Vegetation responses following mountain pine beetle attack in lodgepole pine forests of west-central Alberta

  • Author / Creator
    Steinke, Julie
  • Natural disturbances are an integral part of forest ecosystems and drive successional change. The boreal forest is adapted to stand-replacing fires, which have different ecological impacts than less severe disturbances, such as insect attacks. In recent years, mountain pine beetle (MPB), a bark beetle native to western North America, has undergone an unprecedented range expansion into lodgepole pine forests in west-central Alberta. MPB indirectly influences understory resources and vegetation, but does not necessarily provide suitable conditions for pine regeneration. Lodgepole pine typically requires open, disturbed areas to regenerate, where there are suitable regeneration seedbeds, limited competition, and sufficient heat to open serotinous or semi-serotinous cones. This study explores how vegetation in forests in Alberta will respond to MPB attacks.

    To gauge how forests in Alberta will respond, we visited lodgepole pine-dominated, grey-attack stage, high severity MPB-killed stands throughout a variety of ecosites within west-central Alberta. We collected data to assess the potential for natural regeneration and used model selection to examine which factors best explained pine regeneration. We also conducted an experiment with an untreated control, medium and high severity simulated MPB-attack, and simulated salvage logging to assess responses of understory vegetation seven years after disturbance. We used univariate and multivariate analysis to examine differences in responses of understory vegetation among these treatments.

    Less than half of the post-MPB sites assessed had any evidence of pine regeneration. Rich quality sites, broadleaf advance regeneration, and spruce basal area negatively impacted pine regeneration. These factors likely resulted in a shading of the forest floor and competition with seedlings for resources. Some pine seedlings were found in poorer quality sites, especially those with pre-existing pine advance regeneration. Cones in richer sites were more strongly serotinous and will likely create a forest floor seedbank, while cones in poorer sites had less strongly serotinous cones that may open with sufficient heat from solar radiation.

    The simulated-MPB attack experiment showed that the severity of disturbance determined the magnitude of vegetation changes. Only simulated salvage logging resulted in immediate changes. Seven years post-treatment, vegetation richness and diversity increased along the gradient of treatment severity, while total cover had returned to pre-treatment levels. This was accompanied by a change in species composition; feathermosses were indicative of the untreated control, shade-tolerant species increased their relative abundance in the medium severity (~50%) simulated MPB treatment, both shade-tolerant and -intolerant species increased their relative abundance in the high severity (100% kill) simulated MPB treatment, and disturbance-adapted, shade-intolerant species (including lodgepole pine) were indicators of salvage logging.

    High severity MPB-attack provided a better understory light environment for pine regeneration, but this effect was negated by competitive effects arising from the responses of understory vegetation and continued overstory shading by standing dead and residual live trees. Richer sites are less likely to experience pine regeneration, and higher severity MPB attacks result in more drastic changes in vegetation cover and composition that can result in pine seedlings being outcompeted. Thus, successional pathways may be altered in richer and in high mortality sites; these sites should be prioritized for lodgepole pine rehabilitation in Alberta. Poorer sites that experience some pine regeneration and sites with less severe attacks that have minimal understory impacts may remain on their current successional trajectories. These results can be used to inform management decisions regarding the need for the rehabilitation of lodgepole pine sites in Alberta.

  • Subjects / Keywords
  • Graduation date
    Fall 2018
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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