Review the project overview.pdf ?Submit your Assignment as a .PDF, .DOC, or .DOCX file with a file name ?in this format: your full name.Assignment.? This fil

Wildfires and global change

Author(s): Juli G Pausas and Jon E Keeley

Source: Frontiers in Ecology and the Environment , SEPTEMBER 2021, Vol. 19, No. 7 (SEPTEMBER 2021), pp. 387-395

Published by: Wiley on behalf of the Ecological Society of America

Stable URL: https://www.jstor.org/stable/10.2307/27091477

REFERENCES Linked references are available on JSTOR for this article: https://www.jstor.org/stable/10.2307/27091477?seq=1&cid=pdf- reference#references_tab_contents You may need to log in to JSTOR to access the linked references.

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms

Wiley and Ecological Society of America are collaborating with JSTOR to digitize, preserve and extend access to Frontiers in Ecology and the Environment

This content downloaded from �������������76.33.214.159 on Wed, 05 Feb 2025 03:22:25 UTC�������������

All use subject to https://about.jstor.org/terms

© The Ecological Society of America Front Ecol Environ doi:10.1002/fee.2359

REVIEWS 387

Front Ecol Environ 2021; 19(7): 387–395, doi:10.1002/fee.2359

We live on a flammable planet, with wildfires occurring nearly everywhere there is sufficient biomass (Archibald

et al. 2013). Fires are an ancient phenomenon (Pausas and Keeley 2009; Scott 2018), and many plant species have acquired adaptive traits that help them survive and reproduce under recurrent fire events (Keeley et al. 2011; Lamont et al. 2019). Wildfire regimes vary across ecosystems, especially in relation to productivity (Keeley and Zedler 1998; Pausas and Ribeiro 2013), and also as a consequence of human activities (Balch et al. 2017; Syphard et al. 2017; Keeley and Pausas 2019) and other global changes. While temperature and atmospheric car- bon dioxide (CO2) concentrations are increasing worldwide, other global change drivers (eg population growth, land use

and management, rainfall, invasive species) have differential impacts on global fire activity (Krawchuk and Moritz 2011; Pausas and Paula 2012; Keeley and Syphard 2019). In addition, global change drivers have differing effects depending on the ecosystem vegetation structure and dominant fuel types (grass, litter, or standing wood). Understanding all of these complexi- ties is not always straightforward, yet is key for the sustainable management of ecosystems in a changing world. Global warm- ing is often implicated as the primary driver of accelerated wildfire activity (eg Williams et al. 2019), but anthropogenic factors other than climate change can be as, if not more, impor- tant than climate change. However, in many ecosystems, the relative role of climate may be increasing as warming escalates. Here, we present and discuss the complexities of the role of global change drivers in modifying fire regimes (Table 1) and provide a mechanistic understanding of this topic that is appli- cable to any region worldwide (although many of our examples are drawn from Mediterranean climate regions [MCRs], given that they are among the most well documented).

The occurrence of wildfires in an ecosystem requires the confluence of at least four factors: ignitions, continuous fuels, droughts, and appropriate weather conditions (wind, high temperatures, and low humidity). Wildfires result from the nexus of these factors, all of which are potentially affected by global changes. Thus, fire regimes are changing in many ecosystems, but often for different reasons. Because fire is a spatial process based on connectivity, it is unlikely that the relationship between wildfire drivers and wildfire activity will be linear (Abades et al. 2014; Pausas and Keeley 2014; van Nes et al. 2018). For this reason, abrupt shifts in fire regimes are possible even with small changes in the drivers, making prediction difficult. Fire drivers can therefore be considered as switches: that is, wildfires occur when the four primary drivers (ignitions, fuel, drought, and fire weather) are “switched on” (sensu Bradstock 2010). However, the level at which a driver switches on (or off) may not be fixed; moreover, fire weather acts at a different spatiotemporal scale from the other three drivers (Moritz et al. 2005). Here, we

Wildfires and global change Juli G Pausas1* and Jon E Keeley2,3

No single factor produces wildfires; rather, they occur when fire thresholds (ignitions, fuels, and drought) are crossed. Anomalous weather events may lower these thresholds and thereby enhance the likelihood and spread of wildfires. Climate change increases the frequency with which some of these thresholds are crossed, extending the duration of the fire season and increasing the fre- quency of dry years. However, climate- related factors do not explain all of the complexity of global fire- regime changes, as altered ignition patterns (eg human behavior) and fuel structures (eg land- use changes, fire suppression, drought- induced dieback, frag- mentation) are extremely important. When the thresholds are crossed, the size of a fire will largely depend on the duration of the fire weather and the extent of the available area with continuous fuels in the landscape.

1Centro de Investigaciones sobre Desertificación, Consejo Superior de Investigaciones Cientificas (CIDE- CSIC), Montcada, Spain*(juli.g.pausas@ext. uv.es) ; 2Western Ecological Research Center, Sequoia– Kings Canyon Field Station, US Geological Survey, Three Rivers, CA; 3Department of Ecology and Evolutionary Biology, University of California– Los Angeles, Los Angeles, CA

In a nutshell: • Climate change alone is insufficient to explain current

fire- regime changes for wildfires • Wildfires require the confluence of at least four factors:

ignitions, continuous fuels, droughts, and appropriate weather conditions

• Climate change increases drought frequency, extending the fire season and increasing the frequency of dry years

• Human factors apart from those associated with climate change modify ignition patterns and landscapes in such a way that they increase the probability of ignitions co- inciding with extreme weather in landscapes with con- tiguous fuel beds

• The relative role of climate is increasing as warming con- tinues, but on some landscapes its importance may be outweighed by other global change drivers

This content downloaded from �������������76.33.214.159 on Wed, 05 Feb 2025 03:22:25 UTC�������������

All use subject to https://about.jstor.org/terms

Front Ecol Environ doi:10.1002/fee.2359 © The Ecological Society of America

JG Pausas and JE Keeley388 REVIEWS

propose that wildfires be viewed through a threshold approach: that is, wildfires occur when three thresholds are crossed (ignition, continuous fuel, and drought), and fire weather shifts these thresholds to lower values, triggering the occur- rence and spread of wildfires (Figure 1). Through this lens, we evaluate how global change affects wildfire drivers (igni- tion, fuel, drought, and weather) and emphasize the inter- actions that drive wildfires across the world.

Factors affecting wildfire regimes

Ignition patterns

The primary natural source of fire ignition is lightning. Volcanoes and falling rocks are of secondary importance, and spontaneous ignitions may be considered where there is a high accumulation of organic matter (such as peat). In a world undergoing marked alterations in climate and weather patterns, the spatial and temporal distribution of lightning activity may be changing (Romps et al. 2014), with unforeseen implications for the fire regime.

However, the main causes of changes in fire ignitions are anthropogenic factors (Balch et al. 2017; Syphard et al. 2017; Cattau et al. 2020). Humans cause ignitions directly by acci- dent (eg cigarettes; campfires; sparks from engines, power- lines, and welding equipment) or deliberately (ie arson), but also indirectly by altering fuels in such a way that increases their susceptibility to ignitions. Openings created by humans (through construction of roads and buildings, logging activi- ties, and so forth) in formerly vegetated landscapes increase the availability of fine dry fuels for both anthropogenic and natu- ral ignitions (Figure 1). Consequently, ignitions, including those

by lightning, are often associated with roads and exotic grass- lands, which are more flammable than natural forest vegeta- tion (Calef et al. 2008; Veldman et al. 2009; Barlow et al. 2020). Overall, the number of people living in the wildland– urban interface (WUI) may be an indicator of potential igni- tion points (although under very high population densities fire activity decreases due to quick detection, proximity to suppression resources, and fuel fragmentation) (Syphard et al. 2007). For rural populations in highly developed land- scapes (such as Eurasia), this association may be weaker, given that rural activities (eg grazing, farming, wood gather- ing; see below) often reduce fuels, making ignitions less likely. However, agricultural fires in less developed land- scapes can also ignite adjacent wildlands and produce wild- fires if the other thresholds (Figure 1) have been crossed, as currently occurs in some tropical ecosystems (Barlow et al. 2020). Furthermore, the increase of the rural population in those ecosystems also tends to result in higher levels of burn- ing to convert forests to farmland. In many ecosystems, the number of people living in the WUI has increased in recent decades, and as with many spatial processes, this number tends to increase exponentially. For instance, the population of California has increased by six million since 2000, and most fires in this region are ignited by a diversity of anthro- pogenic factors (Syphard and Keeley 2015), including power- line failures that have been the cause of fires over ~200,000 ha since 2000 (ie five times the amount over the previous 20 years; Keeley and Syphard 2018, 2019). The “California life- style” model, in which development is spread widely across the landscape instead of being contained within more con- centrated settlements, is becoming increasingly common around the world, especially in other highly populated MCRs.

Table 1. Main effects of different global change drivers on fire- regime parameters in ecosystems with different types of fire regimes

Crown- fire ecosystems Surface- fire ecosystems

Global change driver Woody- fueled fires Grass- fueled fires Litter- fueled fires

Drought +flammability +FI, +FS, – FRI

– biomass – FI

+litter fall, – litter decomposition +FI

Urban population growth +ignitions – FRI

+fire exclusion +FRI, +FI

+fire exclusion +FRI, +FI

Rural population growth +fragmentation – FS, +FRI

+(over)grazing and +ignitions +fragmentation, +openings, +ignitions

Atmospheric CO2 Minor effect Encroachment +FI

+litter production, +C/N, – decomposition +FI

Invasive grasses – FRI, – FI +biomass +FI

+biomass +FI

Heatwaves +flammability +FI, +FS

+flammability +FI, +FS

+flammability +FI, +FS

Unnatural fuel loads (fire exclusion, tree plantations) +FI +FI +FI

Examples Mediterranean shrubland, boreal forests

Tropical grasslands, savannas, open woodlands

Pine woodlands, some closed forests

Notes: + indicates positive effect of the global change driver; – indicates negative effect. FI: fire intensity; FS: fire size; FRI: fire return interval.

This content downloaded from �������������76.33.214.159 on Wed, 05 Feb 2025 03:22:25 UTC�������������

All use subject to https://about.jstor.org/terms

© The Ecological Society of America Front Ecol Environ doi:10.1002/fee.2359

Wildfires and global change REVIEWS 389

The relationship between the number of ignitions and the occurrence of wildfires is likely to follow a saturation curve (Figure 1a): that is, if the weather conditions are not severe, few ignitions are unlikely to generate a wildfire, but there is a level of ignitions in which the probability of a wildfire increases abruptly (Bradstock 2010). Some ecosystems may therefore be limited by natural ignition (flammable ecosystems that do not burn because of the lack of ignitions; central Chile may be an example; Keeley et al. 2012). Others are saturated by ignitions (eg areas with large human population densities or frequent lightning strikes), in which case a small reduction in ignitions may not reduce fire activity. However, even a relatively small number of ignitions, if coupled with extreme fire weather (which reduces the threshold value; Figure 1), can generate large wildfires. For instance, anthropogenic ignitions have been declining in recent decades in many MCRs, yet the few ignitions that occur during severe wind events generate large wildfires (Curt and Frejaville 2018; Keeley and Syphard 2018, 2019).

Increasing human population growth enhances the prob- ability of ignitions during severe fire weather; in MCRs, due

to urban sprawl into landscapes with dangerous fuels, it also increases the number of people and properties at risk. For example, the area burned in the 2017 Tubbs Fire in northern California, which was caused by a powerline failures, largely coincided with the area burned by the 1964 Hanly Fire in the same area. Yet there were no fatalities in the Hanly Fire despite the fact that it was substantially larger than the Tubbs Fire, and only 84 structures were destroyed; in comparison, there were 22 fatalities in the 2017 Tubbs Fire, and more than 5600 structures were destroyed (Keeley and Syphard 2019). The Tubbs Fire was far more devastating simply because the regional population had increased fivefold over the intervening 53 years between the two fires, and the elec- trical grid system had been greatly expanded across the landscape, with a consequent increase in the potential for wind- driven powerline failures.

Fuel continuity

The rate at which wildfires spread through plant commu- nities depends on their structure and the flammability of

Figure 1. Conceptual model of relationships between fire parameters and their drivers. (a) Probability of fire occurrence versus ignitions, fire spread ver- sus landscape fuel continuity, and fuel flammability versus drought. In these graphs, dashed vertical lines indicate thresholds. In all cases, fire weather (strong wind, high temperature, or low humidity) shifts the curve and the threshold toward lower values (thick red arrows; ie saturation is reached at lower values along the x- axis), consequently increasing the probability of an ignition resulting in a fire, fire spread (for a given landscape configuration), and veg- etation flammability (fuel dries faster). The flow chart shows the main factors affecting the fire drivers, including human population growth in or near wild- lands, altered fuel loads (fragmentation, oldfields, fire exclusion, among others), and climate change. Arrows indicate positive interactions, with the exception of changes in fuel, which can increase or decrease fuel continuity depending on the system (eg fragmentation versus fire exclusion or increas- ing oldfields). (b) Once all thresholds have been crossed, the size of the fire is determined by the duration of the extreme fire weather and the availability of continuous fuels in the landscape.

(a) (b)

D ha

rm ab

er en

S tu

di o/

w w

w. dh

ar m

ab er

en .c

om

This content downloaded from �������������76.33.214.159 on Wed, 05 Feb 2025 03:22:25 UTC�������������

All use subject to https://about.jstor.org/terms

Front Ecol Environ doi:10.1002/fee.2359 © The Ecological Society of America

JG Pausas and JE Keeley390 REVIEWS

neighboring plants. Fuel continuity and its load are crit- ically important, and both vary with productivity across ecosystems (Pausas and Paula 2012), as productivity con- trols plant growth and decomposition. However, human activities strongly influence fuel patterns. For instance, agriculture and urban infrastructure increase ecosystem fragmentation and reduce landscape fuel continuity. In highly populated areas (eg southern Europe), large fires often cease when they reach farmland. Indeed, a global reduction of fire activity has been detected in recent dec- ades (Marlon et al. 2008; Andela et al. 2017), partially due to increased farming (mainly in tropical areas). Other factors, however, increase fuel continuity and fire activity in many regions of the world.

Fire exclusion is an example of a driver that has enhanced fuel buildup in many ecosystems. For instance, many west- ern US coniferous forests that were once subject to frequent surface fires have experienced a marked increase in under- story fuels as a consequence of a successful fire suppression policy. This has caused a widespread increase in vertical connectivity (ladder fuels) and in the susceptibility to high- intensity crown fires (Covington and Moore 1994; Allen et al. 2002; Swetnam et al. 2016). Similar fire- exclusion poli- cies are altering fuel structure in other ecosystems world- wide (eg Baker and Catterall 2015; Johansson et al. 2019). In Mediterranean Europe, aggressive fire suppression is cur- rently the primary policy; although this approach reduces the number of fires, it increases fuel loads and consequently the susceptibility of the landscape to ignitions and droughts (Pausas and Fernández- Muñoz 2012; Curt and Frejaville 2018).

Forestry plantations are another potential source of increasing fuel amount and continuity. They are on occa- sion established in natural nonforest ecosystems, such as Mediterranean shrublands and savannas (Bosch and Hewlett 1982), which greatly increases fuel loads in these systems. Other plantations are established in native forest ecosystems, but tree density and the degree of homogeneity are usually much higher in plantations (such as on evenly aged plantations, which maximize wood production) than in the native forests they replace and thus they increase fuel continuity (Keeley and Syphard 2019). Fuel management can alleviate several of these problems by generating verti- cal discontinuities and reducing tree density (Knapp et al. 2017), but this is not always practiced because of cost, lim- itations due to air- quality restrictions (eg prescribed burns), and topographic constraints (eg mechanical treat- ments); in addition, silvicultural management is often based on the climate of the 20th century. As a result, mas- sive fires have become increasingly common in exotic tree plantations, such as in Chile and Portugal (Gómez- González et al. 2018).

In many regions, rural abandonment of agriculture (including livestock grazing and wood gathering) has driven increases in early successional (flammable) plant

communities, enhancing fuel loads and continuity (homoge- nization of landscape). This trend has been occurring in part because rural abandonment has not been accompanied by the reintroduction of native herbivores. Examples have been documented in Mediterranean Europe (Pausas and Fernández- Muñoz 2012) and the former Soviet Union (Dubinin et al. 2011). However, this process is occurring in many other regions where urbanization rates are increasing, and it is likely to become more relevant in areas where tourism provides an alternative economic income to traditional rural livelihoods (Chergui et al. 2018).

Invasive plants are also changing fuel patterns, most typi- cally by increasing the amount and continuity of herbaceous fuels, with consequences for the fire regime (Brooks et al. 2004; Pausas and Keeley 2014). For example, the Great Basin Sage Scrub ecosystem in the US is threatened due to exotic grass invasion and subsequent changes in a natural mosaic of patchy burns toward large, continuous burns that threaten recovery of the native ecosystem (eg the 2020 Dome Fire in California’s Mojave Desert; Keeley and Pausas 2019).

Increased atmospheric CO2 enhances plant growth and lit- ter production, and is therefore likely to be contributing to increasing fuel load and continuity; a substantial proportion of Earth greening (Piao et al. 2020) and savanna encroachment (Buitenwerf et al. 2012) has been linked to atmospheric CO2 fertilization. Given that CO2 increases water use efficiency (ie reduced stomata openings for fixing a given amount of car- bon), the greater biomass promoted by higher concentrations of atmospheric CO2 may be more important in dry ecosys- tems, although evidence for this remains limited (Van der Sleen et al. 2015).

Independent of the mechanism that increases fuels, greater fuel continuity clearly enhances the probability of fire spread, a pattern that is not linear but rather associated with thresholds (Figure 1) due to the spatial nature of the processes associated with fire spread (that is, a contagious process; Abades et al. 2014; Pausas and Keeley 2014; van Nes et al. 2018). In other words, there is a level of fuel continuity from which fires can easily “percolate” through the land- scape, and this percolation threshold is modified by fire weather (Figure 1).

Under very high and continuous fuel loads as well as extreme fire weather, fires not only cross the fuel threshold very easily but can generate enormous heat and a tall plume of hot air that drives convection columns (pyrocumulonimbus; Panel 1; Figure 2). In some extreme cases, the heat may be so great that the convection column extends through the tropopause and enters the stratosphere (Figure 2; Dowdy et al. 2019). In such instances, fire– atmosphere feedbacks produce complex, rapid, and unpredictable fires (see “firestorm” in Panel 1). The intensity of these fires is such that flammable material some distance ahead of the fire front is desiccated and easily ignited by embers, enabling the fire to jump fuel breaks. These types of fires can also burn forests that have been traditionally considered resistant to wildfire.

This content downloaded from �������������76.33.214.159 on Wed, 05 Feb 2025 03:22:25 UTC�������������

All use subject to https://about.jstor.org/terms

© The Ecological Society of America Front Ecol Environ doi:10.1002/fee.2359

Wildfires and global change REVIEWS 391

Droughts

Droughts occur in all ecosystems but the average severity and duration of drought events, includ- ing anomalous drought events, differ markedly among systems. The impact of a drought is largely due to deviation from the long- term average, as this is what drives plant adaptations. In MCRs and savanna ecosystems, droughts occur frequently, and as a result these land- scapes are typically fire prone. Globally, how- ever, droughts may occur with less regularity and are often dependent on decadal- scale events. These differing patterns of drought have impor- tant effects on wildfire frequency and intensity.

On a global scale, climate warming is expected to greatly affect fuels, whereas changes in rainfall patterns are more variable; even on land- scapes where rainfall is not declining, however, warming increases evapotranspiration rates, leading to both a drier cli- mate and fuels. Drought has contrasting effects between woody and grassy ecosystems (Table 1). In woody- dominated

ecosystems (eg Mediterranean, temperate, and boreal ecosys- tems), drought not only increases the likelihood of fire, it also creates more standing dead biomass, and increases plant mor- tality and litter, further enhancing fuel connectivity. All of these factors exacerbate ignition success and fire spread. In

Figure 2. Pyrocumulonimbus (a) in La Pampa Province, Argentina, as seen from a satellite (29 Jan 2018) and (b) in Beneixama, Spain, as seen from the ground (15 Jul 2019). In (a), note that the dynamics of the smoke at lower levels (moving toward the south) differs from that at higher levels (pyrocumulonimbus moving west).

(a) (b)

Panel 1. Key concepts

Crown fire: fires in woody- dominated ecosystems that affect all veg- etation including crowns (woody- fueled fires). They are typically high intensity. Examples include fires in some Mediterranean- type forest and shrublands and in closed- cone pine forests.

Fire regime: the characteristic wildfire activity that prevails in a given area at a particular time. It is typically determined by its frequency, intensity, seasonality, size distribution, and type of fuels consumed, and depends on the frequency which all fire thresholds are crossed (Figure 1). Two common fire regimes that represent extremes are surface- fire regimes and crown- fire regimes.

Fire weather: refers to the prevailing weather conditions affecting fire behavior; extreme (or severe) fire weather refers to strong winds, high temperatures, and low humidity.

Firestorm: a generic term used to describe wildfires with extreme, at times erratic behavior driven by high winds. The term has been used to describe wind- driven fires (eg Santa Ana wind firestorms in California) or pyrocumulonimbus plume fires (the 2019– 2020 fires in Southeast Australia).

Flammable: a general term for fuels that easily ignite and contribute to fire spread. Quantifying the propensity to burn (flammability) is complex, as it encompasses several processes and depends on plant traits and struc- ture, weather conditions, and the scale of reference (Pausas et al. 2017).

Megafire: wildfires at the extreme of the frequency size distribution for a given ecosystem; typically megafires are outliers (in the statistical sense) in relation to the historical fire size distribution. They are often

driven by strong winds and/or high and continuous fuel loads (ie wind- driven or fuel- driven wildfires). Sometimes the term refers to several fires that burn simultaneously in a specific region, and then merge as they grow. The size of megafires is determined by the duration of the fire weather and by landscape characteristics (Figure 1b); for example, fires in Alaskan boreal forests often extend over 500,000 ha or more, but in other landscapes, continuous fuels are insufficient to carry fires of this size. The social impacts of a fire are not included in this definition (but see Stephens et al. 2014), as not all megafires are necessarily catastrophic.

Pyrocumulonimbus: a dense, towering, vertical cloud carried by pow- erful upward air currents generated by the heat of a wildfire (Figure 2; also known as plume- dominated fires, superheated wildfires, and wildfire- driven thunderstorms). This phenomenon is typically linked to very high and continuous fuel loads and extreme fire weather that produces great heat and strong convection currents. In most cases, it remains in the troposphere, but when heat produced by a fire is extremely high, it can cross the tropopause and inject a large amount of smoke into the stratosphere. These plumes often collapse at altitude due to colder temperatures, and create extreme winds. As such, these plumes generate feedback processes between the atmosphere and the fire that can produce strong surface winds, tornadoes, and even pyro- genic lightning ignitions that further expand the fire (firestorms).

Surface fire: fires that spread in the herbaceous (grass- fueled fires) or litter (litter

Collepals.com Plagiarism Free Papers

Are you looking for custom essay writing service or even dissertation writing services? Just request for our write my paper service, and we'll match you with the best essay writer in your subject! With an exceptional team of professional academic experts in a wide range of subjects, we can guarantee you an unrivaled quality of custom-written papers.

Get ZERO PLAGIARISM, HUMAN WRITTEN ESSAYS

Why Hire Collepals.com writers to do your paper?

Quality- We are experienced and have access to ample research materials.

We write plagiarism Free Content

Confidential- We never share or sell your personal information to third parties.

Support-Chat with us today! We are always waiting to answer all your questions.