Intensification of fire regimes and forest loss in the Território Indígena do Xingu

For much of its history, forest fires in the Amazon basin occurred at intervals of 100 years (Sanford et al 1985, Meggers 1994, Bush et al 2007). Records from pre-Columbian times and for much of the twentieth century suggest that forest canopies have functioned as ecological firebreaks by creating a moist understory microclimate (Sanford et al 1985, Piperno 1997, Nepstad et al 2006, Bush et al 2007). Although pre-Columbian Indigenous populations used fire to manage their lands (Heckenberger et al 2003, Levis et al 2018), they also employed advanced techniques to prevent them from escaping into primary forests (Heckenberger et al 2003). Such ancient knowledge likely prevented widespread forest fires even during dry warm years, when Amazon forests tend to be more flammable. In recent decades, however, global changes have altered several key drivers of forest flammability, particularly in southern Amazonia (Alencar et al 2004, Brando et al 2014). The resulting intensification of fire regimes in the region may be pushing forests beyond a flammability tipping point, allowing wildfires to spread over vast areas (Pueyo et al 2010, Brando et al 2020a) with negative consequences for the region's local populations.

As climate changes in the Amazon region, forests have become increasingly susceptible to fire—including the protected areas (Indigenous lands and conservation units) that have long worked as large-scale firebreaks. In the past 20 years, extreme drought events have triggered major forest fires in the Amazon region even as deforestation rates have declined (Brando et al 2014, Aragão et al 2018), probably because human sources of fire ignition are so abundant in the landscape. There is growing evidence that forest degradation—areas with lower canopy cover, due in part to fire and drought stress—has now surpassed deforestation throughout much of the Amazon biome, both in terms of area and carbon emissions (Baccini et al 2017, Matricardi et al 2020, Kruid et al 2021). An outstanding question, though, is whether widespread forest fires will occur at the same rate inside Indigenous lands and conservation units, given the projected increases in regional droughts (Duffy et al 2015) and associated decreases in the forest's capacity to maintain cool, moist understory microclimates (Brando et al 2020b).

Conservation units and Indigenous lands have proven effective in preventing deforestation and forest degradation historically, but rapid climate changes and more frequent anthropogenic disturbances now pose serious threats to these protected forests (Walker et al 2020). Such novel disturbance regimes can cause major shifts in forest communities, potentially pushing the ecosystem towards another stable state (Hirota et al 2011, Scheffer et al 2015, Trumbore et al 2015, Flores et al 2017). As a result, forests may lose their capacity to store biomass and other crucial ecosystem services, such as stabilizing local and regional climate, or providing food and medicine for Indigenous communities living in the region (Levis et al 2017).

If such near-term shifts in ecosystem stable states were to occur in Amazonia, the southeastern region would likely experience the first, most drastic changes in vegetation structure, functioning, and diversity. The climate of this region is already relatively drier and hotter than other parts of Amazonia. In addition, large-scale deforestation and global climate changes have already triggered a lengthening of the dry season in the region (Fu et al 2013). Furthermore, several climate models project an even drier and hotter climate for the region in the coming decades (Cox et al 2000, Duffy et al 2015). Such climatic changes increase the likelihood of widespread forest fires that, if left unchecked, could cause abrupt changes in vegetation structure (Brando et al 2014, 2020a). Floodplain forests are particularly vulnerable to such rapid changes in fire regimes, since common understory fires can kill entire tree stands from the root, with limited chances for recovery (Flores et al 2017). Given the intensification of fire regimes in southeastern Amazonia, it is necessary to better understand the extent of forest cover changes and why these changes have occurred.

Forest degradation associated with global changes could have serious consequences for Indigenous communities. In Brazil, for instance, drought-fire interactions now threaten the Xingu Indigenous Territory (TIX, Portuguese acronym)—an iconic 2.8 Mha reserve in the southeastern Amazon, that is home to over 6000 Indigenous people from 16 distinct ethnic groups. The TIX has experienced rapid climatic changes due to high deforestation rates outside the park since the 1970s (Silvério et al 2015). Since the 2000s, it has also contended with more frequent and intense droughts, and widespread forest fires, marking a shift from previous decades. Although climate models predict an average increase in dry season length and a consequent decrease in average precipitation in the TIX, we still know relatively little about how interactions between droughts, fires, and land management by Indigenous populations will affect the future of these forested protected areas.

Here, we evaluated forest cover changes in the TIX from 2001 through 2020, and whether fire occurrence and extreme drought events played a role in the observed changes. Our two hypotheses were that: (a) forest degradation inside the TIX is increasing due to increased frequency of forest fires, drought events, and slash-and-burn agriculture as populations grow; and (b) forest degradation is higher in seasonally flooded forests than in upland forests, independent of ignition sources.

2.1. Study area

The study area was the Território Indígena do Xingu (TIX), located in northern Mato Grosso state, Brazil. The TIX comprises the Xingu Indigenous Park (2.6 Mha) and three other Indigenous Lands (TI): Batovi (5159 ha), Pequizal do Naruvôtu (27 980 ha) and Wawi (150 328 ha). Together, these areas form a mosaic of 2.83 million hectares (figure 1), home to 16 ethnic groups representing seven distinct language families (ISA 2011). The total population of the TIX was 3331 people in 1985 (UNIFESP (Universidade Federal de São Paulo) 1983) and increased to 6326 people by 2017 (SIASI/SESAI 2017) (figure 2).

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The soil of the region is mainly typical dystrophic red–yellow latosol and typical dystrophic floss neossol (Santos et al 2018). The Xingu River basin covers approximately 520 000 km² (figure 1(A)), with an average flow rate of between 2582 and 9700 m³ s⁻¹ (Latrubesse et al 2005). The average air temperature is 26 °C in summer and 28 °C in winter, with annual precipitation varying between 1485 and 2547 mm.

2.2. Datasets and analysis

The forest cover in the TIX dropped 7% from 2001 (25 930.42 ± 1,179.09 km²; ±CI) to 2020 (24 044.34 ± 1,104.05 km²) (figures 1, 2 and S2). The annual rate of forest loss between 2001 and 2015 averaged 0.23% (6512 ha yr⁻¹), but that rate increased to 4.13% (116 941 ha) and 1.93% (54 648 ha) in 2016 and 2017, respectively. Additionally, approximately 16% (453 041 ha) of the TIX area burned at least once between 1984 and 2017, and 12.5% (353 939 ha) burned more than once. Between 1984 and 2006, the average annual burned area was 0.3% (8484 ha yr⁻¹) of the TIX but it increased to 3.4% (96 271 ha yr⁻¹) between 2007 and 2017. The peak annual burned area occurred in 2010, when more than 10% of the TIX's forests burned (figures 1 and 2).

Our analysis suggests that several variables influenced the rate of forest cover loss inside the TIX. All six variables (forest type, fire frequency, population density, distance from rivers, and droughts frequency) were important predictors of forest loss from 2001 to 2020 in our final statistical model (table S5). Fire occurrence was the most important predictor of forest cover loss, but interactions between fire, droughts, and forest type were also significant (table S5). Our full model explained 22% of the observed decline in forest cover (pseudo r² = 0.22; table S5).

The recurrence of forest fires during drought (vs. non-drought) years, and in flooded (vs. upland) forests, strongly influenced the likelihood of forest loss (figure 3). Forests that did not experience extreme droughts, even if they were burned multiple times, had much higher probability of maintaining higher canopy cover compared with forests that burned multiple times during drought years (figure 3(B)). In flooded forests, the interaction between fire recurrence and drought events more than doubled the likelihood of forest losses (figure 3(A)). In the case of upland forests, the incidence of four or more forest fires increased the probability of forest loss to >57%. In the case of flooded forests, just three fire events raised the probability of forest loss to >63% (figure 3(A)). These results indicate that the recurrence of forest fires is a key process driving forest loss in the region and confirms that flooded forests are particularly vulnerable to fire-drought interactions (figure 3).

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Our statistical model indicates that forest cover losses are generally higher for areas with higher population density, particularly in areas that are periodically flooded (tables S5 and S6). Specifically, we observed a higher probability of fire-related forest loss where population density was higher, especially in the case of flooded forests (figure S5). In areas burned once and with low population densities (e.g. ∼5 people km⁻²), the probability of fire-induced forest loss was 20%, but this probability increased to ∼90% when population density increased to 30 people km⁻² (figure S5). For flooded forests, the probability of forest loss was 50% at low population densities and nearly 100% at high population densities (figure S5).

While old forest cover near villages was relatively low in 2001, it declined even further by 2020 (figures 4 and S6). For instance, the forest cover in areas close to the villages (1 km away) decreased from 82% in 2001 to 54% in 2020. Forest cover losses during this interval decreased as distance from the villages increased. For example, areas up to 10 km away from villages showed a reduction of only about 21% in forest canopy between 2001 and 2020 (figure S6).

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Our land cover classification for the entire TIX showed that the area covered by mature forests (e.g. forests with no evidence of recent disturbances) decreased from 70% in 2001 to 54% in 2017 (figure 4(A)). This reduction in mature forest cover was accompanied by an increase in old secondary forest (from 19% in 2001 to 26% in 2017) and in grasslands (from 3% in 2001 to 8% in 2017; figure 4(A)). Young secondary forests also increased from 6% in 2001 to 10% in 2017 (figure 4(A)). These transitions in land cover type were strongly influenced by distance from the villages. We observed more transitions from primary forests into old secondary forests and grassland near villages (figure 4(B)). As distance from the villages increased, likewise, primary forests became more dominant and younger secondary forests and less dominant. Finally, the grassland class increased between 2001 and 2017, but probability of finding it decreased slightly with distance from the villages (figure 4(B)). This shows that areas with greater influence on Indigenous activities may not be recovering fast enough to become mature forests.

Our study investigated recent patterns and processes associated with forest degradation in the TIX during the 21st century. Our results point to an acceleration of forest canopy loss (our proxy for forest degradation) in the TIX between 2001 and 2020. We show that the main drivers of forest loss were associated with drought-fire interactions and distance from larger villages. Our findings also indicated that the probability of forest loss was substantially higher in flooded forests compared with upland ones. These synergistic effects of multiple natural and anthropogenic factors are already driving large-scale forest degradation across one of the world's largest Indigenous lands. Avoiding further large-scale forest mortality as the climate changes will require decisive new approaches to fire management in the region. Indigenous populations of the TIX have a profound knowledge of fire management, including practices to prevent fires from escaping into primary forests (Heckenberger et al 2003). Some of these practices could help to prevent widespread forest fires, including the expansion of the fire brigades in the region.

Projections of future forest cover in the TIX rely mostly on process-based models that lack a realistic representation of forest fires or drought-fire interactions. Nevertheless, fire recurrence during droughts was the primary determinant of forest loss in the TIX in our study. After three consecutive fires, for instance, over half of the forests succumbed to a degraded state that could persist for years or decades. One hypothesis is that reduced forest resilience under a changing climate facilitates the establishment of more open vegetation following recurrent fires, with the predominance of grasses (Trumbore et al 2015, Nobre et al 2016). Given that grasses are highly flammable, new fires could prevent forest regrowth, as demonstrated by controlled fire experiments in which grass invasion triggered high intensity forest fires even during non-drought years (Silvério et al 2013, Brando et al 2014). Our study suggests that a similar process is apparently underway in the TIX. Over 350 000 ha of forests burned more than once during the study period, representing over 12% of the TIX's total forested area. Grass invasion into these burnt forests may create a positive feedback in which fires facilitate the permanent establishment of flammable grasses, especially if the climate changes—associated both with land-use change and greenhouse gas accumulation in the atmosphere—continue to reduce forest resilience to disturbances (Cochrane and Schulze 1999, Silvério et al 2013, Brando et al 2020a). Hence, fire suppression efforts and controlled fires may be key strategies for fostering forest recovery and driving an overall reduction in vegetation flammability inside the TIX.

The likelihood of forest loss by fire was higher where extreme droughts had occurred. This result indicates that the interaction with other degradation factors (climate change and biotic factors) is already putting pressure on the forests in the region. The projected intensity and frequency of droughts may thus be proxies for the potential strength and extent of future forest losses. Global models predict a new climate regime for the Amazon with longer dry seasons (Malhi et al 2008), higher temperatures, and a greater vapor pressure deficit (McDowell et al 2018, Barkhordarian et al 2019), suggesting that the forests of the region will experience greater water stress. The result could be widespread tree mortality, especially when water stress is accompanied by intense fires. Large trees, which store the highest amounts of carbon and contribute to climate regulation through evapotranspiration, are also the most vulnerable to these extreme droughts (Rowland et al 2015). In burned forests, they also become extremely vulnerable to disturbances such as windstorms and other edge effects associated with a more open canopy (Silvério et al 2019). As these trees die and lose foliage during extreme droughts, they add a large amount of fuel to the forest floor, which again favors the occurrence of high intensity forest fires (Nepstad et al 2004, Brando et al 2014, Balch et al 2015). Such interactions between drought events and forest fires can amplify forest degradation, pushing the ecosystem to a 'grassy' stable state, maintained by a positive fire feedback (Hirota et al 2011, Staver et al 2011).

Our study corroborated previous ones documenting the greater vulnerability of seasonally flooded forests compared to upland forests (de Resende et al 2014, Flores et al 2014). The higher flammability of flooded forests during droughts is attributed to a greater amount of fuel material that can easily spread smoldering ground fires (Flores et al 2014, Almeida et al 2016), a process that is consistent with our results. Moreover, in flooded forests, tree recruitment tends to be limited by soil impoverishment and the flooding period may restrict plant growth (Flores et al 2017, 2020). The new precipitation regime, already evident in the southeastern Xingu Basin, thus has immediate repercussions for the future of flooded forests (due to their lower resilience compared to surrounding upland forests) and places them at greater risk of transitioning to herbaceous vegetation after fires (Flores et al 2017).

In our study, human density in the villages contributed to explaining forest losses, as well as reductions in mature forest area closer to villages in the TIX. Two processes may explain these results. First, many Indigenous peoples used to live a semi-nomadic lifestyle centered around small, temporary settlements across the landscape. As these groups have transitioned to living in fixed villages, they have also intensified subsistence agriculture near the villages (Schmidt et al 2021). This process relates partly to limited availability of suitable lands for new villages as the TIX population expands, but also to the fact that some of these groups were resettled in the TIX in the early 1970s. Second, as the fire regime intensifies (i.e. more frequent, intense fires) it has increased the time needed for secondary forests to recover after abandonment from slash-and-burn agriculture, as found in a recent study conducted at the Kawaiwete and the Ikpeng villages in the TIX (Schmidt et al 2021). Deforestation outside Indigenous lands can also increase fire ignition sources and contribute to regional climate changes (Silverio et al 2015), which may overwhelm the inhibitory effect that tropical forests have historically played in reducing sources of ignition and acting as firebreaks in the region (Staver et al 2011, Brando et al 2014). This regional connectivity underscores the importance of preserving tropical forests both inside and outside conservation units and Indigenous territories.

Our results revealed that drought-driven fires already pose a serious threat to forest integrity in the TIX. The flooded forests of the TIX are less resilient and more vulnerable to fire degradation than upland forests, placing them at greater risk of transitioning to herbaceous vegetation. Forest losses were also associated with a higher human density and distance from the villages, since human populations occupy vulnerable areas (flooded forests) and can contribute to increasing ignition sources. If the trends observed here continue, they will exacerbate the negative effects of extreme climatic events on the Xingu region, increasing the intensity and frequency of fires, and transforming the region's forests. The persistence of these forests will ultimately depend on adapting the ancient fire management practices of Indigenous peoples to novel climatic conditions in the region. Reducing ignition sources and the accidental spread of forest fires will be crucial, both from the surrounding landscape (e.g. agricultural lands) and from within Indigenous territories.

Our findings reveal how the TIX is now exposed to unprecedented drought and fire regimes, which have driven cumulative forest cover losses. Moreover, increasing population density inside the TIX implies that protecting the culturally and biologically diverse ecosystems of the Xingu may require the expansion of the area or existing protections on Indigenous territories to help mitigate forest losses in the region. This highlights the need for holistic and interdisciplinary analyses to understand how the interaction between current climate changes and human activities are modifying tropical forest ecosystems. Such an integrated understanding could contribute to the design of new management strategies to prevent vegetation loss and minimize the negative consequences for Indigenous peoples, who depend on forest resources for their livelihoods.

We thank all members of the research team at the Instituto de Pesquisa Ambiental da Amazônia (IPAM), the Instituto Socioambiental (ISA), and the Graduate Program of Ecology and Conservation at the Universidade do Estado do Mato Grosso (UNEMAT). This study was supported by grants from the Conselho Nacional de Desenvolvimento Científico e Tecnológico—CNPq/Prevfogo-Ibama (#442710/2018-6) and the Gordon and Betty Moore Foundation (#9957). CNPq provided a research scholarship to M T F (#380219/2021-2) and the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES/BRASIL; FINANCE CODE 001) provided scholarships to R S O and H K A.

The data that support the findings of this study are available upon reasonable request from the authors.