\chapter{Literature Review}\label{ch:lit_rev}

\begin{refsection}

\raggedbottom


In this chapter, I start by introducing the Brazilian Cerrado, a brief introduction to its recent history, and its ecological, climatic and landscape characteristics, providing the necessary information and context to support this thesis. Then, in the next three sections, I elaborate on the state of knowledge and research gaps that have been addressed in each of the three research papers of this thesis (\textbf{Chapter \ref{ch:paper01} (Research Paper 1)}, \textbf{Chapter \ref{ch:paper02} (Research Paper 2)} and \textbf{Chapter \ref{ch:paper03} (Research Paper 3)}). In each of these three sections, I introduce the subject addressed and the necessary concepts with a general perspective and then focus on the Brazilian Cerrado, detailing the scholarship and unresolved questions. Finally, I give a brief overview of the main methodologies and sources of data used in this thesis.

\section{The Brazilian Cerrado}

Cerrado is the name given to a Neotropical region comprising a wealth of different ecosystems, often referred to as a tropical savanna because this is the dominant vegetation type \citep{gottsberger_life_2006}. Most of this region of savanna-like ecosystems is found within Brazil (Figure \ref{fig:cerrado_daSilva}), which constitutes the study area of this thesis, although a small extension is also found within Bolivia and Paraguay \citep{ribeiro_cerrado_2008}. 

The Brazilian Cerrado is a complex and heterogeneous region at many different levels, its vegetation composition and rich biodiversity, its topography and soils, its patterns of land use area, its patterns of fire, and the varied human communities and economic activities - indigenous groups, traditional pastoral or \textit{quilombola} communities, small and extensive agricultural properties, and some large urban centres. In addition, the Cerrado encompasses the headwaters of three major South American river basins as well as large aquifers, providing important hydrological resources to this continent \citep{oliveira_trends_2014}. The geographical patterns of the vegetation, landscape and environmental characteristics of the Cerrado have been studied and classified into nineteen ecoregions \citep{sano_cerrado_2019} (Figure \ref{fig:cerrado_daSilva}).

The official geographical limits of the Cerrado have varied through time and within the literature \citep{vieira_reviewing_2022}. In this thesis, we use the official geographical limits defined by the Instituto Brasileiro de Geografia e Estatística (IBGE, Brazil's Geography and Statistics Institute) in 2019 \citep{biomas_sistema_2019}, as this is the most recent official version (Figure \ref{fig:cerrado_daSilva}). 

The Brazilian Cerrado has a marked seasonal climate that shapes the phenological cycle of this region \citep{oliveras_effects_2013}, which is dominated by C$_{4}$ grasses: during the wet season, rainfall enables vegetation growth; during the dry season, vegetation is in general subject to hydric stress, and perennial plants dry out while some tree species shed their leaves \citep{oliveras_effects_2013}. As a consequence, particularly in the more open forms of Cerrado vegetation, there is a general accumulation of dry dead biomass - or alive but with little moisture content - over the dry season that can easily burn \citep{simon_recent_2009}. In turn, fire favours the establishment of grasses and shrubs against the proliferation of trees and fire-sensitive species \citep{mistry_fire_1998}. These synergies between fire and certain vegetation types that allow to maintain the open ecosystems of the Cerrado are mediated by the fact that most vegetation species endemic to the Cerrado are adapted to fire \citep{simon_recent_2009, gottsberger_life_2006}.

It has been estimated that the majority of Cerrado vegetation lineages diversified less than 4 million (M) years before present (yrs BP) \citep{simon_recent_2009}. This indicates that Cerrado vegetation started evolving and expanding around this time, coinciding with the global expansion of the savanna biome dominated by C$_{4}$ grasses \citep{simon_recent_2009}. Further, phylogenetic studies indicate that endemic Cerrado species seem to have evolved in situ through rapid and frequent adaptive shifts to resist fire, evolving from nearby fire-free ecosystems' species and lineages \citep{simon_recent_2009, simon_evidence_2012}. Indeed, paleo-pollen and charcoal records evidence the presence of fire in Cerrado from 32,000 yrs BP \citep{salgado1997late} mediated by climatic fluctuations and the corresponding shifts in vegetation \citep{cassino2023millennial}, as well as by the presence of large herbivores \citep{dantas2020megafauna, doughty2016impact}. Historically, fire was ignited by lightning and, later on, as human populations proliferated in South America, it was also ignited by humans \citep{cassino2023millennial}.




\begin{figure*}[t!]
\centering
\includegraphics[width=\textwidth]{_literature_review/daSilva_2024.PNG}
\caption{The location and limits of the Cerrado used in this thesis, defined by the Instituto Brasileiro de Geografia e Estatística (IBGE, Brazil's Geography and Statistics Institute) in 2019. Colours represent the different land use and land cover categories identified by the MapBiomas project (Collection 8.0 \citep{souza_reconstructing_2020}). Black lines represent the different ecoregions of the Cerrado \citep{sano_cerrado_2019}, identified with numbers: 1, Alto Parnaíba; 2, Alto São Francisco; 3, Araguaia Tocantins; 4, Bananal; 5, Basaltos do Paraná; 6, Bico do Papagaio; 7, Centro-norte Piauiense; 8, Chapada dos Parecis; 9, Chapadão do São Francisco; 10, Complexo Bodoquena; 11, Costeiro; 12, Depressão Cárstica do São Francisco; 13, Depressão Cuiabana; 14, Floresta de Cocais; 15, Jequitinhonha; 16, Paracatu; 17, Paraná Guimarães; 18, Parnaguá; 19, Planalto Central; 20, Vão do Paranã. Source: \citet{da_silva_arruda_assessing_2024}.} 
\label{fig:cerrado_daSilva}
\end{figure*}


\subsection{Brief overview of the evolution of the Cerrado in the Holocene (since circa 12,000 yrs BP)}

Over 10,000 yrs BP, the extent of savannas and grasslands in eastern South America was much larger than present-day Cerrado. This larger extension is confirmed by evidence in the paleological records indicating the presence of savannas and grasslands in areas that are currently dominated by forests \citep{dantas2020megafauna}, and by the presence of pockets of savannas and grasslands within currently forested areas \citep{dantas2020megafauna}. This larger extension is also inferred from the evidence in the fossil record showing the presence of megafauna (mammals of very large dimensions, generally over 50 Kg \citep{dantas2020megafauna}) across South America \citep{dantas2020megafauna, doughty2016impact}. 

Megafauna, and particularly mega-herbivores, have a big impact on ecosystems as they consume biomass and shape vegetation structure \citep{dantas2020megafauna}. These large mammals can maintain the open structure of grassy ecosystems in regions that could otherwise sustain forests, playing a similar role to fire as biomass consumers \citep{bond_fire_2005}. Around 12,000-10,000 yrs BP, however, megafauna was in the process of extinction across South America, with archaeological evidence pointing at humans as a major cause \citep{prates2021late}. Humans have been present in South America since at least 15,000 yrs BP \citep{vialou2017peopling}, and population densities in central Brazil show a peak between 12,700 yrs BP and 8,800 yrs BP \citep{araujo2025human, prates2021late}. Some authors relate this peak in human population precisely to their hunting of megafauna, whose extinction would have also prompted the decrease in human population \citep{prates2021late}.

Importantly, before their extinction, the densities of megafauna in present-day central Brazil has been estimated to have been larger than in Africa \citep{doughty2016impact}. Different authors argue that, consequently, such drastic extinctions would have prompted profound transformations on the vegetation of the herbivory-maintained savannas \citep{dantas2020megafauna, doughty2016impact, johnson2009ecological}. Indeed, fossil records indicate increases in arboreal vegetation in many sites in South America following megafauna extinction \citep{behling2003late}. On the other hand, rather immediate surges in fire activity could have also followed megafauna extinction, as fire occurrence would have no longer been limited by megafauna vegetation consumption \citep{johnson2009ecological, dantas2020megafauna}. Indeed, some studies have found evidence of no fire activity during the Late Pleistocene, with the first records of biomass burning being observed during the glacial-interglacial transition from 11,000 to 10,500 yrs BP \citep{escobar2024long}. 

Still, changes in the climate could provide an alternative explanation to the observed shifts in vegetation composition. Indeed, a wetter period took place in South America at the end of the Pleistocene (during the Younger Dryas, 12,900 to 11,700 yrs BP) \citep{escobar2024long}. However, while this episode possibly contributed to vegetation changes towards more forested types, several authors insist that paleoclimate alone is unlikely to explain the replacement of most of these savannas by forests \citep{arruda2018vegetation, dantas2020megafauna}, and that megafauna extinction played a relevant role. 

Throughout the Holocene, there were several episodes of rapid climate change \citep{araujo2025human}. As a general trend, palynological studies support the hypothesis that there was a peak in moisture around 8,000 yrs BP, followed by dryness in the mid-Holocene until about 6,000-5,000 yrs BP, and a return of moisture in the Late Holocene \citep{araujo2006human}. However, these patterns of climate change seem to have varied across different regions in central Cerrado showing, in general, numerous fluctuations between wet and dry episodes \citep{araujo2025human}. Overall, vegetation seems to have responded to the changes in rainfall and seasonality \citep{behling2003late, cassino2020vegetation, salgado1997late, cassino2023millennial}, with Cerrado vegetation undergoing several episodes of expansion and retreat. Across different sites, vegetation became more arboreal in periods when rainfall increased, showing decreased or absent fire activity; while vegetation returned to more open grasslands during drier episodes, with increased fire activity as recorded by charcoal influx in the paleological records \citep{cassino2023millennial, escobar2024long, cassino2020vegetation}.

Overall, in the early Holocene until about 8,500 yrs BP, the landscape seems to have been dominated by savannas and shrub woodlands \citep{behling2003late}. Over time, and following the aforementioned climatic fluctuations, grasslands and savannas gave way to tropical rainforest in the Amazonia and southeastern Atlantic region, as they became wetter \citep{behling2003late}; while they evolved into tropical dry forests in the transition area with present-day Caatinga in the east and northeast \citep{mota2019landscape}.

Between 8,000 and 2,000 yrs BP, a period sometimes referred to as the "Archaic Gap", human population density was generally low and unstable, appearing and disappearing from different areas \citep{araujo2006human}. Some authors argue that these fluctuations in human population were related to the rapid changes in the climate, rather than to the dry episodes \textit{per se} \citep{araujo2025human}. During this period, fire activity possibly had a natural cause and varied with the climate \citep{behling2003late}.

Human population started to increase steadily about 2,300 yrs BP, showing a major peak between 1,400 and 1,200 yrs BP \citep{araujo2025human}, when it started conforming the modern indigenous population - like the Xavante and Xerente societies that occupied extensive areas of Cerrado \citep{klink2002past}. Anthropological studies on indigenous groups show an active use of fire throughout the dry season for different purposes (see Section \ref{sec:anthropic_use_fire} for a more detailed explanation). Interestingly, studies in present-day Minas Gerais report a high fire activity period around 1,600 yrs BP burning arboreal vegetation \citep{cassino2023millennial} that was not coincident with a dry event, while it did coincide with the arrival of ceramists-horticulturalists \citep{araujo2025human}. Therefore, these late peaks in fire activity were most likely related to humans. 

Indigenous population subsequently plummeted with the arrival of European colonisers \citep{araujo2025human}. Furthermore, colonisers established bans to the use of fire to, among other reasons, ensure wood supply for shipbuilding \citep{pivello_understanding_2021}. More recently, the modern occupation of the Cerrado started from the south, in the area of São Paulo \citep{klink2002past}.

Thus, present-day Cerrado and surrounding areas have dynamically changed over the Holocene, evolving in response to climate, megafauna extinction and, more recently, to human expansion across the Cerrado. In particular, authors argue that megafauna extinction probably has had long-lasting legacy effects, and that Cerrado may still be experiencing changes in relation to it \citep{dantas2020megafauna, doughty2016impact}. As such, current Cerrado is the result of processes of change whose consequences are probably still ongoing.

Cerrado vegetation has responded to these changes or disturbances by adapting to new climatic conditions and to changes in herbivory consumption and in fire regimes \citep{araujo2025human}. Furthermore, on the long run, such dynamism has probably contributed to the high levels of biodiversity and endemism of the Cerrado, and to its extraordinary heterogeneity. Still, overall, these perturbations and the consequent vegetation responses took place on scales of hundred to thousands of years. Furthermore, however drastic, none of this changes caused the immediate conversion of large tracts of Cerrado vegetation - as is the case of the recent anthropic expansion over the Cerrado that we will discuss in the next section.


\subsection{Recent patterns of land use expansion}

One of the striking characteristics of the current Cerrado is the large-scale level of land use conversion taken place over the past six decades \citep{sano_land_2010, alencar_mapping_2020, jepson_access_2010, grecchi_assessing_2013, espirito-santo_understanding_2016}. Moreover, the rates of Cerrado conversion have often been faster than those of deforestation in the Amazon rainforest \citep{machado_estimativa_2016, soterroni_expanding_2019, machado_why_2024}. However, despite its ecological importance, Cerrado conversion has attracted much less attention in the media and this region is less protected than the Amazon rainforest \citep{soterroni_expanding_2019}. As a consequence, the patterns of human land uses are an important aspect of the Cerrado, profoundly affecting its ecology, hydrology, and fire regimes \citep{da_silva_arruda_assessing_2024, silva_putting_2021}.

In the 1960s, the capital city of Brazil was established in Brasília, a newly-constructed city in the interior of the Cerrado \citep{soares-filho_cracking_2014}. This event, accompanied by economic development policies from the Brazilian government, triggered the expansion of agricultural lands and human settlements in the Cerrado \citep{soares-filho_cracking_2014}. Given its proximity to the two major urban centres of Brazil, São Paulo and Rio de Janeiro, this conversion to agriculture started in the south, rapidly advancing towards the centre and, later, the northern areas of the Cerrado \citep{gottsberger_life_2006}. At first, Cerrado vegetation was primarily converted to pastures for cattle ranching. In the 1990s and 2000s, however, Cerrado was increasingly converted to commercial croplands, primarily soybean, sugarcane, corn and cotton \citep{rekow_socio-ecological_2019}. In addition, since the 2000s, most of the Cerrado conversion took place in the Matopiba region, an area in the north of the Cerrado comprising parts of the Brazilian states of Maranhão, Tocantins, Piauí and Bahía \citep{machado_why_2024}. The Matopiba is where Cerrado conversion is currently most active \citep{machado_why_2024}, in part because most of the southern areas of the Cerrado have already been converted, especially the regions that are not formally protected.

Apart from the development policies, the low protection levels of the Cerrado also facilitated this rapid large-scale agricultural expansion. For example, Brazil's Forest Code requires only 20\% of the native vegetation in a private land in the Cerrado to be preserved, while in the Amazon this requirement is of the 80\% \citep{klink_conservation_2005}. Further, in 2006 a Soy Moratorium - an agreement prohibiting the commercialisation of soy grown in recently deforested land \citep{soterroni_expanding_2019} - was approved for the Amazon, but it has not yet been extended to the Cerrado \citep{soterroni_expanding_2019}. Currently, only about 3\% of the Cerrado is under strict protection, with a further 4\% of its area classified as indigenous lands and 5\% classified as areas for sustainable use \citep{carranza_protected_2014}. 

This rapid large-scale conversion has flagged the Cerrado as an urgent area for conservation, as it threatens its high levels of biodiversity and endemism \citep{myers_biodiversity_2000}. Further, studies warn about the wider effects of large-scale conversion on the hydrological cycles of the Cerrado \citep{spera_land-use_2016}, as well as on its large quantities of carbon stored in above- and below-ground biomass and in the soil \citep{bustamante_estimating_2012}.

Crucially, the speed and large scale of these land use transformations, together with the speed of current anthropic climate change, are in striking contrast with the previous disturbances that affected the Cerrado during the Holocene. 



\subsection{Current climate}

The Cerrado has a tropical climate classified as Aw by the Köppen climate classification \citep{nascimento_clima_2020}. The most relevant climatic characteristic is its strong seasonality, with a wet season from October to April (austral summer), and a dry season from May to September (austral winter). This seasonality is influenced by a confluence of air masses, primarily an Equatorial air mass coming from the Amazon region, generated by the rainforest evapotranspiration and water bodies \citep{nascimento_clima_2020}, that causes the summer's rains; and a tropical air mass from the Atlantic that brings hot and dry air after traversing various topographical barriers, associated with the winter's dry season. As a consequence, the Cerrado presents a longitudinal gradient of precipitation increasing from east to west, varying between 800 mm and over 1,800 mm on average per year \citep{nascimento_clima_2020, da_silva_arruda_assessing_2024}. The annual average temperatures of the Cerrado vary between 18$^\text{o}$C and 28$^\text{o}$C. Even though there is no strong temperature variation between the wet and dry seasons, the latter presents higher maximum temperatures on average in relation to the lower cloud formation and higher insolation during this period. In addition, Cerrado temperatures also show a latitudinal gradient increasing from the south towards the Equator in relation to the angle of solar radiation \citep{nascimento_clima_2020}.

Over the past few decades, the temperatures of the Cerrado have been increasing in relation to anthropic climate change \citep{hofmann_brazilian_2021}, causing decreases in relative humidity and increases in vapour pressure deficit (VPD) \citep{hofmann_brazilian_2021} - a measure of the drying capacity of the air -, along with generalised increases in evapotranspiration \citep{oliveira_trends_2014}. Using data from climate re-analysis models, some studies report decreases in the dry season rainfall since the 1960s, particularly at the beginning of the dry season, in relation to changes in atmospheric circulation \citep{hofmann_changes_2023}. Therefore, the evidence indicates that the climate of the Cerrado is getting warmer and drier with climate change \citep{hofmann_brazilian_2021}, which expected to increase, in turn, the risk of fire occurrence under all Representative Concentration Pathways (RCP) of the International Panel for Climate Change (IPCC) \citep{silva_impacts_2019}.

 
\subsection{Current vegetation}

\begin{figure*}[t!]
\centering
\includegraphics[width=\textwidth]{_literature_review/physiognomies_fromBook_adapted02.png}
\caption{The different vegetation physiognomies of the Cerrado. Adapted from \citet{ferreira_stochastic_2023}, in turn adapted from \citet{ribeiro_cerrado_2008}.} 
\label{fig:physiognomies}
\end{figure*}

The varied vegetation of the Cerrado comprises grasslands, savannas and forest formations \citep{ribeiro_cerrado_2008}. Within each of these groups, there are in turn different types of vegetation physiognomies (Figure \ref{fig:physiognomies}) presenting different structure, vegetation growth-forms, species composition, and other environmental factors like the soil composition, availability of water and nutrients, the occurrence of fires, or climatic characteristics \citep{ribeiro_cerrado_2008, oliveira-filho_vegetation_2002, henriques_patterns_2002}. Even though the dominating vegetation form is that of a savanna with a grass layer and a varied but generally scarce tree cover (savanna \textit{censu lato}) \citep{oliveira-filho_vegetation_2002}, Cerrado vegetation usually varies continuously in the scale of meters to kilometers between the different physiognomies. As a consequence, a typical Cerrado landscape is a matrix or mosaic of different vegetation types \citep{de_l_dantas_fire_2013}. 

Despite the predominance of savanna ecosystems across the Cerrado, we can distinguish certain areas with distinct dominant vegetation formations \citep{sano_land_2010} (see Figure \ref{fig:cerrado_daSilva}). The northern parts of the Cerrado, which in the past have sometimes been excluded from its official geographical limits \citep{vieira_reviewing_2022}, are mostly dominated by forest formations, comprising both rainforest and dry forests \citep{sano_land_2010}. In the centre-west of the Cerrado, there is a seasonally-flooded region of savanna ecosystems \citep{marimon_floristica_2012}, and there are patches dominated by grasslands in the eastern areas of the state of Tocantins \citep{alencar_mapping_2020}. Finally, along the border with the Amazon there is generally an ecotonal gradient of vegetation, especially in the western-most part of the Cerrado \citep{morandi_tree_2020, marques_redefining_2020}.
 
This characteristic mosaic of generally open ecosystems is the result of a confluence of factors, including the seasonal climate, soil fertility, fire regime and the climatic and natural history of the Cerrado \citep{oliveira-filho_vegetation_2002}. Even though, the Cerrado's climate is considered to be moist enough to sustain forest formations despite the seasonal precipitation and generally low soil fertility \citep{franco_10_2002}, the periodic occurrence of fire has been signaled as a determining factor contributing to creating and maintaining the mosaic \citep{de_l_dantas_fire_2013}. As we have seen, fire acted as an evolutionary force for Cerrado vegetation prompting species from nearby biodiverse fire-free ecosystems to develop varied adaptation strategies to this disturbance agent \citep{simon_evidence_2012} - including thick barks, underground organs and deep roots, fast resprouting capacity, or fire-stimulated flowering and germination \citep{miranda_fire_2002}. This process facilitated the expansion of the Cerrado and is also argued to explain its rich biodiversity \citep{simon_evidence_2012}, as well as the small patches of fire-sensitive species that remain scattered across this region. 

\subsubsection{Vegetation adaptations to fire}

In the Cerrado, different types of vegetation present different adaptations to fire that protect them from this disturbance or allow them to take advantage of this phenomenon \citep{pivello_understanding_2021}. The soil generally acts as an insulation layer against the temperature increase caused by the passage of fire \citep{billings_fire_1990, pivello_understanding_2021}, and vegetation has developed fire adaptation strategies that take advantage of this fact. Soil heating caused by fires depends on the intensity of the fire, as well as on the soil's own moisture content \citep{mistry_fire_1998}. In general, the greater the fuel load, the higher the temperatures that can be attained by the fire and the longer the residence time of the fire, as well as the larger the amount of ashes deposited on the surface, which can cause larger rises in soil temperature \citep{billings_fire_1990}. However, since savanna and grassland fires are usually fast and low intensity, the increase in soil temperature generally affects the most superficial layers \citep{mistry_fire_1998}.  

The species most resistant to fire are found among the herbaceous and undershrub flora \citep{billings_fire_1990}, which present a wide variety of adaptations to this disturbance agent. For example, some species with annual life cycles avoid fire because they are in the form of seeds during the dry season and grow during the wet season \citep{billings_fire_1990}. On the other hand, many perennial species present different underground structures - where they are protected from fire - such as bulbs, underground shoots, rhizomes and xylopodia that allow plants to survive fire \citep{billings_ecological_1982}. In particular, xylopodia are thick woody organs that store nutrients and can present dormant buds close to the soil surface from which plants can resprout \citep{billings_ecological_1982} - which they can do just a few hours to days after a fire. Other woody species in the herbaceous layer protect themselves against fire by developing dense underground systems of trunks and branches, while presenting only a few branches and sprouts aboveground \citep{mistry_fire_1998}. 

Fire triggers flowering in many herbaceous and undershrub flora species, which can take place only a few days to weeks after the burn \citep{oliveira1996influencia}. This strategy allows flowers, fruits, seeds and seedlings to escape fires and facilitates cross-pollination as a result of plants flowering at the same time \citep{billings_fire_1990}. Finally, fire can also promote the germination of certain species for which the high temperatures crack the seeds' coats, allowing the later penetration of water into the seed \citep{billings_fire_1990}.

Regarding Cerrado woody plants, fire rarely kills established individuals \citep{mistry_fire_1998}. Most adult individuals of Cerrado tree species present thick barks in the trunk and branches that isolate the internal tissues protecting them from the fires' high temperatures \citep{medeiros2008post}. Furthermore, these thick barks also protect buds, which allow the plant to resprout from branches \citep{medeiros2008post, mistry_fire_1998}. This process results in a symposial growth of the branches that gives rise to the characteristic tortuosity of Cerrado trees \citep{mistry_fire_1998}. 



\subsection{Fire}

In the savanna-like ecosystems of the Cerrado, fires are usually low intensity and fast spreading. Such fires generally consume the surface layer of the vegetation \citep{miranda_fire_2002}, formed of grasses and the non-woody part of shrubs, and do not necessarily affect the tree canopy \citep{mistry_fire_1998}. Still, fire behaviour and vegetation consumption depend on the type of vegetation physiognomy \citep{miranda_fire_2002} and weather conditions \citep{mistry_fire_1998}. 

Crucially, vegetation physiognomy affects fire behaviour because the structure of the vegetation and its composition determine the abundance and distribution of the fuel, and influence its moisture content \citep{kauffman1994relationships}. Often, denser forms of Cerrado savannas where grasses and shrubs connect with the tree canopy can experience intense fires because the different vertical layers of fuel are connected. On the other hand, patches of closed tree canopy seldom experience fire due to the absence of fuel on the surface layer and a generally moister understorey \citep{miranda_fire_2002}. Further, especially in the more open physiognomies, time since last burn also influences fire behaviour due to fuel accumulation during fire free periods \citep{fidelis_year_2018}. Generally, fires will burn with more intensity after long periods without fire than when they occur regularly \citep{miranda1993efeitos}.

Regarding weather conditions, the timing of fires during the dry season also affects fire behaviour. At the beginning of the dry season, vegetation still has high moisture contents, so fires tend to be patchy and have low intensity \citep{miranda1993efeitos}. As the dry season progresses and the fuel dries out, fires have higher intensity and are more homogeneous, peaking in the late dry season \citep{billings_fire_1990}.


\subsubsection{Ecological effects of fire regimes}

The ecological effects of fires on Cerrado vegetation depend on fire behaviour \citep{henriques_patterns_2002}, the timing of fires during the growing cycle \citep{henriques_patterns_2002} and, particularly, vegetation adaptations to fire \citep{hoffmann_role_2002}. Other factors such as subsequent rainfall \citep{henriques_patterns_2002, konduri_tracking_2023} and herbivory pressure \citep{henriques_patterns_2002} also influence the fires' ecological effects. 

Fire plays an important role on nutrient cycling in the Cerrado \citep{billings_fire_1990, oliveras_effects_2013}, where soils are generally poor and acidic \citep{mistry_fire_1998}. Through ash deposition, fires result in a temporary increase in nutrients on the soil surface layers that accelerates nutrient cycling by causing a shift of nutrient allocation from trees and shrubs to the herbaceous vegetation \citep{billings_fire_1990, oliveras_effects_2013}. However, studies have also shown that a large amount of nutrients are lost in the form of smoke during fires \citep{billings_fire_1990}.

Several studies experimenting with fire exclusion or with increased fire frequencies have demonstrated that the behaviour and frequency of fire can shape vegetation structure and composition favouring certain types of vegetation \citep{pivello_understanding_2021, pinheiro2009dinamica, klink2002past}. On the one hand, periodic fires favour the establishment of grasses and herbs against trees \citep{medeiros2008post}. While adult tree individuals are more prepared to survive fire, seedlings, saplings and younger individuals are vulnerable to fires \citep{medeiros2008post, hoffmann1996effects}. Thus, fire can prevent tree establishment \citep{medeiros2008post} or hamper tree growth preventing saplings from developing completely, which has been referred to as the "fire trap" \citep{stevens_savanna_2017}. However, high fire frequencies can eventually affect the adult specimens too as nutrients can become insufficient after their repeated requirement for tree recovery, resulting in less new sprouts and tree death \citep{medeiros2008post}. In this sense, it has been demonstrated that frequent fires diminish the diversity of Cerrado woody species \citep{mistry_fire_1998}.

On the other hand, suppression of fires for long periods (e.g. a few decades) favours the establishment of trees leading to forested ecosystems \citep{pivello_understanding_2021} - a process termed as woody encroachment \citep{rosan_extensive_2019}. As trees grow and the canopy becomes more closed, light-demanding grasses and herbs start to disappear \citep{pivello_understanding_2021}. As these vegetation types are the most abundant in Cerrado ecosystems, their disappearance leads to acute losses in biodiversity, even if there are increases in diversity among tree species \citep{abreu_biodiversity_2017}. Furthermore, studies found that these fire-free periods can also have negative effects on Cerrado tree specialists: even if established individuals may survive, their seedlings have been found to be almost absent in these encroached forests, which will eventually lead to their disappearance from these sites \citep{abreu_biodiversity_2017}. Furthermore, in the process of encroachment, fuel buildup can lead to larger and more intense fires when these disturbances occur \citep{fidelis_year_2018}. These more severe fires can have negative consequences on fire-adapted species if their protective adaptations (e.g. thick bark, protected buds) become insufficient.

Therefore, both very frequent fires and the complete absence of fires bring about profound changes to Cerrado ecosystems and imply biodiversity losses. Currently, such extreme fire frequencies, both low and high, are widely observed across the Cerrado \citep{pivello_use_2011, pivello_understanding_2021} in association with modern human activities and fire management policies \citep{pivello_use_2011}, as we will see in more detail in the next section. As these fire frequencies are associated with modern human activities and Cerrado species do not seem to be well adapted to them, such patterns of fire are considered to be alterations with respect to those fire regimes to which Cerrado vegetation would be typically adapted \citep{pivello_understanding_2021}. Furthermore, if these changes happen at large scales, they may lead to the loss of the vegetation mosaic that characterises the Cerrado and that enhances its biodiversity \citep{pivello_understanding_2021}.

It is for these reasons that Cerrado ecologists and conservationists insist on the importance of regulating with policies the use of fire in the Cerrado, as well as on the need to implement conservation plans that do not simply suppress all fires \citep{mistry_community_2016, mistry_new_2019, durigan_zero-fire_2020}. Overall, Cerrado ecologists recognise that fire management is a complex task that still has many unresolved questions \citep{pivello_understanding_2021}. Further, fire management decisions and action plans will inevitably target certain goals (be it enhancing biodiversity or certain ecosystem services) and, consequently, may have trade-off effects \citep{pivello_understanding_2021}. In any case, currently, there is agreement that fire prevention is definitely not a conservation goal in fire-adapted ecosystems \citep{mistry_community_2016, mistry_new_2019, durigan_zero-fire_2020, klink_conservation_2005, pivello_understanding_2021}. Rather, as some Cerrado ecologists put it, the question now is, with Cerrado conservation in mind, whether fires should be allowed to occur only if caused by lightning, or if a certain degree of prescribed fires should be used \citep{pivello_understanding_2021}.


\subsubsection{Anthropic uses of fire}\label{sec:anthropic_use_fire}

Even though lightning is a common cause of fire in savanna-like ecosystems of Brazil \citep{menezes_lightning_2022, schumacher_characteristics_2022}, most fires in the Cerrado occur in relation to humans \citep{pivello_understanding_2021}. The range of different human communities and their diverse uses of fire deserves a brief introduction that will support the interpretation of the patterns of fire observed across the Cerrado. 

Indigenous peoples in the Cerrado use fire for a variety of purposes including hunting, stimulating vegetation regeneration and fruit production, clearing areas for planting or as a protection against more intense fires \citep{mistry_indigenous_2005}. In these practices, fires are set throughout the dry season \citep{melo_regime_2011}, depending on the purpose \citep{mistry_community_2016}. These uses of fires are planned by the community and based on in-depth knowledge about the fire ecology of the ecosystems where these communities live \citep{mistry_indigenous_2005, mistry_new_2019}, transmitted orally through generations \citep{mistry_indigenous_2005, melo_regime_2011, pivello_use_2011}. Similarly, although less well studied, fire is also used by non-indigenous traditional communities for pastoral management \citep{eloy_seasonal_2019}. Among these, the \textit{quilombola} communities stand out, formed by descendants of the African diaspora \citep{eloy_seasonal_2019}. These communities also use fire at different times during the dry season, primarily for vegetation regrowth in pastures. Therefore, both traditional uses of fire by indigenous people and by traditional pastoral communities create a mosaic of patches with different fire histories \citep{mistry_community_2016, eloy_seasonal_2019} argued to help maintain the heterogeneous vegetation of the Cerrado \citep{welch_indigenous_2013}.

On the other hand, nowadays, most fires are assumed to be started in relation to modern industrial agricultural activities, either for cattle-ranching to renew pastures, in shifting cultivation or to burn residues \citep{pivello_use_2011}. Often these activities cause large numbers of ignitions and generate high fire frequencies \citep{pivello_use_2011}. In addition, most of these fires are ignited in the late dry season \citep{pivello_understanding_2021, mataveli_satellite_2018}. However, modern agricultural activities can also decrease fire frequency by actively extinguishing fires or introducing fuel discontinuities in highly fragmented landscapes \citep{archibald_managing_2016}. Finally, even though fire is often used in the conversion of native rainforests across Brazil \citep{brando_gathering_2020}, and it has often been assumed to be used in the conversion of Cerrado native vegetation \citep{pivello_use_2011}, recent studies show a rather limited use of fire in Cerrado vegetation conversion \citep{ribeiro_time_2024}. The use of fire seems to be particularly limited in the conversion of savanna and grasslands, either to croplands and pastures, while its use seems more common in the conversion of forest vegetation growth-forms within the Cerrado \citep{ribeiro_time_2024}.

Until very recently, the policies governing the use of fire across Brazil were those of "zero fire" \citep{durigan_zero-fire_2020, pivello_understanding_2021}, irrespective of the type of ecosystem. These policies have been common since colonial times \citep{pivello_understanding_2021}, further reinforced by the use of fire in rainforest deforestation and misconceptions about the role of this disturbance agent in savannas and grasslands \citep{durigan_zero-fire_2020}. However, as the study of Cerrado ecology and tropical savannas advanced throughout the second half of the nineteenth century, the ecological importance of fire began to be understood \citep{durigan_need_2016}. In particular, experiments that completely restricted fire occurrence showed negative impacts on Cerrado vegetation, such as losses in biodiversity and the heterogeneous mosaic of the Cerrado \citep{durigan_need_2016}. In addition, these changes in the structure of the vegetation led to fuel accumulation and increased risk of larger and more intense fires \citep{fidelis_year_2018}. Simultaneously, other studies started to explore the impacts of prescribed fire regimes, observing positive effects on these ecosystems \citep{miranda_efeitos_2010}. Over the past couple of decades, there has been an increasing number of projects raising awareness about the ecological role of fire, practicing prescribed burns that build on traditional knowledge, and preventing irregular burning \citep{schmidt_fire_2018, mistry_community_2016, mistry_new_2019, eloy_seasonal_2019}. In light of these initiatives and the increasing recognition of traditional uses of fire \citep{mistry_new_2019}, since the 2000s there has been a push to (re)introduce the use of fire to manage Cerrado protected areas for conservation. These practices are usually grouped under the term Integrated Fire Management (IFM), which was eventually included in the 2012 Brazilian New Forest Code \citep{pivello_understanding_2021}. Finally, very recently, a new Brazilian National Policy on IFM (Lei 14,944, 31$\text{st}$ July 2024) has been adopted that considers the use of fires by local communities, promotes the use of prescribed burns for conservation and regulates the use of fire for agricultural practices \citep{schmidt_fire_2020}.


\section{On humans and the climate as main drivers of the burned area of tropical savannas}

The growing availability of remote sensing data products with increasingly longer times series, especially from the 2000s, has allowed the characterisation of global patterns of burned area and their changes in time \citep{van_der_werf_climate_2008}. Since then, many studies have noted that most of the world's burned area is concentrated in regions with intermediate levels of precipitation and productivity, and with a long enough dry season \citep{van_der_werf_climate_2008, archibald_what_2009, aldersley_global_2011}, particularly in tropical and subtropical savanna ecosystems \citep{van_der_werf_climate_2008, archibald_what_2009, andela_human-driven_2017}. However, studies also noted that global burned area decreased in the past few decades despite climate change, driven precisely by declines in tropical and subtropical savannas' burned area (Figure \ref{fig:andela_2017}) \citep{andela_human-driven_2017, wu_reduced_2022, jones_global_2022}.

\begin{figure*}[t!]
\centering
\includegraphics[width=\textwidth]{_literature_review/andela_2017.PNG}
\caption{\textbf{a} Average annual burned area and, \textbf{b} trends in burned area between 1998 and 2015. Maps showing the highest levels of burned area in tropical savannas and grasslands, as well as decreases in burned area in some of these savannas (e.g. in the African northern hemisphere). Maps derived using data from the Global Fire Emissions Database version 4 \citep{van_der_werf_global_2017}. Source: \citet{andela_human-driven_2017}.} 
\label{fig:andela_2017}
\end{figure*}

Because the climate is a major driver of fire, these apparently contradicting processes have been an area of active research. Studies exploring climatic effects found that relations between rainfall or seasonality and burned area were often poor in tropical and subtropical savannas, pointing at the relevance of human factors acting as fire controls \citep{van_der_werf_climate_2008, archibald_what_2009}. Among these factors, landscape fragmentation as a consequence of land use expansion was identified as a major driver of declines in savanna burned areas \citep{archibald_southern_2010, archibald_climate_2010, aldersley_global_2011, andela_recent_2014, archibald_managing_2016, andela_human-driven_2017, haas_global_2022}, primarily in relation to the introduction of fuel discontinuities, along with other factors such as active fire suppression \citep{archibald_what_2009}. 

Disentangling the effects of landscape fragmentation and the climate is complex, especially since their interaction seems to vary regionally \citep{andela_recent_2014}. For example, in savannas of the African northern hemisphere, declines in burned area are associated with combined effects of land use expansion and changes in rainfall patterns \citep{andela_recent_2014}; while in the southern hemisphere, upward trends in burned area are related to rainfall changes \citep{andela_recent_2014}, although these climatic effects seem to be constrained by the level of human impact \citep{archibald_climate_2010, andela_recent_2014}.

In the Brazilian Cerrado, similar dynamics have been observed, where the expansion of landscape fragmentation is concomitant to declines in burned area \citep{rosan_fragmentation-driven_2022, da_silva_arruda_assessing_2024}, while burned area increases are concentrated in regions with the largest remnants of native vegetation \citep{mataveli_analysis_2017, silva_drivers_2020, silva_putting_2021, da_silva_arruda_assessing_2024} and correlated with temporal increases in fire weather \citep{silva_drivers_2020}. However, there seems to be an unresolved question regarding the fact that agricultural expansion, which is prevalent across the Cerrado, is correlated with decreases in burned area in some regions, but correlated with burned area increases in others \citep{silva_drivers_2020, silva_putting_2021, da_silva_arruda_assessing_2024}. Often, the simultaneous positive trends in burned area and in land use area have been related to the use of fire in Cerrado's native vegetation conversion \citep{silva_drivers_2020}. However, recent studies point at limited changes in fire activity during conversion of Cerrado's savannas and grasslands to pastures or croplands, except in the case of forest conversion \citep{ribeiro_time_2024}. Finally, studies on Cerrado's burned area have usually characterised climatic and land use effects separately \citep{mataveli_analysis_2017, mataveli_satellite_2018, silva_drivers_2020, silva_putting_2021, da_silva_arruda_assessing_2024}, and few studies have considered their combined effects \citep{conciani_human-climate_2021}. Interestingly, \citet{conciani_human-climate_2021} found no strong relation between rainfall and the burned area of some protected areas, while the explanatory power of their model increased when including human factors. They suggested that human influence buffered the relation between climate and fire \citep{conciani_human-climate_2021}.

Therefore, evidence points at a non-trivial relation between the effects of the climate and land uses on burned area in the Cerrado and other tropical savannas, suggesting human controls may hinder climatic effects \citep{conciani_human-climate_2021, andela_recent_2014, archibald_climate_2010}. However, further research is required to understand to what extent, and how, landscape fragmentation limits burned area responsiveness to climate in the Cerrado, and whether such controls act similarly for different climatic variables.



\section{On the quantification of fire regimes and their changes through time}

The occurrence of fire and its behaviour depends on the climate and type of vegetation \citep{archibald_biological_2018}, as well as edaphic factors, topographic characteristics and the presence of human activities \citep{keeley_twenty-first_2019}. As such, the frequency and characteristics of fires vary across the globe \citep{archibald_defining_2013, chuvieco_global_2008, pais_global_2023}, and have also varied through time \citep{bowman_fire_2009, keeley_evolutionary_2022}. The patterns of fire that typically occur in a certain area over a period of time are known as fire regimes \citep{krawchuk_global_2009, archibald_defining_2013}, and are usually quantified in terms of burned area, frequency, seasonality, fire size and intensity, or the shape of the fires \citep{martin_fires_1992}. From an ecological perspective, analyses of fire regimes are very valuable because both the behaviour of a single fire and its impacts on the vegetation can only be fully understood or accounted for taking into consideration the current and historical patterns of fire of the ecosystem \citep{pivello_understanding_2021}.

\begin{figure*}[t!]
\centering
\includegraphics[width=\textwidth]{_literature_review/archibald_2013.PNG}
\caption{Distribution of different fire regimes at a global scale by a pioneering study mapping fire regimes using a clustering algorithm \citep{archibald_defining_2013}. The map highlights five different fire regimes depending on the characteristics of the fires in each 0.5$^\text{o}$ grid cell in the period 1997 to 2010. Classification used data from the Global Fire Emissions Database version 3.1 \citep{van_der_werf_global_2017}, and the Moderate-resolution Imaging Spectroradiometer (MODIS) MCD45A1 \citep{araujo_satellite-based_2015}. The names of the fire regimes indicate their main characteristics: FIL, frequent-intense-large fires; FCS, frequent-cool-small fires; RIL, rare-intense-large fires; RCS, rare-cool-small fires; ICS, intermediate-cool-small fires. Source: \citet{archibald_defining_2013}.} 
\label{fig:archibald_2013}
\end{figure*}

Burned area is extensively used in the literature because it is a direct indication of the extent of fire occurrence, and it is readily obtained from many remote sensing products \citep{giglio_collection_2018, chuvieco_generation_2008}. However, the development and refinement of geospatial data techniques to extract information at individual fire level \citep{archibald_identifying_2009, andela_global_2019, laurent_fry_2018, finco_monitoring_2012} has opened the avenue to characterise fire regimes altogether \citep{chuvieco_generation_2008, archibald_defining_2013}. Indeed, there is an increasing number of studies characterising fire regimes at regional \citep{cattau_modern_2022, boulanger_fire_2013, elia_uncovering_2022}, continental \citep{archibald_southern_2010, galizia_global_2023} and global scales \citep{archibald_defining_2013, pais_global_2023, garcia_characterizing_2022, chuvieco_generation_2008}, covering different periods of time (see for example Figure \ref{fig:archibald_2013}). These studies are allowing to understand the complexity of fire dynamics, the trade-offs between fire characteristics, and how these different characteristics are shaped by the climate, vegetation and human activities \citep{archibald_defining_2013}. At a global scale, there are some general patterns, with regimes of frequent fires overlapping with tropical savannas and grasslands (Figure \ref{fig:archibald_2013}), while fires are generally infrequent across the arctic boreal regions \citep{archibald_defining_2013, pais_global_2023}. However, these same studies also find that fire regimes imperfectly correspond to unique climates and vegetation types \citep{archibald_defining_2013, pais_global_2023}, pointing at the role of other factors shaping fire occurrence. The importance of these factors is further supported by regional studies, which provide more nuanced accounts of differences in fire regimes, especially uncovering contrasting dynamics at small scales \citep{cattau_modern_2022, boulanger_refinement_2014, elia_uncovering_2022, galizia_understanding_2021, silva_putting_2021}.

Ecosystems adapted to fire, like most of the ecological communities in the Cerrado, are actually adapted to certain patterns of fire frequency, intensity, severity, size distribution or seasonality, apart from burned area \citep{keeley_evolutionary_2022}. That is, these ecosystems are adapted to a certain fire regime and it is the overall deviation from these patterns that can generate changes to the vegetation composition and structure \citep{pivello_understanding_2021}. Even though the characteristics of the natural fire regimes of the Cerrado remains an open question, the study of the current patterns of fire and how they are related to, and are changing with, climatic and human factors can allow a better understanding of the processes undergoing these savannas.

Until now, studies on the fire regimes of the Brazilian savannas have used the Cerrado's ecoregions \citep{sano_land_2010} to study the spatial patterns of fire regimes \citep{silva_putting_2021, da_silva_arruda_assessing_2024}. Across the Cerrado, these studies have found that the different ecoregions present remarkable differences when considering different characteristics of fire individually \citep{da_silva_arruda_assessing_2024}, and when considering fire regimes as a whole \citep{silva_putting_2021}. These studies show an overall tendency of some ecoregions to concentrate large, frequent and intense fires, while other ecoregions present small, infrequent and cool fires \citep{silva_putting_2021}. In addition, the ecoregions with the latter patterns of fire are concentrated in the south of the Cerrado, where both the presence of humans and the landscape fragmentation are very high. In this sense, the fire regimes of the Cerrado seem to vary across a gradient of human land use intensity \citep{silva_putting_2021, rosan_fragmentation-driven_2022, mataveli_satellite_2018, konduri_tracking_2023}, similarly to the patterns of fire observed in the southern African savannas \citep{archibald_southern_2010}. 

The ecoregions in which the Cerrado has been divided were identified based on rainfall patterns, topographic features, soil characteristics, land use and land cover, and conservation units and indigenous lands \citep{sano_cerrado_2019}. All of these variables are factors that affect the occurrence and behaviour of fire, so the characterisation of their fire regimes contributes to understanding the spatial patterns of fire across the Cerrado. However, the regions of the Cerrado with different fire regimes - henceforth fire regime regions - may not necessarily correspond geographically to these ecoregions. This may be because of the inherent heterogeneity of environmental characteristics within the ecoregions, or because of the similarity in patterns of fire between areas belonging to different ecoregions \citep{silva_putting_2021}. Furthermore, the climate and land use area of the Cerrado have experienced profound changes over the past five decades \citep{alencar_mapping_2020, hofmann_brazilian_2021, hofmann_changes_2023}. As a consequence, both the characteristics of the fire regimes (i.e. fire size, frequency, intensity, seasonality, etc.) and their geographical limits may have also changed over time. However, no studies so far have characterised the fire regime regions of the Cerrado without assuming spatial boundaries \textit{a priori} nor studied how these may have changed. This constitutes a clear gap in the literature on the fire regimes of the Cerrado. Further, first delineating the fire regime regions, and only later studying their environmental characteristics, can help identify the main factors that explain the differences and similarities between the fire regime regions \citep{archibald_defining_2013}.



\section{On the causes of fire ignitions and their associated fire regimes}


Lightning has been the main cause of fires for millions of years before the appearance of humans \citep{scott_burning_2018}. As such, lightning played a crucial role on the evolution and expansion of fire-adapted ecosystems \citep{bowman_fire_2009}, ultimately contributing to shaping the geographical patterns of vegetation \citep{bond_global_2005}. Nowadays, however, humans are the main cause of fire in most regions of the world \citep{janssen_extratropical_2023}. In particular, humans ignite the majority of fires in the tropics \citep{archibald_what_2009}, despite these being the regions where lightning densities are highest \citep{christian_global_2003}. However, the attribution of ignition causes in tropical areas bears a high level of uncertainty \citep{janssen_extratropical_2023}, indicating that more detailed regional studies are required to ascertain the relevance of lightning in these regions.

For a fire event to be successful - that is, to start and spread -, it requires an ignition source, a fuel bed with enough continuity, and adequate weather conditions spurring the flammability of the fuel \citep{archibald_what_2009, pausas_burning_2009}. Even though lightning densities are highest in the tropics, most lightning strikes take place during the wet season \citep{schumacher_characteristics_2022}, which limits their ignition probability \citep{veraverbeke_lightning_2017}. In contrast, humans are less restricted than lightning in the timing of ignition throughout the year and the geographical location of their ignition \citep{balch_human-started_2017}, as long as the fuel bed is flammable and continuous enough. In particular, humans tend to generate excess number of ignitions in comparison to lightning, as well as igniting fires in different weather conditions \citep{balch_human-started_2017}, often giving rise to more extreme fires as a consequence \citep{hantson_human-ignited_2022}. In addition, the use of fires in certain anthropic activities like agriculture can increase the recurrence of fire events in specific areas \citep{gottsberger_life_2006, pivello_understanding_2021}.

The extent to which fire regimes caused by humans differ from those created by lightning has implications on the design of fire management strategies for conservation \citep{van_wilgen_fire_2000}, to better understand the pressures posed by human-caused fires on ecosystems - both fire-adapted and fire-sensitive -, but it can also inform questions regarding the natural history of ecosystems. For example, a study in the Kruger National Park in South Africa, formed by savanna ecosystems, found that the fire return interval of lightning-ignited fires was much longer than that of a fire regime of prescribed burns \citep{van_wilgen_fire_2000}. This finding indicated that the frequency of prescribed fires may have been too high, with potentially degrading effects on these ecosystems. However, the lightning-ignited return intervals observed were so long, that it also posed the question on whether, after millions of years of human uses of fire, a regime solely ignited by lightning could also lead to losses in biodiversity and changes in ecosystem structure \citep{van_wilgen_fire_2000}.

In the Cerrado, there is a limited number of studies exploring lightning-caused fires. In particular, the proportion of lightning to human-caused fires in the Cerrado has not yet been quantified, relying on estimates for the Pantanal \citep{menezes_lightning_2022} - a savanna-like wetland south of the Cerrado. Therefore, the fact that humans are currently the main source of ignitions in the Cerrado is a commonly held perception stemming from experts knowledge on modern uses of fire \citep{pivello_use_2011}, and from observations on the mismatch between the lightning season - mostly coinciding with the wet season, October to April \citep{schumacher_characteristics_2022} - and the fire season - during the dry season, May to September \citep{mataveli_satellite_2018, silva_putting_2021, da_silva_arruda_assessing_2024}. A study focused on the south-western most part of the Cerrado carried out an identification of lightning-caused fires and found that these were mostly concentrated in the transition period from the dry to the wet season \citep{schumacher_characteristics_2022}, but it also identified lightning-caused fires during the wet season. Further, different studies in the early 2000s focusing on the Emas National Park \citep{ramos-neto_lightning_2000, pereira_identificacao_2005}, the Serra da Canastra National Park \citep{medeiros_incendios_2004} or the Chapada dos Veadeiros National Park \citep{fiedler_ocorrencia_2006}, also reported the occurrence of lightning-caused fires during the wet and transition periods, indicating that lightning fires may be more frequent in the Cerrado than has been usually assumed \citep{ramos-neto_lightning_2000, medeiros_incendios_2004, pereira_identificacao_2005, fiedler_ocorrencia_2006}. Moreover, they found that, while lightning fires were generally small, had a patchy shape, and extinguished naturally with rainfall, dry-season fires caused by humans were larger, fast spreading and required active extinction \citep{ramos-neto_lightning_2000, medeiros_incendios_2004, pereira_identificacao_2005, fiedler_ocorrencia_2006}.

Therefore, the quantification of lightning fires against human-caused fires in the Cerrado, and the comparison of the fire regimes that each type of ignition cause creates is a major research gap. Addressing these questions can shed light on how and to what extent the current fire regimes of the Cerrado may differ from what would naturally occur, with important implications for Cerrado conservation.


\section{Satellite remote sensing and ancillary data products to study large-scale dynamics of fire}

Remote sensing encompasses a range of techniques for data collection where the sensor gathers data without physical contact with the study object, from a certain distance \citep{chuvieco_fundamentals_2020}. The advent of remote sensing has allowed the study of processes and phenomena taking place on the Earth's surface at a large-scale. Nowadays, remote sensors can be placed on different types of platforms, including, but not limited to, unmanned aerial vehicles (UAVs), airplanes, and satellites \citep{chuvieco_fundamentals_2020}. In this thesis, the major sources of data are based on spaceborne remote sensing products detecting fire occurrence, which is the main focus of this section. Remote sensing observations may be based on active or passive sensors depending on their technology. 

Passive sensors detect the electromagnetic signal (EM) - the light - emitted by an object either due to its own temperature or the light reflected on the object from a light-emitting source such as the Sun. Both types of signals are extensively used to detect fire occurrence. In the first case, as fires are exothermic combustion reactions that emit light and heat, they produce strong signals in the infrared that are recorded on passive sensor images \citep{chuvieco_fundamentals_2020}. Fires can therefore be identified from these images using contextual algorithms that detect thermal anomalies - pixels with hotter temperature than their surroundings - and determine whether these anomalies are likely to have been produced by active fires \citep{giglio_collection_2018, schroeder_new_2014}. In the case of the reflected light, the identification of pixels corresponding to burned areas is based on the analysis of changes in the EM signature of pixels between consecutive satellite overpasses, and can use different parts or combinations of the EM spectrum (i.e. not only in the visible) \citep{chuvieco_fundamentals_2020}. Fire data products obtained in this way are usually referred to as burned area products. 

Active sensors are so-called because they emit energy, be it radio waves (RADAR techonology) or laser pulses (LiDAR techonologies), and record the reflected signal to characterise the study object. These technologies have applications on topographic mapping, detection and tracking of climatic phenomena, and have also been used in fire scar detection \citep{chuvieco_fundamentals_2020}.

The study of fire dynamics at a large-scale has benefited from the increasing wealth of satellites and sensors, covering increasingly longer periods of time and improvements in their quality, and spatial and temporal resolutions \citep{chuvieco_fundamentals_2020}. In addition, the advances in spaceborne fire data have been complemented with the creation of data products measuring various other environmental factors. Because of their relevance to this thesis, we can highlight the creation of land use and land cover maps, based on the classification of pixels from optical sensors; or climatic datasets, either from climate re-analysis models (like ERA5-Land product from the European Centre for Medium-Range Weather Forecasts, ECMWF, \citep{munoz-sabater_era5-land_2021}), from the combination of weather stations' data and satellite remote sensing data (like the Climate Hazards Group InfraRed Precipitation with Station, CHIRPS, \citep{funk_climate_2015}), or from the interpolation of various climate data sources (like TerraClimate \citep{abatzoglou_terraclimate_2018}). Finally, the development of lightning strike datasets should also be highlighted, whether based on satellite detection or from ground sensor networks, as is the case in this thesis (see \textbf{Chapter \ref{ch:paper03} (Research Paper 3)} for a detailed description).


\subsection{Burned area products}

Burned area products are widely used in the study of the spatiotemporal dynamics of fire. Burned area is a direct measure of the extent of fire occurrence in an area, and is related to the number of fires and their size. In addition, the inter-annual variability of burned area responds to climatic variability \citep{jones_global_2022}, and the spatial patterns are related to vegetation, topographic and land use patterns \citep{archibald_climate_2010, archibald_what_2009}. Increasingly, when supported by ground-truth data, the change in the EM signature of the burned pixels is also being used as an indicator of the combustion completeness of the fire \citep{keeley_fire_2009}.

Despite the advances on fire science enabled by spaceborne burned area products, these data bear some limitations that must be taken into consideration. First, most products available until now suffer from a trade-off between the spatial and temporal resolutions related to the satellite's orbit, the sensor's characteristics and the swath width - the horizontal surface on Earth covered by the sensor \citep{chuvieco_fundamentals_2020}. Generally, sensors with a smaller field of view - covering small areas - have high spatial resolution but take long to revisit the same area of the Earth's surface. Analogously, sensors with larger fields of view can have short revisit cycles as they cover larger areas at a time, but have low spatial resolutions as a consequence. Recently, these trade-offs have been addressed by using constellations of satellites with high resolutions, reducing the time span between observations of the same points on Earth.

The presence of clouds at the time of satellite overpass can also obstruct fire identification \citep{libonati_algorithm_2015, pereira_burned_2017}, especially in the case of longer revisit cycles and in ecosystems and times of the year when vegetation recovers swiftly. On the other hand, the spatial resolution of the sensor limits the ability to detect small fires \citep{campagnolo_comprehensive_2021}. This limitation is accentuated in the case of highly fragmented landscapes, where the burned signal may be confused with that of agricultural areas \citep{rodrigues_how_2019}.

\subsubsection{MapBiomas Fogo dataset}

In \textbf{Chapters \ref{ch:paper01} (Research Paper 1) and \ref{ch:paper02} (Research Paper 2)}, the burned area dataset used was the one created by the MapBiomas project (MapBiomas Fogo Collection 1.0 in \textbf{Chapter \ref{ch:paper01} (Research Paper 1)} and Collection 2.0 in \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}, \citep{alencar_long-term_2022}). This dataset is based on Landsat and provides data for the period 1985-2023 (in the most recent version of the product), even though this thesis focused on the period 1985-2020. 

The Landsat programme consists of a series of missions launching satellites for Earth Observation, and has provided satellite images continuously since the 1970s \citep{irons_next_2012}. Landsat sensors have a spatial resolution of 30 m and a revisit cycle between 15-16 days or 7-8 days, depending on the period. The MapBiomas project bases its data products on the Landsat sensors Thematic Mapper (TM), Enhanced Thematic Mapper Plus (ETM+), and the Operational Land Imager and Thermal Infrared Sensor (OLI-TIRS), onboard of Landsat 5, Landsat 7, Landsat 8, and Landsat 9, respectively \citep{alencar_long-term_2022}.

This dataset was chosen for various reasons. First, MapBiomas Fogo provides the longest time series of annual burned area data for the whole of Brazil. This was one of the main advantages for this thesis, as it allowed to explore changes in Cerrado fire dynamics before the 2000s, which has seldom been explored in the literature. In addition, the MapBiomas project offers a suite of geospatial products based on Landsat, highlighting the MapBiomas land-cover and land use data product \citep{souza_reconstructing_2020}. As such, these two datasets offered the opportunity of studying the relation between burned area and land use area not only with the same spatial resolution, but also developed by the same team of experts, granting a degree of coordination in their production. Finally, MapBiomas data are publicly available and easy to access for quick visual inspection on their website, and easily downloadable through Google Earth Engine \citep{gorelick_google_2017}. Further details on the algorithms used by the MapBiomas project are provided in the Methods section of \textbf{Chapter \ref{ch:paper01} (Research Paper 1)} and \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}.

The main limitation of this dataset resides on its omission errors, particularly the detection of small fires and the mapping of burned areas in highly fragmented landscapes \citep{nogueira_can_2016, alencar_long-term_2022}, possibly leading to an underestimation of burned area particularly in the southern regions of the Cerrado. Another main limitation is the high uncertainty on the estimation of the timing of burn occurrence. This limitation stems from the long revisit cycle of the Landsat mission, but is also related to the methodology used by MapBiomas to generate their burned area product, which creates annual maps of burned area without aiming to maximise the precision of the burn date. However, despite the uncertainty of burn date at pixel level, the seasonal patterns of fire detected across the Cerrado with MapBiomas Fogo data \citep{alencar_long-term_2022, da_silva_arruda_assessing_2024} agree with the patterns previously detected using other satellite-based burned area products \citep{mataveli_analysis_2017, mataveli_satellite_2018, silva_drivers_2020, silva_putting_2021}.


\subsection{Mapping individual fire events}

While spaceborne burned area products are readily available for studying patterns of fire, the characterisation of fire regimes requires going a few steps further. In particular, information is required at the individual fire level so that characteristics like fire size or the number of fires can be estimated \citep{nogueira_can_2016}. However, burned area products only provide information on whether a pixel burned, and an estimation of when it burned. Therefore, information at the individual fire level requires further processing of the burned area data designing algorithms to group pixels into fire events \citep{archibald_identifying_2009}.

Different studies have used different approaches to mapping individual fire events. To group pixels from coarse-resolution burned area products with a daily revisit cycle, some studies used a \textit{flood-fill} algorithm in which contiguous pixels are mapped together if the difference between their burn dates is smaller than a certain threshold \citep{archibald_identifying_2009, laurent_fry_2018}. Other approaches have been developed to map fire events based on Landsat data \citep{finco_monitoring_2012, goodwin_development_2014, bastarrika_mapping_2011}. In particular, a \textit{region growing} algorithm identified seed Landsat pixels using active fire data, and then incorporated neighbouring pixels based on spatial and spectral properties \citep{bastarrika_mapping_2011}.

A more elaborate approach to map individual fire events was implemented in the Global Fire Atlas dataset (GFA) \citep{andela_global_2019}, based on the Moderate-Resolution Imaging Spectroradiometer (MODIS) MCD64A1 Collection 6.1 burned area data \citep{giglio_collection_2016}, which has a spatial resolution of 500 m and a daily revisit cycle. Apart from mapping individual fires, GFA also provides an estimation of the day and location of the fire ignition, along with information on the daily expansion of the fire, fire line length, speed and direction of spread, as well as the size of the fire and its duration \citep{andela_global_2019, andela_2024_GFA}. The GFA algorithm first identifies clusters of contiguous pixels based on the spatial patterns of the burn dates, and then ignition point candidates are identified using a multi-step procedure searching for local minima out of each cluster's burn dates \citep{andela_global_2019}. This algorithm was later applied to re-scaled active fire data from the Visible Infrared Imaging Radiometer Suite (VIIRS) \citep{schroeder_new_2014} to produce the Amazon Dashboard dataset \citep{andela_tracking_2022}. VIIRS has a daily overpass and 375 m spatial resolution, and detects active fires based on thermal anomalies \citep{schroeder_new_2014}. The GFA dataset was used in \textbf{Chapter \ref{ch:paper03} (Research Paper 3)} of this thesis. 



\subsubsection{A custom algorithm to map individual fire events for MapBiomas Fogo data}

To enable the characterisation of the fire regimes of the Cerrado in \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}, a custom algorithm was designed to group together pixels that were likely to be part of the same fire event. This algorithm was based on the same principles of the algorithms described above: grouping spatially contiguous pixels with consecutive burn dates - months in the case of MapBiomas Fogo data. In this case, however, the 1) uncertainty on the burn date, 2) the observation of highly fragmented fire events, and 3) an excessive number of very small fire polygons meant that some further adjustments had to be implemented. As such, we grouped together as part of the same fire event those polygons belonging to the same month that were up to 150 m apart from each other, and we only considered those polygons larger than 3 ha. The algorithm steps are described in the Methods section of \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}.


\subsection{Other ancillary data products}

Fire is a complex phenomenon that depends on - and in turn affects - the climate, the type of vegetation and human presence among others. As such, besides data on fire occurrence and behaviour, the study of fire dynamics is usually complemented with ancillary data products providing information on these other factors. The main aim of this thesis is to understand how the patterns of fire of the Cerrado are related to the climate and patterns of land use expansion. Further, in \textbf{Chapter \ref{ch:paper03} (Research Paper 3)}, the aim is to identify the most likely cause of fire ignition, whether lightning or humans. Therefore, there are a few datasets on climate variables, land use and land cover class, and on lightning occurrence that have played a major role in this thesis. Here, we provide some context on the datasets used and comment on their advantages and limitations.


\subsubsection{Land use area data}

Every research chapter of this thesis has used data on land use area for the whole Cerrado from the same product, the MapBiomas land use and land cover dataset \citep{souza_reconstructing_2020}. MapBiomas is a project co-created and led by a network of Brazilian institutions including universities, non-governmental organisations, and technology companies that was launched in the year 2015. Initially, their main objective was to create annual maps of land use and land cover for all of Brazil using remote sensing data in a cheaper and faster manner than other existing methods and projects, as well as providing a longer time series of data into the past. Besides, they intended that this project would be completely open, from the processing scripts to the data, so that it could be easily replicated in other areas of the world \citep{souza_reconstructing_2020}.

With this objective, MapBiomas decided to use Landsat imagery, as it would allow them to produce annual maps as far back as 1985 with medium resolution \citep{souza_reconstructing_2020}. In addition, the project network decided to use Google Earth Engine \citep{gorelick_google_2017} as their processing platform, from which users can also access the processing scripts and data for downloading, which also reduced the costs of producing such dataset. With time, the project has diversified the number and type of data products that they provide, including the burned area product MapBiomas Fogo dataset used in \textbf{Chapter \ref{ch:paper01} (Research Paper 1)} and \textbf{Chapter \ref{ch:paper02} (Research Paper 2)} of this thesis.

MapBiomas released the first version of their land use and land cover map in 2016 providing data for the period 2008 to 2015 using a simplified legend with only 7 land use and land cover categories. Subsequently, every year they have released an updated version of their product covering increasingly longer periods, expanding and refining the legend, and improving their algorithms and methodologies. In 2018 they released the first version covering the full period 1985 to 2017, MapBiomas Collection 3 \citep{souza_reconstructing_2020}. Currently, the last release was in 2024, MapBiomas Collection 9, covering the period 1985 to 2023. In this thesis, MapBiomas Collection 6.0 was used in \textbf{Chapter \ref{ch:paper01} (Research Paper 1)}, Collection 8.0 was used in \textbf{Chapter \ref{ch:paper02} (Research Paper 2)} and in \textbf{Chapter \ref{ch:paper03} (Research Paper 3)}.

MapBiomas produces their annual land use and land cover maps for each year independently. First, annual mosaics of Landsat images are created filtering out pixels containing clouds or smoke \citep{souza_reconstructing_2020}. These mosaics contain summaries of the information of the different Landsat bands throughout the year (average, maximum, standard deviation or minimum value), as well as vegetation indices calculated combining bands \citep{souza_reconstructing_2020}. These different summary values and indices are later used by machine learning algorithms to classify pixels. Then, different teams within MapBiomas are in charge of carrying out the pixel classification for different regions (i.e. Amazon, Cerrado, Caatinga, Pantanal, Mata Atlântica, and Pampa), and for different "cross-cutting themes" (i.e. forest, grassland, agriculture, pasture, urban area, water, etc.) \citep{souza_reconstructing_2020}. In this manner, each team can adapt the algorithm to improve their pixel classification by, for example, using pixel data from the period of the year that may be most optimal to identifying certain types of land use and cover classes in a specific type of vegetation. Furthermore, each team produces their own training dataset by visually identifying the land use and land cover class of a number of pixels - which varies depending on the region and class. Then, the teams classify the pixels using the Random Forests algorithm implementation of Google Earth Engine \citep{gorelick_google_2017}. In this manner, they obtain a classified map for each land use and land cover class and region \citep{souza_reconstructing_2020}. Then, each team applies spatial and temporal filters to these maps to increase the consistency of the classification based on a set of rules \citep{souza_reconstructing_2020}. 

Next, they integrate the maps for the different land use and land cover classes into a single map for each region using a set of rules to decide which classification decision to keep when there are inconsistencies. This step is still done by region so that different criteria can be applied for each different context \citep{souza_reconstructing_2020}. Finally, they integrate together the maps for the different regions applying a few last spatial filters to diminish inconsistencies \citep{souza_reconstructing_2020}, as well as transition filters that will detect and address class transitions that are not reasonable or impossible between consecutive years \citep{souza_reconstructing_2020}.

MapBiomas land use and land cover maps in Collections 6.0 and 8.0 have an accuracy around 75\%. One of the main advantages of this product for the aims of this thesis - apart from the long time series and the fact that it is easily matched with MapBiomas Fogo data - is the fact that MapBiomas land use and land cover data product distinguishes pastures from grasslands and savannas. This turned out to be a key factor for the aims of this thesis as it allowed to disentangle drivers of fire in this region - which has not been possible by other studies using land use and land cover data that did not fully distinguish these two classes in the Cerrado \citep{mataveli_satellite_2018, haas_global_2022}.



\subsubsection{Climate data}

Currently, global and quasi-global gridded climate data products are widely used by the scientific community in a large variety of applications. Over the past few decades, there has been an increase in the number of such data products with increasing accuracy, offering data for longer time periods increasing temporal and spatial resolutions, and offering wider variety of climate variables \citep{abatzoglou_terraclimate_2018}. Their usage in different biogeoscience disciplines has been stimulated by the parallel development of other gridded data products - typically based on remote sensing products - describing other Earth system variables, such as the aforementioned fire data products. 

Indeed, because the climate is one of the main drivers of fire \citep{jones_global_2022}, such climate datasets are commonly used in the study of fire dynamics. The combination of these two types of gridded datasets - along with ancillary data products like land use data - has allowed a better understanding of how spatial and temporal patterns of fire are affected by different climate variables \citep{jones_global_2022}, as well as contributed to improved fire modelling efforts \citep{haas_global_2022, hantson_status_2016}. 

Climate data products may be produced based on meteorological observations from weather stations, satellite remote sensing products, climate reanalysis models and combinations of these \citep{abatzoglou_terraclimate_2018}. In this thesis, climate data from three different products have been used to describe different climate variables.  In this section, we first describe the climate data products used, and then we discuss the rationale for combining data from these three data products.

\begin{itemize}
    \item \textbf{Temperature-related variables}
\end{itemize}

In this thesis, we used temperature-related variables from ERA5-Land \citep{munoz-sabater_era5-land_2021}, a climate reanalysis product that was developed as an enhancement of the land component of the fifth generation European ReAnalysis (ERA5) data product- which in turn is an improvement of the earlier ERA-Interim product \citep{hersbach2020era5}. Climate reanalysis products are a type of gridded climate data product generated by combining past observations from weather stations, in situ meteorological measurements and remote sensing data with physical models of the climate to generate time series of multiple climate variables \citep{hersbach2020era5}. Both ERA5 and ERA5-Land were created and are maintained by the European Centre for Medium-Range Weather Forecasts (ECMWF), and both are part of the Copernicus Climate Change Service of the European Commission \citep{hersbach2020era5, munoz-sabater_era5-land_2021}. 

ERA5 provides data on different climate variables describing the global atmosphere, land surface, as well as on ocean waves from 1940 onward with 31 km spatial resolution, and both monthly and hourly time steps \citep{hersbach2020era5}. ERA5-Land provides climate data over the Earth's land surface with a higher spatial resolution than ERA5 - 9 km -, and was produced with higher resolution in the numerical calculations of the ECMWF land surface model and including topographic elevation corrections \citep{munoz-sabater_era5-land_2021}. An evaluation against in situ data showed that ERA5-Land provides a better description of the hydrological cycle than ERA5 \citep{munoz-sabater_era5-land_2021}. Furthermore, even though ERA5 and ERA5-Land provide very similar estimations of land surface temperature, ERA5-Land shows better correlation with MODIS data in the tropics than ERA5 \citep{munoz-sabater_era5-land_2021}.

In each of the three results chapters, we used monthly averaged ERA5-Land data on air temperature at 2 m above the surface of land (ERA5-Land variable "2 meter temperature"), and the dewpoint temperature at 2 m (ERA5-Land variable "2 meter dewpoint temperature"). Dewpoint temperature is the temperature to which the air would have to be cooled at constant pressure to produce air saturation \citep{munoz-sabater_era5-land_2021}. We used the combination of these two variables to calculate Vapour Pressure Deficit (VPD), a direct measure of the atmospheric demand for water, which we calculated following the procedure detailed in \citet{allen_crop_1998}.

\begin{itemize}
    \item \textbf{Precipitation}
\end{itemize}

To represent precipitation in each of the three results chapters, we used data from the Climate Hazards Group InfraRed Precipitation with Station data (CHIRPS) V2.0 \citep{funk_climate_2015}, created by the United States Geological Survey and the Climate Hazards Group at the University of California, Irvine. CHIRPS is a quasi-global gridded data product providing rainfall data at a 0.05$^\text{o}$ for the period 1981 to present \citep{funk_climate_2015}. This rainfall dataset is produced by blending advanced interpolation techniques of precipitation data from gauge station with satellite remote sensing data to represent locations where gauge stations are sparse \citep{funk_climate_2015}. CHIRPS dataset was created to allow scientists to assess recent extreme rainfall and drought events in a historical context by providing long records of precipitation data with high temporal and spatial resolution \citep{funk_climate_2015}.

\begin{itemize}
    \item \textbf{Potential Evapotranspiration}
\end{itemize}

Finally, in \textbf{Chapter \ref{ch:paper01} (Research Paper 1)} and \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}, we used data on Potential Evapotranspiration (PET) from the TerraClimate dataset \citep{abatzoglou_terraclimate_2018}. This dataset provides climate data on different primary climate variables (like maximum and minimum temperature, precipitation accumulation or wind-speed) along with derived variables (like PET, runoff, or soil moisture). These variables are obtained with an interpolation algorithm of climate data from various sources: high spatial resolution climatological normals from the WorldClim dataset, and coarser resolution monthly data from the Climate Research Unit and the Japanese 55-year reanalysis \citep{abatzoglou_terraclimate_2018}. 

TerraClimate estimates monthly PET using the Penman Monteith approach, that is, based on an energy balance instead of obtaining PET purely from temperature-based metrics and incorporating some modifications to improve PET estimations over snow \citep{abatzoglou_terraclimate_2018}. Overall, TerraClimate shows good performance when validated against station-based observations \citep{abatzoglou_terraclimate_2018}. In particular, annual PET estimations show agreement with station-based PET data from FLUXNET stations \citep{abatzoglou_terraclimate_2018}.

\begin{itemize}
    \item \textbf{Rationale for choosing climate data products}
\end{itemize}

Even though gridded climate data products are widely used in studies of different scales across the world, these products often perform poorly over regions with sparse in situ meteorological observations \citep{burton_inter-comparison_2018}. In this sense, tropical areas present the greatest challenge because they are regions where meteorological stations are sparse and because, particularly over tropical forests, land-atmosphere feedbacks are complex and vary within small spatial scales \citep{burton_inter-comparison_2018}.

Regarding the Brazilian Cerrado, findings on trends in different climate variables across the Cerrado seem to strongly depend on the source of data - whether on-ground weather station data \citep{hofmann_brazilian_2021}, remote sensing data \citep{oliveira_trends_2014}, or climate reanalysis data from ERA5 \citep{hofmann_changes_2023}. First, a study on climate trends based on weather station data from the Brazilian National Meteorology Institute (Instituto Nacional de Meteorologia) found that temperature and VPD had increased across the Cerrado over the period 1961-2019 \citep{hofmann_brazilian_2021}. While this study concluded that the Cerrado is becoming hotter and drier, such drying trends were derived from decreases in relative humidity (that is, derived from temperature-related variables), while the study did not mention changes in rainfall. Similarly, using rainfall data from the Tropical Rainfall and Monitoring Mission (TRMM) - a research satellite providing information on rainfall over tropical and subtropical regions from 1997 to 2015 \citep{liu2012tropical} -, another study over the Brazilian Cerrado found increases in average annual evapotranspiration, but it did not report any trends on rainfall \citep{oliveira_trends_2014}. In contrast, a study based on ERA5 climate data found rainfall reductions of up to 50\% in the period 1960 to 2021 \citep{hofmann_changes_2023}. Therefore, there seem to be disagreements about Cerrado's climate trends in recent decades, particularly when comparing satellite data against climate reanalysis products.

\citet{burton_inter-comparison_2018} carried out an assessment aiming to show which gridded climate data products performed better over tropical areas. Thus, they compared on-ground data from meteorological stations against the climate estimates from various global and quasi-global products \citep{burton_inter-comparison_2018}. Their results indicate that ERA-Interim (the precursor to ERA5 and ERA5-Land) was one of the most reliable datasets regarding temperature-related variables over the tropics. However, they found that ERA-Interim significantly overestimated tropical rainfall \citep{burton_inter-comparison_2018}. In this sense, \citet{burton_inter-comparison_2018} found that CHIRPS and TRMM were the best-performing rainfall dataset in the tropics, as their precipitation data were in close agreement with rainfall gauge data from meteorological stations. Still, their results also showed a generalised tendency to considerably underestimate precipitation in high rainfall months, and to slightly overestimate precipitation in low rainfall months \citep{burton_inter-comparison_2018}.

Therefore, based on the recommendations derived from the inter-comparison of climate data products over the tropics \citep{burton_inter-comparison_2018}, on the findings of no clear trends in rainfall when using TRMM data \citep{oliveira_trends_2014} nor when using CHIRPS (see \textbf{Chapter \ref{ch:paper01} (Research Paper 1)})), which are in contrast to findings of decreasing precipitation when using ERA5 data \citep{hofmann_brazilian_2021}; we decided to use CHIRPS data to inform precipitation, and ERA5-Land to inform temperature-related variables. Finally, because we needed to compare precipitation against PET to identify the dry season months but CHIRPS only provides rainfall data, we decided to use PET from TerraClimate (see \textbf{Chapter \ref{ch:paper01} (Research Paper 1)} and \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}).




\subsubsection{Lightning data}

Lightning strikes are the main natural cause of fire ignitions. Therefore, the identification of fires caused by lightning is an active research field within fire science community \citep{veraverbeke_lightning_2017, moris_global_2023}, from the ecological role of lightning as a cause of disturbance \citep{veraverbeke_lightning_2017, janssen_extratropical_2023} to assessing the risks posed by this natural phenomenon to human lives and infrastructure. As with other aspects related with fire, the study of lightning as a fire ignition cause has benefited from the development of lightning detection technologies over the past few decades and the increasing availability of data \citep{said2017towards}.

Lightning strikes are electrical discharges that can occur in the atmosphere as a consequence of charge separation buildup in clouds produced by particle movement within the cloud, which can in turn cause charge buildup on the Earth's surface underneath the cloud \citep{dwyer2014physics}. These electrical discharges can partially neutralise the charge separation within the cloud (cloud flashes), or lower charge to the ground (cloud-to-ground, CG, strikes) \citep{said2017towards, pohjola2013comparison}. CG strikes can last just a few microseconds and can measure on the order of 10,000 A to 100,000 A \citep{said2017towards}. These current channels produce very high temperatures \citep{dwyer2014physics}, which can in turn spark a fire once they hit the ground if there is enough flammable fuel \citep{rego_fire_2021}.

These current discharges radiate energy in a wide range of the EM spectrum: from radio emissions, through the optical range, and even up to gamma rays (very high frequency EM emissions) \citep{said2017towards, pohjola2013comparison}. These emissions can be detected and located by Lightning Location Systems (LLS) which, depending on their characteristics, will capture specific aspects of lightning discharges \citep{said2017towards}. For example, sensors of the optical and near infrared range of the EM spectrum onboard satellites can detect lightning discharges \citep{said2017towards}. Such sensors are of course limited by their spatial resolution but, crucially, by their overpass cycles - even though lightning detectors are increasingly being placed on geostationary satellites \citep{said2017towards}.

Lightning strikes are commonly detected by dense networks of ground sensors. The technologies of these sensors focus either on Very High Frequencies (VHF), or Medium, Low and Very Low Frequencies (MF, LF, and VLF) \citep{pohjola2013comparison}. VHF signals (30-300 MHz) travel directly through the atmosphere, so they can easily be detected by sensors, but do not reach distances larger than 100-200 km \citep{pohjola2013comparison}. Therefore, a high-accuracy coverage of lightning strikes by VHF sensors requires a large number of sensors \citep{said2017towards}. Finally, CG strokes generate a distinctive signal on the MF to VLF range (3 kHz - 3 MHz) \citep{pohjola2013comparison, said2017towards}. This signal can travel as a ground wave - which can be detected several hundreds of kilometers away -, as well as through the atmosphere bouncing between the ground and the ionosphere - which can be detected thousands of kilometers away \citep{pohjola2013comparison}. 

In \textbf{Chapter \ref{ch:paper03} (Research Paper 3)} of this thesis, the aim was to quantify the proportion of fire ignited by lightning and by humans, and compare aspects of the fire regimes created by each type of ignition. Lightning data for this chapter was provided by co-authors, which in turn was produced by the Vaisala Global Lightning Detection Network GLD360, a network of ground VLF sensors with a global-scale coverage owned by Vaisala Inc. and launched in 2009 \citep{said2017towards}. This company's lightning strikes dataset provides the geolocation of the different strokes, an uncertainty measure of their location, as well as data on the polarity and the peak current of the lightning strike \citep{said2017towards}. However, this company does not disclose the location of their LLS sensors, which hampers the assessment of the overall quality of their network \citep{pohjola2013comparison}.



\begingroup
\setstretch{1.2}
\printbibliography[heading=subbibnumbered]
\endgroup

\end{refsection}