Ten Common Questions About Adaptive Forest Management: III. Is forest thinning alone sufficient to mitigate wildfire hazard?

This is from the Ten Common Questions paper, a synthesis published in Ecological Application. I’m posting each answer separately. If you use the search box and look for Ten Common Questions, you can find them all.  My introduction to the paper is here. Please put any questions for the authors in the comments; I’ll try to get answers from them. I’d appreciate if the tone were respectful (think graduate seminar, not Twitter).

Question 2: “Is forest thinning alone sufficient to mitigate wildfire hazard?”

While “thin the forest to reduce wildfire threat” is commonly cited in the popular media, the capacity for thinning alone to mitigate wildfire hazard and severity is not well supported in the
scientific literature. Thinning treatments require strategic selection of trees to target fuel ladders and fire-susceptible trees, along with a subsequent fuel reduction treatment (Jain et al. 2020).

When thinning is conducted without accompanied surface fuel reduction, short and long-term goals may not be realized.  Thinning from below reduces ladder fuels and canopy bulk density concurrently, which can reduce the potential for both passive and active crown fire behavior (Agee and Skinner 2005). For instance, Harrod et al. (2009) found that thinning treatments that reduced tree density and canopy bulk density, and increased canopy base height significantly reduced stand susceptibility to crown fire compared to untreated controls. Furthermore, large-diameter trees and snags that provide essential wildlife habitat and other ecosystem values can be retained and fuels can be deliberately removed around these structures using this approach (Lehmkuhl et al. 2015). Where wood from treatments can be marketed, revenues from thinning help to sustain broader management goals on public lands. For example, some landscape restoration collaboratives seek to reinvest profits from commercially viable thinning to off-set costs associated with more labor-intensive manual thinning and prescribed or cultural burning needs (Shultz and Jedd 2012).

Some studies show that thinning alone can mitigate wildfire severity (e.g., Pollet and Omi 2002, Prichard and Kennedy 2014, Prichard et al. 2020), but across a wide range of sites, thin and
prescribed burn treatments are most effective at reducing fire severity (see reviews by Fulé et al. 2012, Martinson and Omi 2013, Kalies and Yocom Kent 2016). On most sites, thinning alone
achieves a reduction of canopy fuels but contributes to higher surface fuel loads. If burned in a wildfire, these fuels can contribute to high-intensity surface fires and elevated levels of associated tree mortality (e.g., Stephens et al. 2009, Prichard and Kennedy 2012). When trees are felled and limbed, fine fuels from tree tops and branches (termed activity fuels) are re-distributed over the treatment area, thereby increasing surface fuel loads (Martinson and Omi 2013). Mechanical fuel reduction treatments of these activity fuels are possible, but in many locations, biomass removal and utilization (e.g., for bioenergy) after thinning treatments can be cost-prohibitive due to long hauling distances and the economic and technological challenges of building new biomass facilities (Hartsough et al. 2008). Mastication equipment is sometimes used to shred understory trees and shrubs into smaller woody fragments, which are then redistributed and left on site (Kane et al. 2009). However, following mastication, surface fuels are temporarily elevated, and masticated stands that burn in wildland fires can cause deep soil heating from smoldering combustion and elevated fire intensities (Kreye et al. 2014).

Other unintended consequences of thinning without concomitant reduction in surface fuels can occur. For instance, decreasing canopy bulk density can change site climatic conditions (Agee and Skinner 2005). Wildfire ignition potential is largely driven by fuel moisture, which can decrease on drier sites when canopy bulk density is reduced through commercial thinning (e.g., Reinhardt et al. 2006). Reduced canopy bulk density can lead to increased surface wind speed and fuel heating, which allows for increased rates of fire spread in thinned forests (Pimont et al. 2009, Parsons et al. 2018). Other studies show no effect of thinning on surface fuel moisture (Estes et al. 2012, Bigelow and North 2012), suggesting that thinning effects on surface winds and fuel moisture are complex, site specific, and likely vary across ecoregions and seasons.

In summary, although the efficacy of thinning alone as a fuel reduction treatment is questionable and site dependent, there exists widespread agreement that combined effects of thinning plus prescribed burning consistently reduces the potential for severe wildfire across a broad range of forest types and conditions (Fig. 3, Fulé et al. 2012, Kalies and Yocom Kent 2016, Stephens et al. 2021). Given this broad consensus in the scientific literature, some authors suggest that forest thinning should be considered in the context of wildfire hazard abatement, ecological restoration and adaptation, and revitalization of cultural burning (Lehmkuhl et al. 2007, Hessburg et al. 2015, Huffman et al. 2020). Where restoring resilient forest composition and structure and reducing future wildfire hazard are goals of management (Koontz etal. 2020), combined thinning and burning approaches will provide ecological and wildfire-risk reduction benefits (Knapp et al. 2017).

From Sharon: I didn’t find anything surprising here, did you?

7 thoughts on “Ten Common Questions About Adaptive Forest Management: III. Is forest thinning alone sufficient to mitigate wildfire hazard?”

  1. No, no surprises; I am glad to see the term “canopy bulk density” used as a measure for stratifying potential benefits against known values. Thinning, for thinning’s sake is NOT going to buy anything toward truly managing fuels.

    We used more of a “restoration cut” in Region 3 (see GTR 310) to identify some of the same characteristics mentioned in the articles.

  2. The most acceptable statement here is that effects of fuel-reduction projects are complex, site-specific and vary among ecoregions and treatments. What is glaringly missing is (1) how likely is it that a given treatment project will find itself in the path of a serious fire, and when; and (2) once a project is complete, how does the fuel status change in subsequent years?

    • In the Sierra Nevada, the chances that a fire runs into a previous fuels treatment seems pretty dang high, right now. Remember, thinning projects (in the Sierra Nevada) are less about stopping fires and more about mitigating the effects of an over-crowded forest. Regarding your second question, it depends. Commercial thinning of ladder fuels has a very long-lasting effect on that multi-storied stand.

      My last project before retirement is currently burning, on the Caldor Fire, and it will be interesting to see if the many thinning units survived. If these thinned units survive, then the project was a complete success. If they do not survive, that doesn’t mean the project was a failure. Will it mitigate fire behavior? We’ll see.

      It will also be interesting to see how well an un-thinned Giant Sequoia plantation survives (or not). There is one close by (of many), within the Caldor Fire footprint.

    • I’m with Larry, the way things are going everything burnable is going to burn sooner or later. In another Prichard paper (Prichard and Kennedy 2014), it says:

      “Because surface fuels are critical for wildland fire ignition and spread, prescribed and wildland fires can act as temporary fire breaks and mitigate future wildfire behavior (Peterson et al. 2005, Boer et al. 2009, Stephens
      et al. 2012b). The longevity of this effect depends on how quickly surface fuels accumulate following the fire event (Collins et al. 2007, Miller et al. 2012). For example, Finney et al. (2005) reported that prescribed burns
      within nine years of the Rodeo-Chedeski fires in Arizona were effective at mitigating burn severity, whereas 20- year-old prescribed burns in this study generally remained effective. In a broadscale study of recent
      wildfires in northern California, Miller et al. (2012) reported that the incidence of high severity of fire was lower in areas that burned within 30 years of a previous wildfire. Similarly, Boer et al. (2009) found recent
      prescribed burns (,6 years old) reduced the incidence and extent of wildfires in eucalypt (Eucalyptus spp.) forests of southwestern Australia.”

      As far as I can tell, these are about “natural” burn severity (a variable of interest) not utility of the treatments for suppression efforts.

  3. I would agree with the majority of the expert comments that were cited. I would only add that future solutions must also include the creation of broad, landscape-scale patch mosaics of different species and ages that better reflect natural, historic disturbance conditions (including fire regimes), which will better partition fire events into smaller, more localized, and manageable events in the future. Current logging and suppression conditions are promoting massive, wide-ranging fires that result in extensive areas of impact and severe mortality levels, not to mention excessive expenditures on reactive fire-fighting responses, which ultimately limit funds that could be made available for fuel management projects instead. Focusing exclusively on dispersed, stand-scale thinning ‘treatments’ of mainly economically profitable trees will not suffice. Understandably, landscape-scale treatments are problematic given existing private/public land ownership objectives and vested commercial interests.

    Optimally, selected areas would be mechanically thinned, with non-fire adaptive species and ladder fuels targeted for selective removal to decrease canopy densities. This would be followed by timely applications of prescribed fire to remove surface fuels and slash. But, that formula may not work in many remote or sensitive areas, and treatments require repeated treatments within short vegetation regrowth time frames. We generally do not have the time or financial resources to treat millions of acres on short repeat rotation schedules.

    Some large wildland fires cannot be suppressed, no matter how much we try, and those should be “allowed” to burn to reset natural fire regimes (e.g., 1989 Yellowstone Fire); all we can do is try to direct and manage these fires to restrict their extent and to enhance their beneficial aspects. The goal is to reconstruct a semblance of more natural forest landscapes that will include fire-resilient species that are not in crowded conditions, all unsuccessfully competing for scarce resources (i.e., protect the strongest, weed out the weakest).

    Ultimately, the solutions may require imposing regulatory restrictions on where people can build, live, or what or how much they are allowed to exploit/degrade. Our best option is to identify the highest-value sites (i.e., communities, critical infrastructure, cultural sites, etc.) and prioritize them for heightened levels of defensive fire protection.

    • A BIG question remains. How do we get Congress to allocate funds, hire qualified people and add necessary streamlining, to get more ‘boots on the ground’? Clearly, as a whole, Congress is on a different page, and does not see this as an emergency situation. It’s always the same old story, with politicians clinging to their flawed partisan politics.

  4. In this thinning unit, you get to see what it looked like before it was logged. I led the marking crew through this thinning unit, and the loggers hadn’t quite gotten to it before this image was collected. It is directly in the path of the Caldor Fire, and is probably burning right now. You can see in the image that many of the crowns are touching other crowns. Additionally, there are too many trees for a west-facing slope to support. This stand had a higher level of old growth decadence, but the 30 inch rules did not allow mistletoe-infected white firs to be cut.


    I was hoping that we would get to see a post-logging, pre-wildfire view of this unit.


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