Ten Common Questions About Adaptive Forest Management: II. Are the Effects of Fire Exclusion Overstated?

Please see previous post for links and background on this paper.
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1.”Are the effects of fire exclusion overstated? If so, are treatments unwarranted and even counterproductive?”

Concerns about forest thinning and other forms of active management are sometimes based on the assumption that contemporary conditions and fire regimes in dry pine and mixed-conifer forests are not substantially departed from those maintained by uninterrupted fire regimes (Hagmann et al. this issue). This perspective does not accurately reflect the breadth and depth of scientific evidence documenting the influence of over a century of fire exclusion. Support for the suggestion that ecological departures associated with fire exclusion are overestimated has repeatedly failed independent validation by multiple research groups (Hagmann et al. this issue). In addition, these arguments fail to consider widespread Indigenous fire uses that affected landscape scale vegetation conditions linked to valued cultural resources and services, food security, and vulnerability to wildfires (Lake et al. 2018, Power et al. 2018). As is explored in the following sections, a number of forest management and treatment strategies are shown to be highly effective. Site conditions and history are always important considerations. Moreover, there is no one-treatment-fits-all approach to forest adaptation.

Evidence from a broad range of disciplines documents widespread, multi-regional 20th-century fire exclusion in interior forested landscapes of wNA (see a detailed reference list and discussion in Hagmann et al. this issue). Collectively, these studies reveal extensive changes in tree density, species and age composition, forest structure, and continuity of canopy and surface fuels. Forests that were once characterized by shifting patchworks of forest and nonforest vegetation (i.e., grasslands, woodlands, and shrublands) in the early 20th-century gradually became more continuously covered in forest and densely stocked with fuels (Fig. 4).

However, for over two decades, a small fraction of the scientific literature has cast doubt on the inferences made from fire-scar based reconstructions and broader landscape-level assessments to suggest that estimates of low- to moderate-severity fire regimes from these studies are overstated. Hagmann et al. (this issue) examine this counter-evidence in detail and identify critical flaws in reasoning and methodologies in original papers and subsequent re-application of these methods in numerous geographic areas. Subsequent research shows that studies relying on Williams and Baker (2011) methods for estimating historical tree densities and fire regimes overestimate tree densities and fire severity (see also Levine et al. 2017). Moreover, established tree-ring fire-scar methods more accurately reconstruct known fire occurrence and extent. Other studies, also based on the methods of Williams and Baker (2011), conflate reconstructed low-severity, high-frequency fire regimes with landscape homogeneity. These interpretations disregard critical ecosystem functions that were historically associated with uneven-aged forests embedded in multi-level fine-, meso- and broad-scale landscapes. By extension, claims that low-severity fire regimes are overestimated then imply that large, high-severity fires were a regular occurrence prior to the era of European colonization. Such interpretations may lead to the conclusion that recent increases in high-severity fire are still within the historical range of variability, and that there is no need of restorative or adaptive treatments (Hanson and Odion 2014, Odion et al. 2014, Baker and Hanson 2017).

Indeed, research from across wNA has shown that high-severity fire was a component of historical fire regimes, and that fires of all severities are currently in deficit (Parks et al. 2015b, Reilly et al. 2017, Haugo et al. 2019, but see Mallek et al. 2013). Reanalysis of the methods of Baker and others shows that their methods inherently overestimate fire severity and the frequency and area affected by high severity fire (Hagmann et al. this issue, Fulé et al. 2014). In addition, high-severity patches in recent fires are less heterogeneous and more extensive than the historical range of variability for forests characterized by low- and moderate-severity fire regimes (Stevens et al. 2017, Hagmann et al. this issue). Finally, research across wNA reveals key climate-vegetation-wildfire linkages, where fire frequency, extent, and severity all increase with increasing climatic warming, suggesting that observed trends in fire patterns are commensurate with predicted relationships with ongoing climate change (McKenzie and Littell 2017, Parks and Abatzoglou 2020).

Another perspective on this debate contends that whether historical records can be agreed upon is of ancillary importance. Adaptive forest management and fuel reduction treatments are primarily aimed at increasing forest resilience and/or resistance to climate change, fire and other disturbances, which has positive societal and ecological impacts that do not require justification based on historical conditions, particularly given the no-analog present and future that climate change presents (Freeman et al. 2017). For example, the most concerning contemporary high-severity fire events are associated with large patches of complete stand replacement (Miller and Quayle 2015, Lydersen et al. 2016). In some cases, high-severity fire events convert forests to shrubland and grassland assemblages as alternative stable states in uncharacteristically large patches (Falk et al. 2019, Kemp et al. 2019, Stevens-Rumann and Morgan 2019). As such, a critical forest management concern is that high severity wildfires are accelerating rates of vegetation change, forest conversion, and vulnerability of native habitats in response to a warming climate.
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I bolded that another perspective, as the authors state, is that “management does not require justification based on historic conditions.” They have a citation for that, but I don’t think it requires a scientist to say that. As the authors state (and I have also) the point is to increase forest resilience/resistance to climate change, and that with climate change the future is unlikely to be like the past, so the past has limited relevance and is certainly not a target. Certainly scientists from a discipline, say historic vegetation ecology, may think that their research is extraordinarily relevant- but scientists from other disciplines- not to speak of managers and the public, may not agree.  The relationship of “what was” or “what is” to “what should be”, is a societal question, as is the question of “what should be” itself.