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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

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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

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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

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

Heatwaves

+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.

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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)

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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.

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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- fueled fires) layer, such as the understory of some forests
(understory fires) and in savannas and grasslands. These fires are usu-
ally of relatively low intensity and high frequency.

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

JG Pausas and JE Keeley392 REVIEWS

consequence, fire seasons are becoming longer (Westerling
et al. 2006; Flannigan et al. 2013), and this increases the annual
frequency of weather conditions appropriate for fire spread. In
many forest ecosystems where past climatic conditions were
rarely conducive to fire, the frequency of dry years (that is,
years with appropriate conditions for fire spread) is now
higher.

Although drought is often a good indicator of the probabil-
ity of fire spread in woody- dominated ecosystems, this rela-
tionship is not linear, as there is a drought threshold from
which fire spread increases abruptly (Figure 1; Westerling and
Bryant 2008; Pausas and Paula 2012). This threshold is not uni-
versal, and the degree of drought necessary for it to be crossed
(ie to switch from a nonflammable to a flammable state)
depends on the vegetation type (ie landscape fuel amount and
continuity). For example, the drought threshold in productive
ecosystems (where fuels are more dense) is associated with
conditions that are less dry, as compared to the threshold in
dry ecosystems (where fuels are more sparse) (Pausas and
Paula 2012).

In contrast, in seasonal grassy communities (note that
grass- fueled fires account for most burned area globally;
Van der Werf et al. 2006), drought is associated with reduced
(rather than increased) fire activity. This is because grasses
become highly flammable each year during the dry season,
and fire activity is dependent mainly on grass biomass and
continuity; in such ecosystems, drought reduces fuel con-
tinuity and subsequently fire activity. Indeed, grass biomass
and continuity are affected positively by the previous year’s
rainfall (antecedent climate). The contrasting effect of drought
between woody and grassy communities was evident after
the particularly long and severe 2019 drought in Australia
(Figure 3a), which resulted in greatly increased areal extent

burned in forests but decreased areal extent burned in
savannas (Bowman et al. 2020). A positive effect of ante-
cedent rainfall has also been detected in some ecosystems
dominated by woody species, suggesting that drought prior
to onset of the fire season may to some extent reduce fire
spread in the herbaceous or litter layer (Swetnam and
Betancourt 1998; Pausas 2004; Keeley and Syphard 2017).

Fire weather

Fire weather refers to the weather conditions directly affecting
fire behavior (ie conditions that lower fire thresholds in a
given ecosystem and make them easier to cross) (Figure 1a).
In addition, the duration of fire weather can contribute to
fire size (Figure 1b); as examples, the longer- than- average
Santa Ana wind event during 2017 contributed to the mas-
sive Thomas Fire in California, and the very long and intense
heat wave in that state during August– September of 2020
led to the highest annual burned area on record.

Wind is a critically important fire weather factor and facili-
tates the crossing of all three fire thresholds; generally, wind (1)
increases the chances of successful ignition of a fire because it
supplies oxygen for combustion; (2) affects flame length and
depth, as well as the dispersal distance of embers, and therefore
greatly influences fire spread rate and the degree to which a fire
can bridge fuel discontinuities; and (3) increases evapotranspi-
ration rates, exacerbating vegetation dryness (and therefore
enhancing its flammability). Wind can shift fire thresholds
downward (Figure 1), and may offset or overwhelm all other
factors (eg wind- driven fires). In addition, changes in wind
direction determine the shape of the fire, and the duration of
wind events may determine the size of the fire (Figure 1b). The
importance of wind is conspicuous in many fire- prone

Figure 3. The 2019– 2020 bushfires in southeastern Australia were driven by the combination of an extended drought and severe fire weather conditions.
(a) Mean temperature anomaly and rainfall anomaly in Australia for the month of December for all years with available climatic records (1910– 2019). (b)
Smoke plumes showing the importance of wind in driving multiple fires (8 Nov 2019). Graph in (a) elaborated from data by the Australian Bureau of
Meteorology.

(a) (b)

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Wildfires and global change REVIEWS 393

ecosystems (Figure 3b; Keeley et al. 2012); for instance, in
California’s chaparral, the largest fires typically occur in
autumn, when foehn winds – dry, warm winds that blow
downslope in the lee of a mountain range – blow more fre-
quently (eg Santa Ana and North winds; Keeley and Syphard
2019). Notably, these severe wind events do not usually accom-
pany extreme fires, because other thresholds must be crossed
simultaneously to generate those fire events. In fact, large
wind- driven fires in MCRs are typically ignition- limited and
require a combination of extreme wind conditions and human-
caused ignition (see “Ignition patterns” above; Keeley and
Syphard 2019).

The location and direction of topographically determined
winds (eg foehn winds) are unlikely to be affected by global
change, although there is evidence of shifts in seasonal peaks,
frequency, and intensity (Guzman- Morales and Gershunov
2019). However, windstorms that are linked to ocean temper-
atures could be more susceptible to change (Elsner et al.
2008); the unprecedented approach of Hurricane Ophelia
toward western Europe in 2017, where it fueled fires in
Portugal and Spain and transported the smoke to the UK, is
one prominent example (Figure 4). Although difficult to pre-
dict, new wind regimes may be key for future fire activity in
many regions.

High temperature and low atmospheric humidity are the
other key factors of fire weather; they exacerbate evapotran-
spiration rates and warm and dry fuels, which shift ignition
and fuel thresholds downward (that is, warm and dry fuels
ignite and burn faster because they require less heat energy to
reach ignition temperature; Figure 1). Foehn winds are usu-
ally very hot and dry, and can therefore greatly enhance igni-
tions, flammability, and fire spread even when they are not
especially strong. High temperatures can also contribute to
the formation of strong convections, which typically develop
when fuel loads are very high (but not often in wind- driven
fires), increasing the probability of generating pyrocumulo-
nimbus clouds and severe fire behavior (Panel 1). More fre-
quent heatwaves (Figure 3; Wang et al. 2020) resulting from
climate warming may increase the occurrence of these
events. The recent massive wildfires in Southeast Australia
(2019– 2020) and California (2020) were both driven by
extreme weather conditions consisting of strong winds and
high temperatures after extraordinarily dry years (Figure 3).

Conclusions

Although none of the factors mentioned above generate wild-
fires in and of themselves, combinations of these factors – ignition,
drought, continuous fuels, and suitable fire weather – typically
act in concert to produce wildfires under certain conditions.
The fire regime of an ecosystem is determined by the fre-
quency with which all thresholds are simultaneously crossed,
and its current variations depend on how global change
factors affect the various fire drivers (Table 1; Figure 1).
When these thresholds are crossed, the size and duration

of a fire will largely depend on how long the fire weather
lasts and the extent of the area containing suitable fuel
material (Figure 1b). The former can be influenced by cli-
matic change, whereas the latter largely depends on landscape
constraints (including topography) and human activities (eg
fragmentation, rural abandonment, fire exclusion, tree plan-
tations). Wildfires in Spain, for example, are typically smaller
than those in Australia or Canada not because of differences
in climate and weather conditions but rather because of the
lower availability of extensive natural vegetation. Large wildfires
(see “megafire” in Panel 1) are not necessarily novel events
in many ecosystems, but they may be increasing in occurrence
as thresholds are more frequently crossed due to higher num-
bers of ignitions (Syphard and Keeley 2015), as well as land-
scape modifications (eg Covington and Moore 1994; Pausas
and Fernández- Muñoz 2012) and more severe drought con-
ditions (Figure 3; eg Boer et al. 2020).

Human factors drive many of these changes in fire regime
(Figure 1). One of these factors is climate change, which
increases the frequency of conditions conducive to fire: that is,
it increases the fire season (in fire- prone ecosystems) or the
frequency of fire- prone years (in typically non- fire- prone eco-
systems). However, climate- related factors do not explain all
the complexity of the changes in global fire regimes, as altered
ignition patterns (eg human behavior) and fuel structure (eg
land- use changes, fire suppression, fragmentation) are also
important. For instance, in many Mediterranean ecosystems,
the drought threshold is crossed annually, and vegetation cover
is usually high enough for fire spread; as such, ignitions are a

Figure 4. Hurricane Ophelia originated in the Caribbean and eventually
reached Europe, where it fueled fires in Portugal and Spain, and covered
the UK with smoke from those fires (16 Oct 2017). It is considered the
easternmost Atlantic hurricane on record. Red circles are fires as detected
by the Visible Infrared Imaging Radiometer Suite (VIIRS).

N
A

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

JG Pausas and JE Keeley394 REVIEWS

key factor. Larger populations of humans in the WUI will likely
lead to increased ignition rates, and consequently higher prob-
ability of ignitions coinciding with extreme weather events to
generate wildfires. In other ecosystems, changes to forest man-
agement (through policies, rural depopulation, and so forth)
may be more relevant for fire activity than changes in climate.
Overall, with respect to climate- change- associated increases in
fire activity (eg due to increasing drought frequency), cold and
moist ecosystems and fuel- driven fire regimes are likely to be
more susceptible than warm and dry ecosystems and wind-
driven fire regimes; however, other drivers may outweigh cli-
mate in controlling fire regimes.

Thresholds can be quantified for each system with statistical
models constructed from empirical data (Westerling and
Bryant 2008; Pausas and Paula 2012), but this approach can be
challenging because estimations of one threshold requires con-
trolling for the others. An alternative approach is to use simu-
lation models and sensitivity analysis (Abades et al. 2014;
Pausas and Keeley 2014; van Nes et al. 2018). Further research
is also required for using thresholds and early warnings for fire
risk management, as has been done previously for other natu-
ral hazards. Because wildfires require the crossing of several
thresholds simultaneously (Figure 1), managing with the goal
of preventing any single threshold from being crossed could
potentially reduce fire activity considerably. Although the
occurrence of weather conditions conducive to fire may be
increasing as a result of climate change, managing ignitions
and fuel loads have been shown to be effective tools for main-
taining fires within an acceptable regime. Application of these
tools will vary across ecosystems, however; for instance, areas
in which fuel- driven fires dominate require very different
management approaches from those dominated by wind-
driven fires (fuel versus ignition control, respectively; Keeley
and Syphard 2019). Likewise, grassy and woody ecosystems
respond in different ways and therefore require distinct man-
agement strategies (Table 1). In areas where management is
likely to be ineffective, the primary option should be to delimit
danger zones in which human activities are minimized, in
much the same way as is done in areas at risk of flooding or
volcanic activity.

Acknowledgements

This work was performed under the framework of the
FIROTIC project (PGC2018- 096569- B- I00) of the Spanish
Government and the PROMETEO/2016/021 project of
Generalitat Valenciana. Any use of trade, product, or firm
names is for descriptive purposes only and does not imply
endorsement by the US Government.

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Wildfires as an ecosystem service

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

Source: Frontiers in Ecology and the Environment , JUNE 2019, Vol. 17, No. 5 (JUNE
2019), pp. 289-295

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CONCEPTS AND QUESTIONS 289

Front Ecol Environ 2019; 17(5): 289–295, doi:10.1002/fee.2044

Humans gain a variety of benefits from the proper func-
tioning of natural ecosystems, commonly referred to as

“ecosystem services” (MA 2005). Wildfires are a globally
important and integral part of many ecosystems, playing key
roles in ecosystem dynamics and the retention of species that
have evolved in response to fire (Pausas and Keeley 2009). We
contend that most wildfires are natural processes that provide
a variety of benefits to humankind, and that in wildfire-
dependent ecosystems, disturbance is manifested by disrup-
tions in the natural fire regime, often when fires are eliminated
or the frequency of burning increases (Keeley and Pausas
2019). Although conservation of natural processes should not
be strictly tied to their usefulness (Silvertown 2015), it is
important to review the services that wildfires provide, given
that they are often seen, incorrectly, as destructive distur-
bances.

Fire was certainly used by early hunter–gatherer societies,
although there is evidence that the use of fire originated with
even earlier hominids (Burton 2011; Gowlett 2016). Ancestral
hominids were frequently exposed to fire from the time they
moved into open savanna environments more than 2 million

years ago. Eventually, humans learned to control fire and it
became embedded in human behavior; as such, use of fire may
be considered an outgrowth of far older, natural fire regimes
(Gowlett 2016). In addition to cooking, as a means of improv-
ing the digestibility of plant and animal foods, early human
societies used fire for many other purposes, including removal
of pests from campsites, provision of heat and light, and for
social events (eg bonfires, rituals). Indeed, evening campfires
likely played a key role in the development of communication
among early humans. Agricultural societies later used fire for
deforestation to create farmland and, more recently, for forest
management (eg slash- and- burn). Although learning how to
use and control fire is considered to be a key step in the evolu-
tion of both humans and human societies (Wrangham 2009;
Burton 2011), and being a pyrophilic (fire- dependent) species
radically changed the ecological niche of humans, most people
today have a negative perception of wildfires.

We aim to summarize the benefits to humans of living in a
flammable world – that is, in a world that experiences recur-
rent wildfires. The multiple benefits (including direct fitness
benefits) of fire use for early humans are not discussed here
(see Wrangham 2009; Burton 2011); instead, we focus on wild-
fires. Prescribed fires are often intended to mimic the benefits
of wildfires, so they are also briefly discussed below, but we
focus primarily on the benefits of natural (ie historical) wild-
fire regimes. Perturbations in these fire regimes (eg an exclu-
sion policy, or increase in the frequency or intensity of fires)
would feed back to the functioning of the ecosystem and
reduce these services in the same way that major anthropo-
genic changes in a rainfall regime would reduce the services
that precipitation provides to humans. We believe that a thor-
ough understanding of the benefits of wildfires requires a
framework that describes impacts at the evolutionary scale (at
which recurrent fires shape landscapes and the biodiversity
they support), as well as at the socioecological scale (at which
anthropogenic modifications of fire regimes feed back to
humans; Figure 1). In this framework, we define “historical fire
regimes” as those in which human influence is absent (includ-
ing those before humans evolved; ie those operating on evolu-
tionary scales; Figure 1), and “anthropogenic fire regimes” as

Wildfires as an ecosystem service
Juli G Pausas1* and Jon E Keeley2

Wildfires are often perceived as destructive disturbances, but we propose that when integrating evolutionary and socioecological
factors, fires in most ecosystems can be understood as natural processes that provide a variety of benefits to humankind. Wildfires
generate open habitats that enable the evolution of a diversity of shade- intolerant plants and animals that have long benefited
humans. There are many provisioning, regulating, and cultural services that people obtain from wildfires, and prescribed fires and
wildfire management are tools for mimicking the ancestral role of wildfires in an increasingly populated world.

1Centro de Investigaciones sobre Desertificación (CIDE-CSIC), Montcada,
Spain *(juli.g.pausas@uv.es); 2US Geological Survey, Western Ecological
Research Center, Sequoia–Kings Canyon Field Station, Three Rivers, CA

In a nutshell:
• We live in a flammable world and humans have benefited

from fires for millennia
• Wildfires generate open habitats for a diversity of light-lov-

ing plants and animals; these species offer a range of
goods and services (food, fiber, pollination, tourism, hunt-
ing) to humans

• Additionally, wildfires help to control pests and catastrophic
fires, contribute to the regulation of biogeochemical cycles,
and can benefit plants in adapting to novel climates

• Prescribed fires can sometimes be used to replace the
original role of wildfires

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Front Ecol Environ doi:10.1002/fee.2044 © The Ecological Society of America

G Pausas and E Keeley290 CONCEPTS AND QUESTIONS

those modified by human activities (ie those operating on
socioeconomic scales; Figure 1). Ecosystem services deriving
from wildfires are grouped into four broad categories (MA
2005), consisting of supporting, provisioning, regulating, and
cultural services (Figure 1). Several of these services have been
poorly studied and as such are presented here as hypotheses to
be tested. We also separate the services that were important to
early societies (eg hunter–gatherer societies) from those that
are relevant to contemporary societies (Table 1).

Supporting services

Supporting services are usually defined as those necessary
for the production of all other ecosystem services (MA 2005).
The most basic ecosystem service provided by wildfires is
the formation of open habitats that enable the evolution of
a diversity of shade- intolerant plants and animals (Keeley et al.
2012, Parr et al. 2014; Andersen 2019). Fire is likely to

contribute to a range of ecosystem services
(Figure 1), given that biodiversity provides
many services to humans (Isbell et al. 2017)
and fire explains a major proportion of the
variability in biodiversity (Ponisio et al. 2016;
Pausas and Ribeiro 2017). Two general pro-
cesses associated with fire are known to enhance
biodiversity: evolutionary processes (via natural
selection and evolution) and ecological pro-
cesses (via habitat heterogeneity). The biodi-
versity effects of both types of processes occur
because fire generates vegetation gaps.

Fires create new habitat with increased
resources and reduced competition. To take
advantage of such fire- generated habitat, many
plants have evolved a diversity of adaptive strate-
gies for persistence under recurrent fires (Keeley
et al. 2012). Because fire often shortens genera-
tion time and reduces the overlap between gen-
erations (in non- resprouting plants), it promotes
evolutionary novelties (Pausas and Keeley 2014).
Ecologists increasingly recognize that fire may
also influence several traits in animals, though
these effects are less studied (Koltz et al. 2018;
Pausas and Parr 2018).

At the ecological scale, fires generate habitat
heterogeneity by opening gaps and creating
snags and deadwood patches; as such, fires
increase the number of potential ecological
niches, which enhances evolutionary pro-
cesses. There is plenty of evidence that under
natural conditions many species require the
open habitats that are formed and maintained
by recurrent fires and grazing, with the species-
rich savannas and prairies serving as obvious
examples. The “pyrodiversity begets diversity”
hypothesis is based on this idea (Martin and

Sapsis 1992), and many prescribed fire programs focus on spe-
cies conservation, including the conservation of some ecologi-
cal processes (eg pollination; Brown et al. 2017).

Provisioning services

Wildfires were the primary natural agent for generating and
maintaining the open spaces that were often used by early
human societies for gathering food and for hunting, and
later for farming. Wildfires still sustain open grazing areas
and create wildlife habitat, both of which benefit recreational
activities such as tourism and hunting. Anthropogenic fire
regimes, including the prescribed burning programs com-
monly used to enhance pasture quality or to facilitate hunting,
have replaced many historical regimes (Figure 2).

Wildfires maintain diversity and genetic variability, thereby
also contributing to the creation of a range of natural products
for human consumption. For instance, to be able to resprout

Figure 1. Schematic representation linking factors occurring at the evolutionary (green
square) and at the socioecological (yellow square) scale associated with fire regimes and eco-
system services. Natural (historical) wildfire regimes create open habitats that can promote
specific adaptations, biodiversity, and overall functioning in fire- prone ecosystems; these are
the supporting services necessary for the production of all other services (Table 1). Decisions
and policies may modify fire regimes (anthropogenic fire regimes) modulating ecosystem func-
tioning and services (socioecological feedback); that is, policy decisions may switch between
maintaining ecosystem services (stabilizing feedback) and generating unsustainable fire
regimes (disruption of the feedback). Decisions and policies (bottom- right corner) include fire
and landscape management decisions but also socioeconomic changes that have implications
for fire regimes (eg rural abandonment; Pausas and Fernández- Muñoz 2012).

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Wildfires as an ecosystem service CONCEPTS AND QUESTIONS 291

following a fire, fire- prone ecosystems harbor a rich flora with
a diversity of underground plant structures (Pausas et al.
2018); these species typically protect themselves from fire by
storing buds and carbohydrates belowground, which humans
use for food (Figure 3). Examples include many bulbs, tubers,
and rhizomes that were consumed by early hominids (Dominy
et al. 2008), and it has been suggested that dependence on
these widely spaced plant resources contributed to the evolu-
tion of bipedalism (Lieberman 2013). Today, highly selected
versions of these plant structures are commonly employed in
gardening and horticulture. In addition, post- fire resprouts of
several species were used extensively in the past as craft and
basketry material, and are still in use in many rural popula-
tions and indigenous societies (eg hazel [Corylus spp], willow
[Salix spp], and beargrass [Xerophyllum tenax] reprouts are
often used by tribes in Northern California).

For plants, an annual (as opposed to perennial) life cycle is
also an adaptive strategy for persisting in disturbed ecosys-
tems, and many annual food crops originated from open,
fire- prone ecosystems (Khoury et al. 2016). Moreover, open
habitats are rich in flowers, pollinators, and herbivores, which
led to the evolution of different chemical compounds to attract
pollinators and deter herbivores; chemical compounds may
have also evolved to enhance flammability (Pausas et al. 2016).
Many of these compounds are now the basis for large indus-
tries (eg perfumes, drugs). All of these services would be dras-
tically limited in a world without a long history of wildfires.

Regulating services

Regulation of pest populations

Early human societies used fire to clear campsites of pest spe-
cies. For example, the first Europeans to explore what is
now California reported that Native Americans commonly
burned campsites when flea infestations became intolerable
(Bolton 1927). Parasites are still a problem for humans and
livestock, and fire has often proven effective in reducing parasite

Table 1. Examples of ecosystem services provided by recurrent
wildfires to both early and contemporary human societies

Type Service Society type

Provisioning Provide open spaces for
pastures, agriculture, and
hunting

Early, contemporary

Stimulate germination of
desirable annual “crops”
post- fire

Early

Provide carbohydrates from
underground plant organs

Early

Provide craft and basketry
material (resprouts)

Early

Maintain open spaces for
grazing and hunting

Early, contemporary

Provide essences, medicines,
and flowers (ornamental)

Contemporary

Regulating Pest control for humans and
livestock

Early, contemporary

Reduce catastrophic wildfires Early, contemporary

Accelerate species
replacement under changing
conditions

Early, contemporary

Enhance flowering and
pollinator activity

Contemporary

Water regulation Early, contemporary

Carbon balance Early, contemporary

Cultural Spiritual Early

Ecotourism in open
ecosystems

Contemporary

Recreational hunting Contemporary

Scientific knowledge about the
origin of biodiversity

Contemporary

Information about ancestral
fire management techniques

Contemporary

Notes: “Contemporary” refers mainly to Western societies; services listed as “early”
may also be “contemporary” to some indigenous or rural societies.

Figure 2. (a) Wildfires and burns have been used to facilitate hunting for millennia; here, Nola Taylor, a member of the Martu People in Western Australia, burns
spinifex grass (Spinifex sp) to flush out small game during a hunt. (b) In forests, frequent understory wildfires reduce vertical fuel buildup and prevent the
occurrence of large, high- intensity fires that are often catastrophic to both the forest and human communities (shown here is a mixed Pinus ponderosa and
Calocedrus decurrens forest in California’s Sequoia National Park).

(a) (b)

R
Bl

ie
ge

B
ird

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G Pausas and E Keeley292 CONCEPTS AND QUESTIONS

loads. In British Columbia, Canada, for instance, lungworm
(Protostrongylus spp) loads were up to 10 times smaller in
native Stone’s sheep (Ovis dalli stonei) that had access to recently
burned areas than in sheep that did not (Seip and Bunnell
1985). There is evidence that fire suppression increases the
risk and transmission of infectious disease pathogens (eg includ-
ing those carried by organisms such as ticks, chiggers, fleas,
lice, mosquitoes, and a diversity of flies) and modifies host–
parasite dynamics in a range of systems (Scasta 2015). For
instance, burns are currently used to control the spread of
some pest- related problems, such as mosquito- borne diseases.
Scasta (2015) referenced 24 studies that document the effects
of fire, 23 of which demonstrate fire as an effective tool for
managing vegetation, parasites, and disease. Wildfires therefore
often serve as a natural control for many diseases and pests
that affect wildlife, forests, livestock, and human populations.

Regulation of catastrophic fires

In many forest types, frequent surface fires reduce the prob-
ability of large, high- intensity wildfires that can be cata-
strophic to ecosystems and human societies (Figure 2). This
issue emerged after the highly successful fire suppression

policy that was introduced in the US in the 20th century
(eg in many coniferous forests in western states), and that
contributed to dangerous increases in understory fuels.
This novel fuel accumulation is driving the conversion from
frequent, low- intensity surface fire regimes to large high-
intensity crown fires (Covington and Moore 1994; Allen
et al. 2002; Keeley and Pausas 2019). The high frequency
of natural wildfires in these forests had resulted in less
hazardous fires (Walker et al. 2018). Elimination of this
important ecosystem service increased the fire hazard and,
due to the massive increase in tree density, is likely to have
led to the extensive dieback of trees following the extreme
2012–2014 drought in the Sierra Nevada Mountains of
California (Keeley and Syphard 2016). Both the “natural
burn” policy (allowing wildfires to burn naturally and sup-
pressing them only under defined management conditions;
Boisramé et al. 2017) and prescribed fires are often designed
to reduce the frequency of catastrophic fires.

Role in ecosystem water regulation

The smaller amount of woody vegetation after a wildfire event
greatly reduces water consumption by plants and thereby

Figure 3. Fire- prone ecosystems are rich in geophytes that flower very quickly after a fire event (fire- stimulated flowering). These species are the first to
enhance pollination activity; their underground organs were also an important source of carbohydrates for early humans, and they are the ancestral spe-
cies of many common contemporary garden plants. Pictured here are geophytes native to Spain that flower quickly after a wildfire: (a) Narcissus triandrus
pallidulus, (b) Asphodelus cerasiferus, (c) Gladiolus illyricus, and (d) Iris lutescens.

(a) (b)

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Wildfires as an ecosystem service CONCEPTS AND QUESTIONS 293

increases water availability in wells and springs, which several
researchers consider to be an important reason for the tra-
ditional use of fire by Native Americans in California (Anderson
and Keeley 2018). More recently, local farmers in Chile reported
increased stream flow following large fires in anuary 2017
that mainly burned large forest plantations. This is consistent
with observations that burned forests are less susceptible to
drought mortality than unburned forests (van Mantgem et al.
2016; Boisramé et al. 2017), and that surviving trees grow
faster (Alfaro- Sánchez et al. 2016). In addition, forest clearings
created by fires often support deeper winter snowpacks, which
melt later in spring than snow in densely forested areas
(Lundquist et al. 2013); this means that water is released
more slowly in the spring and summer as opposed to rushing
down as winter floods. Wildfires can therefore help to alleviate
water shortages for humans in dry years.

Acceleration of species replacement in changing conditions

In response to a warming world, many plant species may
need to shift to sites where environmental conditions are
more suitable (eg moving poleward or uphill). However,
long- lived plants (eg trees) are unlikely to relocate quickly
enough because of the difficulty of colonizing areas that are
already occupied by other plant species (low population
turnover). By opening gaps, wildfires may help species
replacement to occur more quickly than in closed conditions,
especially in ecosystems dominated by non- resprouting (fire-
killed) species (eg Wang et al. 2019). In addition, these new
gaps for recruitment select for individuals (ie genotypes)
better suited to the new climate, thereby enhancing adap-
tation to drier conditions. We suggest that recurrently burned
ecosystems may be better positioned for tracking global
warming than non- fire- prone ecosystems, although this
hypothesis requires further testing.

Carbon regulation

Although wildfires consume organic matter and release car-
bon (C) to the atmosphere, this occurs in the short term,
whereas over the longer term C fixation can be high during
post- fire regeneration. From a broader temporal perspective,
fire emissions are balanced out by previous C sinks (Yue
et al. 2016), and under a natural (sustainable) fire regime,
long- term C balance should therefore be relatively stable.
Given that charcoal is a very recalcitrant C contributor to
the soil (ie resistant to microbial decomposition), fires could
increase C- sink capabilities by increasing soil charcoal (Santín
et al. 2015); however, the proportion of C retained in the
soil – and its longevity – likely varies as a function of
ecosystem type. Recurrent fires also enhance belowground
C allocation in resprouting plants (Pausas et al. 2018), which
is another way soil C can increase. Overall, frequent wildfires
reduce very large and high- intensity fires, and thereby reg-
ulate the C cycle by smoothing C source–sink dynamics.
However, gaining a better understanding of the conditions

and thresholds whereby changes in fire regime could switch
a system from C- source to C- sink, and to what extent pre-
scribed fires could be used to modulate this switch, will
require further research.

Pollination enhancement

In many regions, a shortage of flowers limits natural pol-
linator abundance and therefore constrains the pollination
service these species provide to crops (Goulson et al. 2015;
Winfree et al. 2018). Populations of both flowers and pol-
linators are larger in open spaces than in forests and closed
ecosystems (Campbell et al. 2007; Hanula et al. 2015), and
people often generate disturbances (eg clearing) to enhance
pollination activity (Goulson et al. 2015). In many ecosys-
tems, however, wildfire is the primary natural factor that
creates open ecosystems. There is increasing evidence that
landscape mosaics with a diversity of fire regimes and post-
fire ages (pyrodiversity) promote floral and pollinator diversity
(Ponisio et al. 2016; Brown et al. 2017; Lazarina et al. 2019),
which could enhance crop pollination (Winfree et al. 2018).
Fire can also extend the flowering period, and therefore
increase the frequency of pollination (Mola and Williams
2018). Given the current global pollination crisis, this service
provided by wildfire may become more important; using
prescribed fires to create clearings adjacent to crops may
help to boost pollination by a wide variety of pollinators.

Cultural services

By promoting biodiversity and habitat heterogeneity, wildfires
generate opportunities for recreation and ecotourism; for
instance, some savanna ecosystems are hotspots for ecotour-
ism and hunting tourism. Wildfires also provide researchers
with opportunities to study the way in which disturbance
selects for adaptations and influences biodiversity (thereby
contributing to scientific knowledge). Furthermore, many
native and traditional societies have a long experience of
living with fire (ie cultural knowledge) and may therefore
be sources of knowledge for Western societies (Fowler and
Welch 2018).

Bipedalism in humans evolved in open landscapes, and
these types of environments are still preferred by many people
and societies, who consider them to be more hospitable than
closed forests (Buss 2015). However, this preference is often
masked by more recent cultural values, at least in Western
countries (eg the 19th- century timber culture; Marsh 1865;
Pausas and Bond 2019). Despite such shifts in values, the pres-
ervation of natural open environments and their drivers may
still provide an emotional and aesthetic service.

Conclusions

Wildfires are natural phenomena, and are important for the
distribution, biodiversity, and functioning of many ecosystems

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G Pausas and E Keeley294 CONCEPTS AND QUESTIONS

worldwide (Bond et al. 2005; Keeley et al. 2012; Pausas and
Ribeiro 2017). Natural fire regimes also provide an assort-
ment of ecosystem services, although many of these remain
largely unnoticed (Table 1). However, it is also true that
many current anthropogenic fire regimes are susceptible to
catastrophic fires. This is especially relevant in ecosystems
for which fire events were extremely rare throughout their
evolutionary history (eg rainforests), and for which fire
regimes fall outside of the range of their historical variability
(Allen et al. 2002; Keeley and Pausas 2019). Fire manage-
ment could focus on shaping fire regimes that provide balance
between ecosystem services and natural resources protection,
given that a variety of tools are available for fire manage-
ment (eg prescribed fires, wildfire management, fire sup-
pression, fuel treatments; Stephens et al. 2013; Boisramé
et al. 2017) and that each one may be appropriate for dif-
ferent settings. Actively shaping fire regimes is becoming
even more necessary as the planet becomes warmer and
drier. However, in many fire- prone ecosystems, landscape
development patterns put human communities at risk, and
thus, even ecologically sustainable fire regimes can be socially
unsustainable.

Fire regimes vary around the world, and the full range of
services provided by fire within the diversity of global ecosys-
tems remains to be quantified. Some services are well sup-
ported (eg regulation of fire intensity, pest control) but others
require formal validation or quantification; for instance, can
we design experiments to evaluate the role of fire in accelerat-
ing species replacement in a changing climate? To what extent
have fire suppression policies increased the presence of pests?
And can wildfires be managed to optimize ecosystem services?
Addressing questions like these would greatly improve our
understanding of the ecosystem services provided by wildfires.

Acknowledgements

This work was performed under the framework of the Spanish
Government’s FILAS project (CGL2015- 64086- P) and the
PROMETEO/2016/021 project of the Generalitat Valenciana.
We thank M Kane and T Kimball for comments on the
manuscript.

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