Near-complete loss of fire-resistant primary tropical forest cover in Sumatra and Kalimantan

Indonesia, specifically Sumatra and Kalimantan, has experienced alarming rates of deforestation over the past five decades, driven largely by commodity production. This deforestation has dramatically increased the frequency and intensity of fires, especially in carbon-rich peatlands. The 1997-1998 and 2015 fire events resulted in substantial greenhouse gas emissions and severe regional haze, posing a significant challenge to Indonesia's commitment to reduce emissions. Intact tropical forests typically resist fire due to their closed canopy, high humidity, and evaporative cooling. However, deforestation alters this, creating drier, warmer microclimates more susceptible to ignition. The conversion of peatlands, containing a massive carbon store accumulated over millennia, to agricultural land through drainage further exacerbates this risk. While the impacts of deforestation on fire susceptibility are well-known, a region-wide assessment of remaining primary forest's fire resistance has been lacking. This study aimed to fill this gap by assessing primary forest loss and fragmentation in Sumatra and Kalimantan (2001-2019) and analyzing its correlation with fire occurrences, to determine the extent to which primary forests still function as a fire barrier and protect peatland carbon stocks.

The existing literature highlights the rapid rate of deforestation and the associated increase in fire occurrence in Sumatra and Kalimantan. Studies such as those by Curtis et al. (2018), Austin et al. (2019), and Margono et al. (2014) quantify the extent of forest loss and its drivers. Research on peatland degradation (Miettinen et al., 2012; Page & Hooijer, 2016) emphasizes the vulnerability of these carbon-rich ecosystems to fire. The role of El Niño and the Indian Ocean Dipole in influencing drought conditions and fire severity has also been extensively studied (Field et al., 2016; Pan et al., 2018). However, a comprehensive assessment of the fire resistance of the remaining primary forest cover at a regional scale, integrating forest cover changes with fire occurrence data, was lacking prior to this study.

This study used a combination of publicly available datasets to assess primary forest cover change and fire activity in Sumatra and Kalimantan from 2001 to 2019. Primary forest extent in 2000 was determined from a dataset based on multi-temporal Landsat imagery analysis, defining primary forest as old-growth natural forest in contiguous blocks of at least 5 hectares, not deforested recently. Annual forest cover loss from 2001 to 2018 was derived from a global forest cover loss dataset, identifying tree cover loss as stand replacement disturbance. The analysis was performed at both the Landsat pixel level and aggregated to a 1km resolution grid for easier correlation with fire data. Four primary forest cover categories were defined based on percentage cover within each grid cell: undisturbed (over 99%), partially deforested (50-99%), mostly deforested (1-50%), and fully deforested (under 1%). MODIS Collection 6 fire locations (MCD14ML) data from NASA were used as a proxy for fire activity. To reduce false positives, only high-confidence detections and those part of larger, persistent fire events were included. The study also used high-resolution maps of Indonesian peat distribution to distinguish between peatland and non-peatland areas, and ECMWF ERA5 reanalysis precipitation data to determine monthly precipitation anomalies. Distance from the forest edge was calculated for undisturbed forest grid cells to assess edge effects.

The study found a dramatic reduction in primary forest cover in both peatlands and non-peatlands of Sumatra and Kalimantan between 2001 and 2019. While primary forest loss rates decreased in the later years of the study period, potentially due to policy changes such as peatland restoration plans and moratoriums, a significant amount of the remaining forest is severely fragmented or degraded due to proximity to forest edges. The analysis showed a strong correlation between primary forest cover and fire occurrence. Undisturbed primary forests (over 99% cover) experienced very low fire occurrence rates (0.9% in peatlands and 0.09% in non-peatlands annually), while fire-affected areas increased drastically in areas with less forest cover. In drought years, fire-affected areas significantly increased in deforested areas but remained low in undisturbed forests. The study revealed that fires rarely penetrated beyond 2 km from the forest edge, even during severe droughts, highlighting the resilience of undisturbed primary forests to fire. However, only a small fraction of the remaining primary forest falls into this category (3% of peatlands and 4.5% of non-peatlands by 2018). This implies that despite the seemingly significant extent of remaining primary forest, the majority is under high fire risk due to fragmentation and edge effects. Up to 97% of peatlands are now considered high-fire-risk areas, significantly exacerbating the risk of carbon emissions from these crucial ecosystems.

This study's findings underscore the significant threat posed by deforestation and fragmentation to the fire resilience of Indonesian primary forests. The near-complete loss of fire-resistant primary forests, particularly in peatlands, highlights the urgent need for effective conservation and restoration efforts. The limited extent of undisturbed forests decoupled from the influence of regional climate variability in driving fire occurrence, emphasizing that the majority of remaining forests are vulnerable. The high correlation between fire occurrence and proximity to the forest edge demonstrates the crucial role of edge effects in amplifying fire risk, even in areas with significant remaining primary forest cover. The results also suggest that policy measures implemented in recent years have shown some effectiveness in slowing down deforestation but not enough to overcome the already vast and highly flammable regions of converted forest. This study's results are relevant to global efforts to mitigate climate change, biodiversity conservation, and human health, given the regional and global impacts of peatland fires.

This research demonstrates the near-complete loss of fire-resistant primary tropical forest in Sumatra and Kalimantan, leaving a highly fragmented landscape vulnerable to fire. The resilience of undisturbed forests emphasizes the vital role of forest protection and restoration in mitigating fire risks and protecting peatland carbon stocks. Future research could focus on the effectiveness of different restoration techniques in enhancing fire resilience and identifying optimal strategies for managing fragmented landscapes. Continued monitoring of forest cover change and fire activity, combined with detailed investigation of specific drivers of fire ignition, are also vital for effective conservation management.

The study relied on satellite-based data for fire detection, which may have limitations in accurately capturing all fire events, particularly small or low-intensity fires. The use of MODIS data, with its inherent limitations in spatial resolution, might have resulted in some underestimation or overestimation of fire extent in smaller, highly fragmented forest patches. The analysis focused on fire occurrence, not the severity or duration of the fires, which are crucial factors influencing ecological impacts and carbon emissions. Future studies with higher resolution data and more detailed analysis of fire severity will be useful.