Fragmentation of natural forest cover generates significant biodiversity loss and negatively affects ecosystem functioning [1
]. Forest fires, in addition to their fragmentation, are one of the main factors driving the degradation of tropical forests, especially in dry years when these regions become a net source of carbon [2
]. In South America, a significant increase in forest fires and burning activity has been detected during the period 2001–2018, and in 2019 a severe fire crisis was recorded, strongly linked to deforestation and forest degradation [3
]. It should also be noted that in some regions the severity of fires has been linked to severe drought events [4
Prior investigations conducted in Brazil [7
] have shown that forest edges resulting from human-induced landscape fragmentation are highly vulnerable to forest fires as a consequence of stronger water limitations and the accumulation of higher fuel mass compared to the areas located in the interior of natural forest. In addition, forest edges are close to potential ignition sources in regions managed for agricultural and livestock uses [5
In recent decades, anthropogenic disturbances have increased in the forested areas of Eastern Bolivia, especially in the Department of Santa Cruz. This is related to the expansion of agriculture and livestock [15
], which has caused an increase in landscape fragmentation [18
]. Using measures based on a landscape hypsometric curve, Maillard et al. (2020b) [22
] determined that the fragmentation of forest areas in the period from 1986 to 2016 increased by 1.4%, while the structural connectivity of the forest core area decreased by 11.8%, due to human activities.
In 2019, due to Bolivian agricultural and economic policies implemented in recent years [23
] and in connection to the occurrence of a very severe meteorological drought, it was a high frequency of forest fires. As a result, the extent of the burned area was three times greater than that in 2018, and 51% higher than the average for the period 2001–2018 [3
]. In the Department of Santa Cruz, the most impacted areas were located in the Chiquitania region. Forest fires mostly impacted ecosystems of the Chiquitano Dry Forest, Cerrado, Chaco and vegetation associated with the Pantanal wetlands [24
], with different levels of fire intensity [25
So far, no studies explored the relation between the effect of forest cover loss and the incidence of forest fires at a landscape scale in Bolivia, particularly at the frontiers of active deforestation, where interactions between deforestation, forest fragmentation and fires are more evident. This becomes more relevant in tropical dry forests due to their climatic seasonality, accumulation of dry biomass during the dry season and interactions with human activities; all these characteristics generate favorable conditions for the occurrence of forest fires [26
In this study we relate the metrics of spatial configuration of forest cover and the severity of meteorological drought with the incidence of forest fires in 2019. The main objective of the study is to identify the relationships between forest fragmentation and the incidence of forest fires during a particularly dry year in the Department of Santa Cruz, Bolivia.
This study analyzed patterns of fire in 2019 with respect to forest fragmentation and spatial variation of drought severity in the Department of Santa Cruz. The forests fires of 2019 were one of the most devastating since the beginning of records [22
]. Given the total forest size structure, the forest fires mostly impacted the three largest forest patches (>2,000,000 ha). Nevertheless, proportionally the fragmented forests (<20 ha) were the most affected by forests fires. Some studies have shown that selective logging increases accessibility as well as fire risk within intact forests [45
]. Consequently, there is an urgent need for better land use planning and management through public policies in order to maintain the largest forest fragments intact [47
]. Furthermore, forest fragmentation has a strong impact on the overall connectivity across the landscape and therefore presents an increased survival risk and a decrease of habitat quality for many species, especially those that are most vulnerable to changes (e.g., Jaguar) [22
Nevertheless, and for many years, prescribed burning has been a common and cheap practice to clear crop areas or renew pastures in Bolivia [44
]. In some natural areas, fire is necessary to maintain ecological conditions, as in the case of the Cerrado [50
]. However, we found that 60% of the areas burned in 2019 were part of forest formations and this may have negative consequences, especially for forest formations [44
]. Historically, in some regions such as the Chiquitano forest there is a very strong pattern of recurrent fire in anthropogenic areas [52
], being one of the main causes of the forest degradation [17
]. In addition, degraded forests increase the risk of forest fire spread.
Four conditions have been characterized as switches for forest fires to occur [53
]: the presence of fuel (biomass), the dry or humid conditions of the biomass available to burn, weather conditions (low humidity combined with high temperatures and high wind speeds) and an ignition source. However, in the Amazon region [8
], it is known that the probability of the occurrence of forest fires can change with the distance to the edge of the forest fragment in relation to the core of that fragment. Within a few days of logging operations, drastic changes in humidity and temperature levels are evident in relation to the interior of the forest [55
]. Low fuel moisture content increases the probability of ignition, the rate of fire spread and the fire intensity [56
]. Intact forests show microclimates with less penetration of wind and solar radiation. Forest edges show high tree mortality, while fragmented forests are more susceptible to the occurrence of fires due to the edge effect [7
]. This would explain that in the Santa Cruz region we have found that distances of less than 500 m to the forest edge (range of 5 km) show a higher percentage impacted by forest fires (71%). A study conducted by Bounoua et al. (2004) [61
] in the Santa Cruz lowlands suggested that the conversion of forests into farmland involves morphological changes in the vegetation, which generates a decrease in the conductance of the canopy and consequently an increase in the local temperature of approximately 2 °C, which could additionally increase the risk of the forest fire expansion.
We should note with an increase of the fire risk in dry forests, such as Chaco and Chiquitano forest, the risk of fire increases given the climate trends, but also because of the introduction of non-native grasses and shrubs into the livestock and agricultural production matrices that surround dry forests [62
]. Their presence increases the combustible biomass [63
] and might change forest fire regimes [64
]. Probably most of these changes would mostly affect the edges of the forest, which would explain the higher frequency of burned areas found in the Santa Cruz region.
Small, isolated forest fragments are more likely to disappear compared to larger fragments [59
]. In the Department of Santa Cruz, small forest fragments (<20 ha and 20–200 ha) were severely impacted by fires in 2019 (27.2%) and many of these are fragments resulting from the advance of the agricultural frontier. Among these fragments, there are those that are left as strips of ecological easements (forests near rivers, streams or lakes) or windbreaks. These windbreaks are strips of trees with natural or planted vegetation between clearings. Directed in a perpendicular direction to the dominant wind and separated by planks (the distance between one windbreak and another). In the property management plans (POP), instruments that zone the lands of a property according to their different capacities of use, both curtains and planks are planned. In the POPs, the current legal administrative resolutions of Bolivia (R.A. ABT 185/2017) stipulate that these curtains should have a minimum width of 30 m (exception for the case of reforestation with >10 m), while the width of the plank is based on the height of the dominant trees and should not be greater than ten times the height of those trees. The relationship between forest fragmentation and the probability of burning found in this study for the year 2019 demonstrates that the current regulations, regarding the configuration of forest fragments (windbreaks, strips of ecological easements and forest blocks in agricultural areas) should be reviewed. An amended regulation may provide improved preventive measures, appropriately addressing forest vulnerability to fire resulting from deforestation of forest fragmentation events. Previous studies show that in the Chiquitano Dry Forest, the reduction of small forest patches generates changes in the composition, structure and floral diversity [68
] demanding for a revision of related public policies.
Considering the new scenario posed by the evolution of the Great Forest Fires that have occurred in the year 2019 in Santa Cruz [69
], there is an increasingly urgent need for adaptation strategies to climate change since droughts are expected to be more frequent and severe [70
]. In 2019, we found a relation between drought severity and the spatial patterns of forests fires, suggesting that drier conditions favor the propagation of forest fires. Due to the accumulation of dry materials, droughts increase the fire risk, in particular in the years of an El Niño event [71
]. In 2010, one of the most extreme droughts was recorded in the Amazon [72
], which also expanded to the Chiquitano Dry Forest [44
]. At the same time an alarming concentration of fires was reported [42
], representing the largest area known to be impacted by fires before 2019.
Consequently, these facts should give rise to agricultural production models that promote integrated practices [77
], prioritizing the maintenance of forest blocks with a larger scale than that currently used. This might imply the revision and rethinking of current regulations to better address and adapt to future climate change projections and predicting increasing forest degradation, which may increment areas burned by forest fires [4
]. For certain areas of the Brazilian Amazon, they are projected to double by 2050, affecting up to 16% of the forests [79
]. Meanwhile, for the Chiquitania region in Bolivia, the interactions between extreme drought conditions and the rapid expansion of the agricultural frontier make the probability of increased fire risk by 2025 up to 1.8 times higher, compared to the 2010 fire event [76
In Bolivia, the ecology of forest fires is a subject that we are trying to understand and there is still much to investigate. We believe that future studies should be aimed at learning more about the dynamics of forest fires, considering the predominance of the direction and intensity of the winds, as well as the combustibility, flammability, calorific value and age of the vegetation, as this will influence the intensity and speed of propagation and, indirectly, the size of the fires.