Habitat Use in Undisturbed Forest and the Effect of Variable Retention Harvesting on Pollinator Assemblages (Apoidea, Syrphidae) in the Boreal Forest of Alberta

  • Author / Creator
    Amos, Jared J. H.
  • While pollination in agricultural areas has been well studied, pollinators in forests have received much less attention, particularly in the boreal forest. Bees, especially native bees, and hoverflies are the two most important groups of pollinators providing crucial ecological and economical services worldwide. However, many pollinator populations are in decline, and their conservation depends on an understanding of how they are distributed in undisturbed forest and how they respond to disturbances. Variable retention harvesting is a form of logging that aims to better emulate natural disturbance, e.g., stand replacing wildfire. The objectives of this thesis were: 1) to establish what species of bees and hoverflies are present in the boreal forest of northwest Alberta, 2) to describe how bees and hoverflies are distributed within undisturbed forest stands, and 3) to determine the effect of variable retention harvesting on bees and hoverflies in deciduous and coniferous forest cover. This was accomplished by sampling bees and hoverflies by net and pan trap in 2012 and 2013 at the Ecosystem Management Emulating Natural Disturbance (EMEND) study site. Pollinators were studied in uncut control compartments in four forest types: deciduous-dominated (DDOM), deciduous-dominated with spruce understory (DDOMU), mixedwood (MIXED), and coniferous dominated (CDOM). Additional sampling was conducted in clearcut, 20% retention, and 50% retention compartments in the DDOM and CDOM forest cover types and along roadsides in forested and agricultural areas within the EMEND landscape. I identified 57 species of bees and 64 species of hoverflies at EMEND. There was no significant difference in their abundance among the four forest cover types within uncut forest. Species richness was also similar among DDOM, DDOMU and MIXED compartments. However, CDOM compartments had a higher bee and hoverfly species richness as well as more species exclusive to a cover type. Bees did not differ significantly in abundance between forest and agricultural roadsides, but species richness was higher for agricultural roadsides. They were more abundant and diverse along forest roadsides than forest compartments. Hoverflies also did not differ in abundance between forest and agricultural roadsides but species richness was higher for forest roadsides. They were more abundant along forest roads than in forest compartments but species richness was higher in forest compartments than along forest roadsides. Within previously harvested compartments, post-harvest retention had no overall significant effect on the abundance of bees or hoverflies. Nevertheless, I observed that bee abundance increased with decreasing harvest retention in CDOM compartments, hoverfly abundance increased with increasing retention in DDOM compartments, and hoverfly response differed between subfamilies in CDOM compartments. Bee species diversity generally decreased as harvesting retention increased. However, hoverfly species diversity was highest in compartments with 50% retention. Forest cover type significantly affected bee abundance within harvested compartments with more bees caught in CDOM than in DDOM compartments. Canopy cover, shrub cover, and flower abundance all significantly explained variation in bee abundance, with canopy cover and shrub cover included in the best supported linear model and canopy cover and flower abundance significant in an RDA analysis. Canopy and shrub cover were both significantly related to hoverfly abundance, however, only canopy cover was included in the best supported linear model. Abundance of particular plant families had no significant effect on total bee or hoverfly abundance. In contrast, flower colour was important, with white and purple flowers having a significant positive effect and pink flowers having a significant negative effect on total bee abundance. White flowers, in contrast, had a significant positive effect on total hoverfly abundance. Several species of bees and hoverflies were identified as indicators, including Bombus rufocintus (agricultural roads), B. terricola (all roadsides), and Megachile relativa (low retention CDOM compartments). While B. terricola is in decline in parts of its range, it was the fourth most abundant species at EMEND. Additionally, I recorded 56 flowering plant species, including one previously unrecorded at EMEND (Lycopus uniflorus). Flower abundance and diversity did not significantly differ among treatments, though abundance decreased with increasing harvest intensity. Canopy and shrub cover were lower in CDOM than DDOM compartments and increased with retention in CDOM. Forest management wishing to minimize impacts on pollinators should take forest cover type into account and aim to increase retention to benefit groups such as Megachilidae and Eristalinae.

  • Subjects / Keywords
  • Graduation date
    Spring 2019
  • Type of Item
  • Degree
    Master of Science
  • DOI
  • License
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