishment of lodgepole pine forests that show greater resistance to mountain pine beetle attacks in western Alberta.

• Subjects / Keywords

  • mountain pine beetle
  • post-outbreak stand condition
  • Pinus contorta var. latifolia
  • beetle outbreak
  • predatary beetles
  • lodgepole pine trees
  • bark beetles
  • secondary beetles
  • survival probability
  • residual pine trees
  • resin ducts
  • anatomical defense
  • Dendroctonus ponderosae
  • woodboring beetles
  • growth-defense relationship

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/r3-qzb9-kp28

• License

  Permission is hereby granted to the University of Alberta Libraries to reproduce single copies of this thesis and to lend or sell such copies for private, scholarly or scientific research purposes only. Where the thesis is converted to, or otherwise made available in digital form, the University of Alberta will advise potential users of the thesis of these terms. The author reserves all other publication and other rights in association with the copyright in the thesis and, except as herein before provided, neither the thesis nor any substantial portion thereof may be printed or otherwise reproduced in any material form whatsoever without the author's prior written permission.