Study: Is fire severity increasing in the Sierra Nevada?

new study published in the International Journal of Wildland Fire found that, contrary to what has been claimed in some of the news coverage of recent forest fires, there is not a trend toward increasing fire severity in the Sierra Nevada. Previously, those who claimed that fire severity was increasing relied primarily on two publications by Jay Miller of the Forest Service (Miller et. al 2009, Miller and Stafford 2012).

However, Dr. Chad Hanson and Dr. Dennis Odion found that the Miller studies left out hundreds of thousands of acres of fire data from their analysis. In contrast, Hanson and Odion used all of the available fire severity data for the Sierra Nevada, and that data showed no trend toward increasing fire severity.

Furthermore, they found that rate of high severity fire since 1984 has been lower than it was historically. These results refute some of the main claims we see on this blog and elsewhere.

Research in the Sierra Nevada range of California, USA, has provided conflicting results about current trends of high-severity fire. Previous studies have used only a portion of available fire severity data, or considered only a portion of the Sierra Nevada. Our goal was to investigate whether a trend in fire severity is occurring in Sierra Nevada conifer forests currently, using satellite imagery. We analysed all available fire severity data, 1984–2010, over the whole ecoregion and found no trend in proportion, area or patch size of high-severity fire. The rate of high-severity fire has been lower since 1984 than the estimated historical rate. Responses of fire behaviour to climate change and fire suppression may be more complex than assumed. A better understanding of spatiotemporal patterns in fire regimes is needed to predict future fire regimes and their biological effects. Mechanisms underlying the lack of an expected climate- and time since fire-related trend in high-severity fire need to be identified to help calibrate projections of future fire. The effects of climate change on high-severity fire extent may remain small compared with fire suppression. Management could shift from a focus on reducing extent or severity of fire in wildlands to protecting human communities from fire.


  1. Matthew.. this all seems very academic to me. How do you think it relates to policy?

    Do we think that models of any kind accurately predict what’s going to happen? People on this blog are probably united in thinking “not really.”

    There are many ways to think of fire as “severe”.. one of which is to talk about its impacts on things you care about. Depending on the things you care about, the answer could be different.

    Do you think that the estimate of folks on this blog, that there seem to be enough dead trees from fires for BBW habitat, is contradicted by this paper?

    • Hello Sharon, Seems like you questions would be better directed at Dr. Hanson and Dr. Odion. Contact info is clearly listed at the International Journal of Wildland Fire link.

      But I’ll try and answer.

      You asked, “How do you think it relates to policy?”

      As previously stated, “there is not a trend toward increasing fire severity in the Sierra Nevada” and “the rate of high severity fire since 1984 has been lower than it was historically.” If so, seems pretty clear how it relates to policy, such as efforts to greatly mandated more national forest logging by excluded public input, enviro analysis, setting NEPA time-lines, gutting ESA and have one branch of the US Government simply prevent citizens from access to the US Federal Court system.

      You asked, “Do we think that models of any kind accurately predict what’s going to happen? People on this blog are probably united in thinking “not really.”

      What models are you talking about? And I have no idea how “we” think.

      You asked, “Do you think that the estimate of folks on this blog, that there seem to be enough dead trees from fires for BBW habitat, is contradicted by this paper?”

      It’s difficult for me to even understand what you are asking. In general, I pay little attention to what “the estimate of folks on this blog” estimate, or what they “seem” to think is true.

      Again, best to direct your questions about this study to the authors. Thanks.

    • I think the applications to policy are pretty clear, even if the paper doesn’t attempt to make policy recommendations from their results (I only read the abstract since the pdf was pricey). A lot of policy/legislation/management recommendations are being based at least on part on an assumption of overall increasing fire severity danger in recent years (e.g., “thin the threat”), and so studies that either support or contradict that assumption are important and have implications for policy (assuming we want to make rational decisions). I don’t see anything about predictive models (?) in the abstract, all I see is analysis of data. Fire severity isn’t some vague warm-and-fuzzy concept, it’s extensively quantified by USFS, USGS, others, hopefully you’re not saying “well it’s all subjective anyway, so it really doesn’t matter”? Realizing this study is just the Sierra Nevadas, would be interesting (for me) to see more analysis of Region 1 fires, and elsewhere too of course. In the end, it’s just more evidence to consider, like the old law school cliché says, “a brick is not a wall”

      • Guy, I’m so glad you raised this… we have heard that fires are “worse” and will become even worse in the future in that :

        They cost more money to fight
        They are larger
        There are more fuels and they are drier, so they lead to fires that have more negative impacts on soils, release more CO2 into the atmosphere, have more smoke that impacts peoples health, have more evacuations, etc.

        BECAUSE OF
        fire suppression
        climate change
        more folks with houses (or communities?) in the woods.

        None of these impacts is denoted by “severity” so I looked up what “severity” is and here’s this discussion in Keeley 2009 :

        Several recent papers have suggested replacing the terminology of fire intensity and fire severity. Part of the problem with fire intensity is that it is sometimes used incorrectly to describe fire effects, when in fact it is justifiably restricted to measures of energy output. Increasingly, the term has created confusion because some authors have restricted its usage to a single measure of energy output referred to as fireline intensity. This metric is most useful in understanding fire behavior in forests, but is too narrow to fully capture the multitude of ways fire energy affects ecosystems. Fire intensity represents the energy released during various phases of a fire, and different metrics such as reaction intensity, fireline intensity, temperature, heating duration and radiant energy are useful for different purposes. Fire severity, and the related term burn severity, have created considerable confusion because of recent changes in their usage. Some authors have justified this by contending that fire severity is defined broadly as ecosystem impacts from fire and thus is open to individual interpretation. However, empirical studies have defined fire severity operationally as the loss of or change in organic matter aboveground and belowground, although the precise metric varies with management needs. Confusion arises because fire or burn severity is sometimes defined so that it also includes ecosystem responses. Ecosystem responses include soil erosion, vegetation regeneration, restoration of community structure, faunal recolonization, and a plethoraof related response variables. Although some ecosystem responses are correlated with measures of fire or burn severity,many important ecosystem processes have either not been demonstrated to be predicted by severity indices or have been shown in some vegetation types to be unrelated to severity. This is a critical issue because fire or burn severity are readily measurable parameters, both on the ground and with remote sensing, yet ecosystem responses are of most interest to resource managers.

        But anyway you define it seems like fire effects = some function of current vegetation conditions, weather and suppression efforts.

  2. The amount of variables involved in proving this is immense. Two giant factors during this time period are the bark beetle explosion during the early 90’s and the timber management rules enacted under the Sierra Nevada Framework and CASPO. Sure, if you include lightning fires that occur in the higher elevations, outside of areas where timber management can occur, yes, you might get data that says wildfires are less severe. Hanson continues to push for zero forest management, on ALL lands.

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