Basten Gokkon – In their latest study, a group of researchers from Indonesia, the U.K., and the U.S. find that heath forests (locally known as kerangas) span 16,586 square kilometers (6,400 square miles) – almost the size of Hawaii's land area – across Kalimantan.
This figure suggests about a 33% loss of forest cover since a 1996 estimate of 24,750 km (9,550 mi ), they write in a paper published July 16 in Cambridge University Press' Journal of Tropical Ecology.
The authors say their research offered a baseline by clarifying the mosaic structure of lowland kerangas and highlighting their distinct role as a unique habitat within the landscape, given limited research on these forests and the resulting gaps in understanding the effects of human pressures and conservation measures.
"Research into kerangas forest coverage in Kalimantan was inspired by our discovery of the Rungan forest landscape's fascinating forest and exceptional conservation value," Namrata Biligeri Anirudh, a Ph.D. ecologist from the University of Indonesia who is the paper's lead author, told Mongabay in an email interview.
Anirudh noted that the landscape was nestled between the Rungan and Kahayan rivers in Central Kalimantan, serving as home to an extraordinary variety of endangered plants and animals, thanks to its rare mix of heath forests on sandy soils, swamps, lowland forests and abundant Borneo ironwood (Eusideroxylon zwageri).
"The fact that such a pristine and diverse ecosystem exists so close to the provincial capital, Palangka Raya, and that this landscape has no forest areas with strict protection status and is characteristic of several different land use types, further highlighted its significance," she wrote.
From 2016-20, the scientists studied 56 plots in the Mungku Baru Education and Research Forest, a 50 km (19 mi ) site in Central Kalimantan managed by Muhammadiyah University of Palangkaraya. The team also mapped the current spread of heath forests in Indonesian Borneo by combining forest cover, soil data and protected area maps.They classified the area's habitats into kerangas, mixed swamp and low pole forest, identifying tree species in the local Dayak Ngaju language and confirming their scientific names. Their findings identified kerangas as the dominant type in both the study site (33.9%) and the broader landscape (27.5%) while tree species diversity varied significantly across habitat types, with mixed swamp habitats showing the highest diversity and low pole forests the lowest.
"We engaged several botanists to review and validate species identifications, ensuring the most up-to-date and accurate taxonomic resolution. In some cases, we revisited specific plots to confirm their locations and cross-check previous records," Anirudh said.
However, the researchers found only 3.7% of total kerangas forest span fell within strictly protected areas, and the vast majority of the extent fragmented across six reserves in Central Kalimantan. They noted that social forestry schemes since 2016 have allowed some villages to manage heath forest patches, providing short-term conservation through community use of non-timber forest products.
In Indonesia, kerangas forests are rapidly disappearing due to land use change, deforestation and other threats, yet they remain largely overlooked in conservation plans despite their urgent need for protection and restoration. The latest paper says the Mungku Baru Education and Research Forest itself is surrounded by oil palm plantations, logging and artisanal mining activities.
Anirudh noted forest fires in 2019 affected parts of the study area and illegal logging targeting Agathis borneensis (a Bornean endemic and characteristic species of the kerangas habitat) arose as a threat to the forest area. She added that artisanal and small-scale gold mining activities began expanding along the bordering rivers, raising the risk of encroachment into the research forest area. She said the sandy soils of these forests attract gold mining that uses mercury, a practice that contaminates land and water, endangering both wildlife and human health.
She said these issues were managed through strong collaboration with the Mungku Baru community members, support from the university's forest management unit and assistance from a local NGO.
The researchers call for more studies on lowland kerangas to be done to guide conservation efforts and push for stronger policy recognition and protection in Indonesia and beyond. They also suggest raising greater awareness of these habitats among local communities, highlighting their ecological and cultural value.
"To date, [kerangas] have received relatively little attention in science, conservation policy, or legal frameworks for forest protection," Anirudh said. "This is particularly urgent given that many kerangas forest areas remain unprotected and are under increasing threat from artisanal and small-scale gold mining."
She said kerangas forests were often overlooked in land use planning because they occurred alongside other habitats and had historically been described from stunted hill areas, masking the taller and richer lowland versions found in places like Rungan. Their sandy, nutrient-poor soils have also fueled the mistaken belief that they are less valuable, she added.
"However, as we highlight in our study, kerangas forests support several highly specialised and unique plant species, including Bornean conifers, pitcher plants, orchids, and others, which make significant contributions to Kalimantan's overall biodiversity," Anirudh said.
The researchers also call for more efforts to develop alternative sources of income and enforce more sustainable practices of artisanal and small-scale gold mining to reduce the acceleration of lowland heath forest degradation. Ultimately, they suggest increased formal protection for kerangas to support national climate change mitigation efforts.
"The lack of formal protection of these habitats may further have implications on human well-being due to compromised ecosystem services," the researchers wrote.
[Basten Gokkon is a senior staff writer for Indonesia at Mongabay. Find him on X @bgokkon.]
Citation
Anirudh, N. B., van Veen, F. J. F., Ripoll-Capilla, B., Buckley, B. J. W., Erb, W. M, Niun, M. A., Maimunah, S., Makur, K. P., Armadiyanto, Estrada, E., Boyd, N. S., Cheyne, S. M., Santiano, Namaskari, N., Husson, S. J., Randi, A., Seaman, D. J. I., Deere, N. J., Supriatna, J., Struebig, M. J., & Harrison, M. E. (2025). Lowland heath forests of Indonesian Borneo: Ecological value and conservation challenges. Journal of Tropical Ecology, 41(e19), doi:10.1017/S0266467425100084