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?