D’oh! Fire suppression makes wildfires more severe

SCIENCE®!!!! discovers what my dad (a civil engineer) taught me more than 50 years ago.

New paper published in Nature Communications.

Here is the abstract and introduction to this open access paper, which of course has the obligatory nod to “climate change” no matter how insignificant to the result.

Abstract

Fire suppression is the primary management response to wildfires in many areas globally. By removing less-extreme wildfires, this approach ensures that remaining wildfires burn under more extreme conditions. Here, we term this the “suppression bias” and use a simulation model to highlight how this bias fundamentally impacts wildfire activity, independent of fuel accumulation and climate change. We illustrate how attempting to suppress all wildfires necessarily means that fires will burn with more severe and less diverse ecological impacts, with burned area increasing at faster rates than expected from fuel accumulation or climate change. Over a human lifespan, the modeled impacts of the suppression bias exceed those from fuel accumulation or climate change alone, suggesting that suppression may exert a significant and underappreciated influence on patterns of fire globally. Managing wildfires to safely burn under low and moderate conditions is thus a critical tool to address the growing wildfire crisis.

Introduction

Wildfires are becoming more destructive and deadly around the world^1,2,3,4. The societal and ecological impacts of fires^5,6 are in our collective consciousness—from Australia’s 2019–2020 megafires⁷, to destructive wildfires in the Mediterranean⁸, to beloved giant sequoias killed by fire in California⁹—prompting widespread calls to address the wildfire crisis^{8,10,11,12,13,14}. We understand the broad drivers of increasing fire activity: changes in climate^1,15,16, vegetation and fuel accumulation^8,17,18, and ignition patterns¹⁹. However, humans also play a direct role in modifying fire activity across much of the globe by engaging with fires minutes to hours after ignition (i.e., initial attack^20,21), and subsequent suppression of escaped fires^22,23,24. While weather, fuels, topography, and ignitions determine how fires might burn, humans strongly shape this into when, where, and how fires do burn (Fig. 1).

Wildfires only burn if they are not extinguished through suppression. Thus, suppression is a “filter” that allows certain types of fire to pass through while removing other types of fire (Fig. 1b). In some locations (e.g., a remote wilderness area) this filter may be relatively porous, and many fires may burn with only minimal suppression²⁵. In most landscapes, however, aggressive suppression of fire is a cultural expectation, and the suppression filter is much less permeable, with only the most extreme fires escaping (e.g., Maximum suppression in Fig. 1)^20,21. Intentional fires (i.e., prescribed fires and cultural burning) are the only types of fires that do not pass through the suppression filter, as they are allowed to burn unimpeded if they are within prescription (Fig. 1a). Area that does not burn because it was “removed” through suppression, results in fuel accumulation and ultimately increases the likelihood and intensity of future fires^26,27 (Fig. 1a). This well-known consequence has been termed the “fire suppression paradox”^28,29: by putting out a fire today, we make fires harder to put out in the future^26,30,31,32 (Table 1).

Suppressing wildfires also has an additional and poorly quantified consequence that we define as the “suppression bias”: fire suppression extinguishes some types of fire (e.g., surface fire) more than others (e.g., crown fire), and thus skews the resulting fire activity toward those types less likely to be removed (Table 1). In contemporary fire management, which easily suppresses and removes low-intensity fire, this bias is inevitably toward higher-intensity burning occurring under extreme weather^{20,21,33,34,35}. Thus, the fires which ecosystems, species, and people experience are skewed towards the most severe and destructive.

We define management approaches that suppress lower-intensity fire more heavily than higher-intensity fire as “regressive suppression,” borrowing language from economics (e.g., a regressive tax rate decreases as taxable income increases) (Table 2). In some instances, however, management could contain and suppress relatively higher-intensity fire more heavily than lower-intensity fire, an approach we term “progressive suppression” (e.g., a progressive tax rate increases as taxable income increases) (Table 2). Both regressive and progressive suppression are subject to the same upper limit, above which fires are simply too intense to suppress (Fig. 1b)³⁶; however, within the domain where suppression is possible, regressive and progressive approaches can have profoundly different impacts (i.e., biases) on the way fires burn.

Although the fire suppression bias has been referenced tangentially in the literature^{8,28,33,37,38,39}, the emergent impacts of the suppression bias have not been assessed. This is largely due to the difficulty of isolating the impact of suppression with empirical data. Suppression is so ubiquitous that we have virtually no control landscapes where fire is completely unsuppressed; even in remote wilderness areas, some fires are still suppressed⁴⁰. Furthermore, it is difficult to measure the magnitude of suppression efforts because even relatively direct proxies such as suppression cost are confounded by other factors, including terrain accessibility, human infrastructure at risk, and availability of suppression resources⁴¹. Finally, data on suppression efforts are generally only available for larger fires⁴², obscuring the many ignitions that are quickly and easily suppressed during initial attack^20,21. To overcome these constraints, we used a simulation approach to assess and quantify the magnitude of the fire suppression bias on fire behavior and ecological impacts, relative to the influence of climate change and fuel accumulation.

Our modeling framework simulates fundamental components of fires: weather and fuel moisture; ignitions; fire growth; fire suppression (through initial attack and containment of escaped fires); and ecological effects. To isolate the effect of fire suppression, we simulated thousands of fires with identical biophysical conditions, but which differed only in their suppression scenario, including three “regressive suppression” scenarios (Moderate, High, and Maximum; Fig. 1b), one “progressive suppression” scenario (Progressive; Fig. 1b), and a control scenario with no suppression. For each fire, we calculated the proportion burned at high severity, average fire severity, daily and total fire size, and the diversity of fire severity⁴³. To compare the influence of suppression to that of climate change and fuel accumulation, we simulated fires across a range of plausible current and future fuel aridity (vapor pressure deficit; VPD) and fuel loading conditions in forest ecosystems in North America. These ranges represent a 240-year time period of modeled increases (e.g., increased VPD based on RCP 8.5 climate scenario⁴⁴; fuel loading rates based on historical fuel modeling⁴⁵).

Using this modeling framework, we show how the suppression bias directly influences fire activity and subsequent fire effects. Specifically, we asked: 1) How does fire suppression influence patterns of area burned, ecological impacts (i.e., fire severity), and the diversity of both factors over space and time? 2) How does the magnitude of this influence compare to that from climate change and fuel accumulation?

The title of the first paragraph of the result section is pretty straightforward

Results

Regressive fire suppression makes fires more severe

You can read the full, obvious, known forever, realizations about forest and land management article here.

https://www.nature.com/articles/s41467-024-46702-0

H/T Duane