Ten Common Questions About Adaptive Forest Management: IV. Can forest thinning and prescribed burning solve the problem?

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 3: “Can forest thinning and prescribed burning solve the problem?”

Fire has been a tool that has been actively used for millennia. Indigenous burning practices maintained prairies, oak and pine savannas, riparian areas, mixed-conifer, hardwood, and dry forests,
and high mountain huckleberry and beargrass assemblages for food, medicine, basketry and other resources (Trauernicht et al. 2015, Roos et al. 2021). Following prolonged fire exclusion, many
seasonally dry forest landscapes that were once frequently burned now are densely stocked with multi-layered canopies that often require thinning prior to restoring fire (North et al. 2012, Ryan et al. 2013). Prescribed burning on its own and in combination with mechanical thinning are essential fuel reduction treatments with demonstrated effectiveness in reducing fire severity, crown and bole scorch, and tree mortality compared to untreated forests (Safford et al. 2012, Kalies and Yocom Kent 2016).  Thinning and burning in partnership with local Indigenous knowledge and practice can support culturally-valued practices, traditions, livelihoods, and food and medicine security (Sowerwine et al. 2019).

Although the use of prescribed burning, often in combination with mechanical thinning, has been shown to be highly effective at mitigating wildfire severity and increasing forest resilience to drought, insects and disease (Hood et al. 2015), these treatments alone cannot address forest management challenges across wNA. Fuel reduction treatments are not appropriate for all conditions or forest types (DellaSala et al. 2004, Reinhardt et al. 2008, Naficy et al. 2016). In some mesic forests, for instance, mechanical treatments may increase the risk of fire by increasing sunlight exposure to the forest floor, drying surface fuels, promoting understory growth, and increasing wind speeds that leave residual trees vulnerable to wind throw (Zald and Dunn 2018, Hanan et al. 2020). Furthermore, prescribed surface fire is difficult to implement in many current mesic forests since fire readily spreads into tree crowns via abundant fuel ladders and can result in crown fires. In other forest types such as subalpine, subboreal, and boreal forests, low crown base heights, thin bark, and heavy duff and litter loads make trees vulnerable to fire at any intensity (Agee 1996, Stevens et al 2020). Fire regimes in these forests, along with lodgepole pine, are dominated by moderate- and high-severity fires, and applications of forest thinning and prescribed underburning are generally inappropriate. However, landscape burning and maintenance of high elevation forests and meadows is part of cultural burning, and high-intensity
crown fire is used operationally on national forests and parks within the US and Canada for landscape restoration objectives (Table 2).

Even where socially and ecologically appropriate, thinning and low-intensity prescribed burning generally require repeated treatments to meet fuel reduction objectives. For example, without prior
thinning, low-intensity prescribed fire, on its own, may not consume enough fuel or cause enough tree mortality to change forest structure and reduce crown fire hazard (e.g., Lydersen et al. 2019b). In contrast, prescribed burns in heavy slash may result in high tree mortality. The first harvest entry into fire-excluded stands often leaves high surface fuel loads and dense understories that require one or more prescribed burning treatments to reduce surface and ladder fuels (Goodwin et al. 2018, Korb et al. 2020). Thus, it often takes multiple treatments and/or fire entries, as well as ongoing maintenance, to realize resilience and adaptation goals (Agee and Skinner 2005, Stevens et al. 2014, Goodwin et al. 2020). Given the extent and variability of forest ecosystems that have experienced prolonged fire exclusion, active forest management can be only one tool to increase adaptation to climate and future fires.

Although thinning and prescribed burning have been shown to be highly effective, the current scale and pace of these treatments do not match the scale of the management challenge (Barnett et al. 2016b, Kolden 2019). Mechanical treatments are constrained by land management allocations and their enabling legislation (e.g., wilderness and roadless areas), operational constraints (e.g., steep slopes, distance to roads, costs), and administrative boundaries (e.g., riparian areas, areas managed for species of concern). In the central Sierra Nevada for example, these constraints – combined with large areas of non-productive timberland that are unsuitable for commercial treatment due to steep slopes or distance from roads – left only 28% of the landscape available for mechanical thinning and prescribed burning treatments (North et al. 2015a). In the remaining area, prescribed burning alone and/or use of managed wildfires may be suitable replacement treatments (Boisramé et al. 2017, Barros et al. 2018). However, prescribed fire-only treatments are frequently limited by cost, liability, air quality regulations, equipment availability, personnel capacity and training, and the need for ongoing maintenance treatments (Quinn-Davidson and Varner 2012, Schultz et al. 2019).

In light of these constraints, some researchers and managers have called for the expanded use of landscape-scale prescribed burns and managed wildfires in addition to fuel reduction treatments as a promising approach to expand the pace and scale of adaptive management (see below). Increasingly collaborative restoration partnerships with Indigenous cultures can increase opportunities for reinstating tribal stewardship practices (Lake et al. 2018, Long and Lake 2018). Under appropriate weather and safety conditions, and where infrastructure is not at risk, managed wildfire may serve as a useful and cost-effective tool for reintroducing wildfire to fire-excluded forests and achieve broadscale management goals.

Sharon’s reflection: In policy studies, we always say that framing the problem is crucial. These scientists ask the question “can thinning and prescribed burning solve “the problem”?” In the abstract, the authors state “We review science-based adaptation strategies for western North American (wNA) forests that include restoring active fire regimes and fostering resilient structure and composition of forested landscapes.”..perhaps how to restore fire regimes and foster climate resilience.  If I were framing the problem I might call it “how can humans in western North America live with fire, including restoring/designing fire resilient forested landscapes.”  It’s not exactly the same thing, but perhaps the answer (thinning and PB can’t do it alone) is the same.