\chapter{Introduction}\label{ch:intro}
\pagenumbering{arabic}


\begin{refsection}

\section{Thesis rationale}

\subsection{The changing fire regimes of tropical savannas and grasslands, a general perspective}

Changes in fire dynamics have become a major concern because of environmental, societal, health and economic impacts \citep{bowman_vegetation_2020, shlisky_fire_2007}. In recent years, there has been an increase in the occurrence of large and uncontrollable fires in some areas of the world \citep{jones_state_2024, lapola_drivers_2023}, with devastating consequences for humans and ecosystems \citep{brando_gathering_2020, jones_state_2024}. This has added to the already common negative perception about fire occurrence of the general public, as well as to the notion that fire activity overall is being exacerbated by climate change \citep{doerr_global_2016}. These perceptions have often reinforced zero-fire policies that already existed in many areas of the world as blanket solutions to dealing with fires \citep{durigan_zero-fire_2020}, which implement active suppression as fire management practices \citep{moritz_learning_2014}. However, fire is a recurrent natural disturbance agent that has been occurring for millions of years \citep{glasspool_charcoal_2004}, it plays fundamental roles in the Earth system \citep{bowman_fire_2009}, and many ecosystems depend on it for their functioning and continuity \citep{shlisky_fire_2007}. In addition, perhaps paradoxically, global burned area has actually declined over the past two decades driven by decreases in burned area of tropical savannas and grasslands \citep{andela_human-driven_2017, jones_global_2022, wu_historical_2021}, which otherwise are the most flammable ecosystems concentrating around 70\% of total burned area \citep{giglio_analysis_2013}. 

Tropical savannas are heterogeneous ecosystems with open canopies and a continuous understory layer of shade-intolerant C$_\text{4}$ grasses \citep{sankaran_determinants_2005}. In these ecosystems, most vegetation species have co-evolved with fire developing adaptation strategies to resist or even thrive with periodic fires \citep{bond_fire_2005}. In fact, fire is a crucial factor maintaining their characteristic open canopies by preventing tree overgrowth \citep{bond_global_2005, oliveras_many_2016, pausas_alternative_2020}. However, the patterns of fire in savannas across the tropics are undergoing profound changes not only in their burned area, but also in their frequency, typical sizes and seasonality \citep{archibald_climate_2010, arruda_alternative_2021} - that is, their fire regimes are changing as a whole. Moreover, at regional scales, both increases and decreases in burned area and fire frequency over time are being reported \citep{andela_recent_2014, silva_putting_2021}, pointing at the complexity of processes underpinning these changes. In these ecosystems, departures from the historical fire dynamics in either direction can have damaging effects  \citep{pivello_understanding_2021} including changes in ecosystem composition, structure or function \citep{stevens_savanna_2017, rosan_extensive_2019} and biodiversity losses \citep{durigan_need_2016}. Such impacts on these ecosystems are particularly relevant because tropical savannas and grasslands are ancient and highly biodiverse ecosystems \citep{veldman_toward_2015, williams_australias_2017}, they are the second most productive biome \citep{beer_terrestrial_2010}, are home to about one fifth of the human population \citep{scholes_tree-grass_1997}, and support local livelihoods \citep{rekow_socio-ecological_2019, ryan_ecosystem_2016} and the production of agricultural commodities and livestock \citep{putz_importance_2010, rekow_socio-ecological_2019}. Thus, understanding how savanna fire regimes are changing in relation to humans and the climate is instrumental to better address their conservation, quantify their carbon emissions, and predict how these regimes may be in the future and their impacts on ecosystems and human populations \citep{wu_historical_2021}.

The two main drivers of changes of the fire regimes of tropical savannas and grasslands, and globally, are climate change and human activities \citep{jones_global_2022}. On the one hand, climate change is increasing the frequency of droughts and heatwaves and lengthening the duration of the dry season \citep{jolly_climate-induced_2015}, thus increasing the flammability conditions of vegetation \citep{simpson_determinants_2016, zanzarini_flammability_2022} and creating longer windows of opportunity for fires to start and spread \citep{flannigan_global_2013}. Thus, under climate change, some studies predict increases in fire incidence over some savannas \citep{silva_impacts_2019, silva_future_2016}. Human activities, however, have a more complex relation with changes in savannas' fire regimes. Overall, the expansion of human activities brings about landscape fragmentation, argued to reduce savannas' burned area by creating fuel discontinuities \citep{archibald_managing_2016, andela_recent_2014}. However, this expansion has sometimes been accompanied by the introduction of alien species, which can increase fuel load and flammability \citep{hoffmann_impact_2004, setterfield_turning_2010}. Further, humans also increase the number of ignitions during the dry season \citep{archibald_evolution_2012, menezes_lightning_2022}, in contrast to wet season lightning fires \citep{ramos-neto_lightning_2000, menezes_lightning_2022}, potentially enhancing fire occurrence and their intensity. Finally, inadequate fire management policies enforcing fire suppression have been common across tropical savannas \citep{durigan_need_2016, stevens_grassy_2022}, contributing to woody encroachment and fuel buildup \citep{stevens_savanna_2017, durigan_need_2016}, processes that can eventually lead to large and intense fires in some areas \citep{fidelis_year_2018}.


\subsection{The Brazilian Cerrado as a case study}

The Brazilian Cerrado is a remarkable example of the confluence of these processes and a notable case study to understand many aspects of contemporary savanna fire regimes and their changes in recent times. Extending over 2 million km$^\text{2}$, the Cerrado is the most biodiverse savanna in the world \citep{klink_conservation_2005}, with high levels of endemism. Despite its ecological importance, the Cerrado is highly threatened by agricultural expansion and is thus considered to be a biodiversity hotspot for conservation \citep{myers_biodiversity_2000}. One of the most remarkable characteristics of this region is the heterogeneity of its vegetation \citep{sano_land_2010}, emphasised by the multiple ecoregions identified within the Cerrado \citep{sano_cerrado_2019}. In fact, the Cerrado is more often described as a mosaic of open grasslands and savannas with different levels of tree cover, intermingled with patches of dry and gallery forests \citep{ribeiro_cerrado_2008}.  Fire plays a crucial role maintaining these characteristic mosaicking patterns of vegetation \citep{miranda_fire_2002} and their biodiversity \citep{durigan_no_2020}. 

In the Cerrado, fire occurs naturally because of lightning as well as in relation to human activities. Among the human uses of fire there is a wide variety of different practices, including traditional uses by indigenous peoples \citep{pivello_use_2011, mistry_community_2016} and pastoral communities \citep{eloy_seasonal_2019}, the use of fire as part of fire management practices \citep{schmidt_fire_2018}, or the use of fire associated with modern agricultural activities \citep{pivello_use_2011}. All these human practices create fire regimes that differ from the naturally ignited ones, particularly in terms of frequency and seasonality \citep{miranda_fire_2002}. The extent to which these different regimes contribute to maintaining the Cerrado or act as degrading agents is a matter of active research. In fact, until recently, the use of fire was prohibited across Brazil regardless of the type of ecosystem \citep{schmidt_fire_2020, durigan_zero-fire_2020}. These policies were the result of a mix of colonial legacies \citep{miranda_efeitos_2010, pivello_use_2011}, the extended use of fire as a deforestation tool in fire-sensitive ecosystems \citep{barlow_clarifying_2020, lapola_drivers_2023, pivello_use_2011}, and misunderstandings about the ecology of savannas \citep{durigan_zero-fire_2020}. Over the last few decades, however, there has been a push to (re)introduce the use of fire as a conservation and fire management tool \citep{miranda_efeitos_2010, schmidt_fire_2018, mistry_new_2019, mistry_community_2016, durigan_zero-fire_2020, durigan_need_2016}, primarily after the observation of degrading woody encroachment processes \citep{rosan_fragmentation-driven_2022, stevens_savanna_2017, durigan_need_2016}, the occurrence of devastating fires in some protected areas as a consequence of fuel build up \citep{fidelis_year_2018, batista_evaluation_2018}, and the increasing recognition of indigenous and traditional knowledge about Cerrado's ecology and their uses of fire \citep{eloy_seasonal_2019, mistry_new_2019, welch_indigenous_2013}. These processes and efforts have recently materialised in a Brazilian National Policy on Integrated Fire Management (Lei 14,944 from 31$^\text{st}$ July 2024) promoting the use of fire to manage fire-adapted ecosystems for conservation, as well as regulating its use in private lands. Still, the use of fire associated with industrial agriculture is considered to be the main cause of fire in the Cerrado \citep{pivello_use_2011}, with complex effects on fire dynamics \citep{silva_drivers_2020, silva_putting_2021}. 

Indeed, over the past five to six decades, the Cerrado has undergone a rapid conversion to agricultural land uses, totalling a loss of around 50\% of its original native vegetation \citep{sano_land_2010, alencar_mapping_2020}. This expansion has brought about widespread increases in landscape fragmentation \citep{rosan_fragmentation-driven_2022, da_silva_arruda_assessing_2024} and nowadays only a handful of regions with continuous Cerrado vegetation remain - including the areas with some degree of protection, about 12\% of the Cerrado \citep{carranza_protected_2014}. Simultaneously, the Cerrado has also experienced a warming and drying process in relation to climate change \citep{hofmann_brazilian_2021, hofmann_changes_2023, oliveira_trends_2014}. These changes have been particularly acute during the dry season \citep{hofmann_brazilian_2021, hofmann_changes_2023}, when most Cerrado fires take place \citep{mataveli_satellite_2018, silva_putting_2021, da_silva_arruda_assessing_2024}. 

Concomitantly, the fire regimes of the Cerrado have also changed over the past few decades \citep{mataveli_satellite_2018, silva_putting_2021, da_silva_arruda_assessing_2024}. When considering the Cerrado as a whole, no clear trends in burned area are observed over the past four decades, while fire size seems to have increased over this period \citep{da_silva_arruda_assessing_2024}. However, when exploring changes at smaller scales, a wealth of different patterns become apparent \citep{mataveli_satellite_2018, silva_putting_2021, da_silva_arruda_assessing_2024}. In particular, many studies report opposing trends in burned area, increases and decreases, in different parts of the Cerrado \citep{mataveli_satellite_2018, silva_putting_2021, da_silva_arruda_assessing_2024}. In addition, there are remarkable differences among Cerrado's ecoregions in terms of fire size and intensity \citep{silva_putting_2021} or fire frequency \citep{da_silva_arruda_assessing_2024}. These observations stress the need to characterise the Cerrado's fire regimes at small scales to prevent the fire dynamics of a particular area from dominating the overall signal of the Cerrado. Further, these regional differences also suggest a complex interplay between the two drivers of fire, humans and the climate, and other landscape characteristics like the type of vegetation \citep{silva_drivers_2020}. 

Overall, the current state-of-the-art indicates that the ongoing alteration of Cerrado's fire regimes are associated with the large-scale conversion of Cerrado's native vegetation to croplands and pastures in confluence with climate change \citep{mataveli_satellite_2018, silva_putting_2021, da_silva_arruda_assessing_2024}, putting further pressures on this highly threatened region. There remain, however, many open questions regarding the mechanisms by which humans and the climate are altering the Cerrado’s fire regimes, the extent of the human footprint on the patterns of fire across the Cerrado, and how the current patterns of fire may differ from naturally ignited ones. The fire regimes of the Cerrado and the processes they are undergoing in relation to modern human activities and the climate are the focus of the present thesis.



\subsection{Knowledge gaps}

Several studies across savannas indicate that the patterns of decreases in burned area observed in some regions are related to the expansion of agricultural area and the concomitant fuel discontinuities and fire controls it brings about \citep{andela_recent_2014, rosan_fragmentation-driven_2022}. On the other hand, increasing temperatures or changes in precipitation are argued to be driving increases in burned area in other areas \citep{andela_recent_2014, silva_drivers_2020}, especially where most savanna vegetation remains \citep{mataveli_satellite_2018}. Further, it is suggested that, in general, human controls on fire activity through landscape fragmentation are stronger than climatic effects, both in the Cerrado \citep{conciani_human-climate_2021} and other savannas \citep{andela_recent_2014, archibald_climate_2010}. However, the interaction between these two drivers of fire has not yet been quantified. 

Disentangling the effects of land use and the climate appears key to understanding the mechanisms underpinning the puzzle of spatial and temporal patterns of burned area observed in the Cerrado and other savannas. In addition, a better comprehension of whether these two determinants of fire are really pushing burned area in opposite directions and how their effects combine can also help the interpretation of changes in other characteristics of fires in the Cerrado. Quantifying the effects and the interaction between land use area and various climatic variables on Cerrado's burned area will be the focus of \textbf{Chapter \ref{ch:paper01} (Research Paper 1)}.

The fire regimes of the Cerrado have been previously characterised for each of its different ecoregions \citep{silva_putting_2021, da_silva_arruda_assessing_2024}, finding a striking heterogeneity of fire frequencies, sizes, intensities and burned areas \citep{silva_putting_2021, da_silva_arruda_assessing_2024}. However, these ecoregions may not correspond to the actual areas of the Cerrado presenting similar characteristics of fire. Moreover, studying fire regimes based on pre-established regions can hamper the analysis of the environmental factors that may explain the patterns of fire \citep{archibald_defining_2013}. In contrast, methods to identify and delineate regions with common fire regimes based solely on the spatial patterns of fire characteristics can overcome these limitations \citep{archibald_defining_2013, pais_global_2023, chuvieco_global_2008}, but have not been used to study the Cerrado's fire regimes. Further, the profound changes the Cerrado has experienced over the last four decades may imply that the geographical configuration of its fire regimes may have changed over this period, something that has also not yet been explored. Identifying the different regions of the Cerrado with different fire regimes and their changes in time will be the focus of \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}.

Finally, there is a major outstanding question in the study of fires in the Cerrado regarding the cause of fire ignitions. The fact that humans are currently the main cause of ignitions in the Cerrado and other savannas is a commonly held perception \citep{archibald_what_2009} stemming from expert's knowledge about modern uses of fire \citep{pivello_use_2011}, and the temporal mismatch observed between the fire and lightning seasons \citep{schumacher_characteristics_2022}. However, studies in some Cerrado protected areas reported the occurrence of lightning fires and suggested that these may be more frequent than usually assumed \citep{ramos-neto_lightning_2000, pereira_identificacao_2005, medeiros_incendios_2004, fiedler_ocorrencia_2006}.

Identifying whether lightning or humans are the ignition cause of fires would represent a major advance in the study of Cerrado's fire regimes. First, it would allow to quantify by how much the number of fires ignited by humans exceeds that of fires caused by lightning strikes, as well as comparing the timing, location and behaviour of the fires caused by each type of ignition.  In short, the identification of fires by ignition cause would contribute to understanding how much human-caused fire regimes differ from those naturally ignited. At the same time, it would shed light on the causes of high fire occurrences in the more intact areas of the Cerrado, with implications for fire management policies beyond the Cerrado's protected areas. The quantification of lightning and human-caused fires and the analysis of the corresponding fire regimes will be the focus of \textbf{Chapter \ref{ch:paper03} (Research Paper 3)}.



\section{Thesis aims and objectives}

The overarching aim of this thesis is to improve our understanding of how fire has been occurring across the Cerrado over the past few decades shaped by human activities and the climate, the two main drivers of fire. This research aim is addressed through three guiding objectives, each corresponding to a research article:

\textbf{Objective 1: Determine how land use area and the climate interact to affect the burned area of the Cerrado.}

This objective is the main aim of \textbf{Chapter \ref{ch:paper01} (Research Paper 1)}, and was addressed through the following research questions:
\begin{enumerate}[\bfseries 1)]
    \item In the Cerrado, what is the causal effect of the extent of land use area in a landscape on the percentage of burned natural area?
    \item How does land use area percentage interact with the climate to affect the burned natural area of the Cerrado?
    \item What are the burned natural area trends across the Cerrado? Are these trends caused by land use expansion and climate change?
\end{enumerate}
    
\textbf{Objective 2: Explore the geographical variation in the patterns of fire across the Brazilian Cerrado, their temporal evolution, and their relation to the climate and human activities.}

This objective is the main aim of \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}, and was addressed through the following research questions:
\begin{enumerate}[\bfseries 1)]
    \item What are the regions of the Cerrado presenting different fire regimes?
    \item What are the environmental variables that characterise these spatial patterns of fire?
    \item How have these fire regimes changed through time?
\end{enumerate}
    
\textbf{Objective 3: Investigate the contribution of lightning and human-ignited fires to the Cerrado's current patterns of fire and compare the fire regimes created by each type of ignition.}

This objective is the main aim of \textbf{Chapter \ref{ch:paper03} (Research Paper 3)}, and was addressed through the following research questions:
\begin{enumerate}[\bfseries 1)]
    \item What proportion of fires in the Cerrado are ignited by lightning strikes and by humans? Do these proportions change throughout the year?
    \item How does the balance between lightning and human-ignited fires change across the Cerrado?
    \item What are the differences in fire behaviour and fire frequency between lightning and human-caused fires?
\end{enumerate}


\section{Thesis structure}

This thesis is presented according to the requirements for the University of Oxford's Doctor of Philosophy in Geography and the Environment and follows the article-based format. This thesis is organised into 6 chapters. This chapter (\textbf{Chapter \ref{ch:intro}}) introduces the thesis background and sets out the main overarching research aim, and the specific objectives and research questions formulated to address the general aim. \textbf{Chapter \ref{ch:lit_rev}} contextualises this thesis by first introducing the Brazilian Cerrado, the study area on which this thesis is focused, and then providing the background knowledge needed to understand the context and interpret the results of this thesis, and identifying the research gaps that motivate the research questions addressed in this thesis.

\textbf{Chapter \ref{ch:paper01} (Research Paper 1)} focuses on the question of how the extent of land use area and the climate interact to explain burned area and the heterogeneous patterns of increases and decreases in burned area across the Cerrado in the period 1985 to 2020. This objective is addressed using a causal inference framework and a Bayesian multilevel modelling approach using geospatial data on burned area and various climatic variables on a 0.2$^\text{o}$ grid over the Cerrado.

Once we have gained an understanding about the interplay between the climate and the extent of human land uses in the landscape, we expand our view by quantifying fire regimes as a whole - that is, including measures of frequency, fire size and seasonality, apart from burned area. Thus, in \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}, we identify the different regions of the Cerrado presenting different fire regimes focusing on two different periods, 1985-1993 and 2012-2020, with the aim of understanding how the patterns of fire vary across the Cerrado, as well as through time. We delineate the different regions using a clustering algorithm with a spatial contiguity constrain on a 30 km grid over the Cerrado. Then, we use statistical ordination methods to explore the characteristics of these regions in terms of fire, climatic and anthropic characteristics, and how they have changed through time.

Then, in \textbf{Chapter \ref{ch:paper03} (Research Paper 3)}, we use an ignition-cause attribution method to identify those Cerrado fires most likely caused by lightning in the period 2019-2023 . The aim is to determine the contribution of lightning-caused fires to the Cerrado's current fire regimes. As well, we investigate whether and how the patterns and behaviour of fires ignited by humans may differ from those ignited by lightning. 

Finally, \textbf{Chapter \ref{ch:conclusion}} concludes this thesis by presenting a synthesis of results and the implications of this research, and briefly discusses future research avenues and outstanding questions identified by the thesis.

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