

\begin{abstract}
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Over the past few decades, the Brazilian Cerrado has experienced profound changes as a consequence of rapid large-scale agricultural expansion and climate change. These processes, coupled with inadequate fire management policies, are altering the fire regimes of these neotropical savannas. In this region of fire-adapted ecosystems, with unique species and extraordinary biodiversity, changes in the patterns of fire could result in ecosystem degradation and irreversible transformations of the native vegetation. 

In this context, understanding how fire occurs in different areas of the Cerrado, how it has changed over time, and how these patterns are affected by humans and the climate, is essential to identify the challenges the Cerrado faces, and to design landscape and fire management strategies that are effective at conserving the unique ecosystems and heterogeneity of this region. This thesis aims to contribute to such understanding by studying in detail different aspects of the Cerrado's fire regimes that remain as major outstanding questions in the literature. This thesis has three objectives, each of which are addressed through a research article.

In \textbf{Chapter \ref{ch:paper01} (Research Paper 1)}, titled \textit{'Human land occupation regulates the effects of the climate on the burned area of the Brazilian Cerrado'}, the objective was to determine how land use area and the climate interact to affect the burned area of the Cerrado's native vegetation. To address this objective, we used geospatial data on burned area, land use categories and various climatic variables covering the period 1985 to 2020, summarised to a 0.2$^\text{o}$ grid covering the whole Cerrado. Using a causal inference framework to design statistical models, this work found that, first, a larger anthropic land use area in the landscape reduces the proportion of Cerrado native vegetation that burns. Further, we showed that landscapes dominated by human land uses ($\geq$40\% land use area) limit the responsiveness of burned area to climatic effects. Therefore, hotter and drier climatic conditions cause noticeable increases in burned area only in the least converted landscapes. Finally, we showed that changes in burned area over the study period took place in those regions of the Cerrado that were mostly unconverted in 1985. In these regions, agricultural expansion caused reductions in burned area over time, while increases in burned area in relation to climate change took place in those areas of the Cerrado that remained more intact.

The objective of \textbf{Chapter \ref{ch:paper02} (Research Paper 2)}, titled \textit{'The fire regimes of the Brazilian Cerrado and their changes through time'}, was to explore the geographical variation in the patterns of fire across the Cerrado, their temporal evolution, and their relation to the climate and human activities. Based again on geospatial burned area data and a 30 km grid cell over the Cerrado, this study used a spatially constrained hierarchical clustering algorithm to identify the different regions of the Cerrado presenting different fire regimes in four consecutive 9-year periods between 1985 and 2020. Focusing on the first (1985-1993) and last (2012-2020) periods, we found that there were considerable differences in the number and geographical limits of the regions identified in each period, with the exception of a region in the north of the Cerrado that was identified in both periods. In addition, in both periods the overall factor differentiating the fire regime regions was their level of fire activity, with some regions presenting large, frequent and numerous fires, while other regions presented intermediate and low levels of these fire characteristics. Further, over time, some regions increased or maintained high levels of fire activity, while other regions experienced striking reductions in their fire activity. Finally, results showed that the level of fire activity was inversely related to the level of human activities in the landscape, and directly related to climatic flammability conditions. 

Finally, the objective of \textbf{Chapter \ref{ch:paper03} (Research Paper 3)}, titled \textit{'Decoupling natural and human-caused fires in the Brazilian Cerrado'}, was to investigate the contribution of lightning and human-ignited fires to the Cerrado's patterns of fire, and compare the fire regimes created by each type of ignition. In this last study, we used a well-established ignition attribution method to identify those fires most likely caused by lightning and by humans. This study found that, between 2019 and 2023, 7.5\% (95\% confidence interval, CI, [6.7, 8.3]\%) of fires were ignited by lightning, 89.1\% (95\% CI, [86.9, 91.3]\%) were ignited by humans, and 3.4\% (95\% CI, [2.7, 4.1]\%) did not have a clear ignition cause. Further, humans were the main cause of fire across the Cerrado, particularly in the regions with larger remnants of native vegetation. In addition, results showed that humans extend three-fold the fire season by creating ignitions throughout the dry season, while lightning-ignited fires were concentrated in the transition period from the dry to the wet seasons. In addition, we found that humans tend to start fires in more flammable conditions than lightning, even when comparing fires occurring in the same month. Finally, we also found humans to cause the most extreme fires observed in this period, about one order of magnitude larger or two times more intense than the most extreme lightning-caused fires. Still, a small but relevant fraction of fires were ignited by lightning, and this type of ignition also caused large and intense fires, even if less so than the human-caused counterparts.

In this thesis, we conclude that humans are the main factor shaping the current fire regimes of the Brazilian Cerrado, both modifying the fuel continuity in relation to land use activities, and through the patterns of fire ignition. Particularly remarkable are the human controls on fire through land use area that limit both fire activity and its responsiveness to climatic effects. In addition, this thesis highlights the opposing directions of change in fire activity undergoing different areas of the Cerrado, both directly related to human activities: agricultural expansion brings about decreases in fire activity, while human ignitions during the dry season may be coupling with climate change giving rise to intensified fire activity in the largest remnants of native vegetation. Finally, this thesis also presents a range of innovative methodologies to harness the possibilities offered by the increasing wealth of data products describing different Earth system factors and processes, particularly satellite-derived datasets.


\end{abstract}