A Regional Synthesis of Post-Glacial Fire History in the Eastern Cascades, Washington, using Macroscopic Charcoal Analysis

Author(s)

Brynn Harrison

Faculty Mentor(s)

Megan Walsh (Geography)

Abstract

The recent rise in severe wildfires in the Pacific Northwest (PNW) has created a heightened sense of urgency and reignited public interest in wildfire risk. In order to put this rise into a broader spatial and temporal context, a more in-depth look at fire histories from frequently burned areas in the eastern Cascades is needed. The first major objective of this study was to determine if CharAnalysis, a statistical program developed to reconstruct fire history from macroscopic charcoal-based records, is appropriate for use with charcoal records from the eastern Cascades of Washington. The second major objective was to develop a regional synthesis of post-glacial fire activity for the eastern Cascades using the seven existing charcoal-based records by developing a regional biomass burning curve, which was then compared to the existing PNW synthesis. Results from the first objective determined that CharAnalysis is either an inappropriate, moderately appropriate, or appropriate tool for use with charcoal records from the eastern Cascades depending on the site’s vegetation and fire regime. Results from completing the second objective indicate that biomass burning in the eastern Cascades rose throughout the post-glacial period, likely due to climate shifts, the establishment of modern forests (particularly after ca. 7,000 cal yr BP), and increased human use of fire in the late Holocene. By completing this analysis, much needed data points eastern side of the Cascades now contribute to our understanding of past and future drivers of fire activity in the PNW.

Keywords: Fire History, Forest Management, Macroscopic Charcoal Analysis

Presentation

13 thoughts on “A Regional Synthesis of Post-Glacial Fire History in the Eastern Cascades, Washington, using Macroscopic Charcoal Analysis”

  1. Stephanie Bartlett

    Hi Brynn,
    I thought you did a fantastic job on your presentation! I was slightly lost on the slide with the Steps of the Biomass Burning Curve Methods, but I think I pretty much understood the rest. I thought your detailed explanation of the biomass burning curve was really well done, and I was surprised to see the drop in fire activity over the last 500 years. However, it made sense as you were explaining it.
    I am doing research with Dr. Kaspari this summer relating to the effects of light absorbing particles (including black carbon and dust) on snow albedo. I am curious if the albedo of the snow in Washington would be more affected by the normal burning cycle that the eastern cascades should have gone through or the high intensity fire events that we are currently experiencing. Thank you for a very interesting presentation!

    Stephanie Bartlett

    1. Hi Stephanie,

      Thank you! Yeah, it was really tough to squish the whole explanation into just a few seconds, but I can try to answer any questions you might have if needed! That sounds like super interesting research. Dr. Kaspari was on my thesis committee, so I’m curious what your results will say!
      I feel like there is so many factors to take into account for your research (when the burning occurs/seasonality, vegetation, elevation especially, fire regime type, etc.), and I would absolutely love to chat with you more about it! Shoot me an email if you’d like: Brynn.Harrison@cwu.edu

  2. That was a fantastic presentation! Including those images of pre-settlement vs. modern forests was an excellent way to drive your point home.

    My only question is just something about the statistics here: what is meant by a Z-score of the transformed charcoal index? I understand that it correlates to how much burning was going on over an area, but I just don’t quite get what exactly a Z-score is. Thank you!

    1. Thank you so much!
      Okay so, a z-score is used to measure the deviation a data point has from the mean, or how unusual an occurrence is based on some long-term average. So, the closer the value is to 0, the less unusual the occurrence. In this case, because the amount of charcoal that can accumulate in a specific lake is related to the lake’s size, slope, vegetation, etc., and these factors differ between lakes, we have to scale and transform the data to standardize them and make them comparable to one another. Each of the seven lakes mentioned were standardized to be included in this one biomass burning curve, and now we can see how much the burning at some point in time differed from the average, long-term trend for the eastern Cascades.
      For example, this means that the level of burning around 14,500 cal yr BP (towards the end of the late glacial) was much lower/further from the mean than what was considered “typical” at these 7 sites (z-score of -4). Conversely, burning around 500 cal yr BP was closer to the long-term mean (z-score of 0) before it deviated from the mean once more at present day (z-score of -1).

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