Beyond mountain pine beetle: soil carbon storage a decade after tree mortality and the possible influence of soil fungi

• Author / Creator

  Lau, Nicole

• Mountain pine beetle (MPB; Dendroctonus ponderosae) disturbances, amplified by climate change, have led to extensive tree mortality and ecosystem succession in boreal forests across western Canada. Often following attack, former ectomycorrhizal (EM) pine stands in Alberta are replaced by arbuscular (AM) mycorrhizal shrubs and forbs. There has been growing interest in the varying ways that EM versus AM fungi influence critical soil processes, particularly concerning carbon cycling. While our comprehension of the specific impacts that MPB-induced mycorrhizal community succession has on long-term soil carbon storage remains somewhat limited, emerging frameworks offer broad yet valuable initial insights into this complex dynamic. Specifically, given the functional differences between mycorrhizal fungal types, a shift from EM- to AM-dominance in affected stands may promote soil organic matter formation and carbon stabilization, potentially enhancing soil carbon stores. The objective of this study was to address the following questions: 1) Will the dominant mycorrhizal type change with disturbance? 2) Does extensive tree mortality change the amount of carbon and nitrogen stored in forest floors and mineral soils? I sampled soil and conducted vegetation surveys at 80 lodgepole pine-dominated sites across west-central Alberta, split by disturbance severity (forests with >70% lodgepole pine basal area killed by MPB versus intact forest) and soil texture (coarse versus fine). I used density fractionation to separate soils into mineral-associated organic matter (MAOM) and particulate organic matter (POM) pools. I explored whether disturbance, mycorrhizal type, soil texture, and sampling depth affected carbon and nitrogen concentrations in these pools. There was a pronounced shift in mycorrhizal community composition from EM to AM dominance following MPB- induced forest transformations. However, overall carbon and nitrogen concentrations in bulk mineral soil, and carbon and nitrogen stocks in forest floors, remained largely unaffected. In addition, I confirmed that carbon and nitrogen storage are strongly dependent on soil texture and depth. Though there were no changes in soil carbon concentrations in the bulk soil, carbon in the MAOM fraction was relatively higher in the disturbed than intact sites. Greater carbon and nitrogen allocation into the MAOM pools is consistent with emerging hypotheses on the importance of AM vegetation to long-term carbon storage in soils. Taken together, these patterns in carbon storage may point to the resiliency of these soils, supported by soil organic matter interactions with the mineral matrix and potentially the buffering effects from the increase of AM fungi and their unique nutrient dynamics. The findings from this study may enhance our understanding of soil processes, offering opportunities to optimize soil carbon stocks and ecosystem services in recovering boreal forests.

• Subjects / Keywords

  • Carbon
  • Boreal forest
  • Mountain pine beetle
  • Mycorrhizal fungi
  • Lodgepole pine
  • Soil
  • Ectomycorrhizal
  • Arbuscular mycorrhizal

• Graduation date

  Spring 2024

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-0a1p-5z06

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.