Heath forests, known locally in Borneo as Kerangas, a term that translates roughly as land that cannot support rice cultivation, are among the most distinctive and least studied forest types in Southeast Asia. They occur on acidic, nutrient-poor sandy soils and are characterised by stunted vegetation, pole-sized trees packed tightly together and tough, sclerophyllous leaves. In Brunei Darussalam, heath forests cover approximately 3,455 hectares, only about 0.6 percent of the country’s forest area, occurring as patches along the coastline or inland adjacent to mixed dipterocarp and peat swamp forests. Given their high levels of plant endemism, Brunei’s intact heath forests are of considerable conservation significance, yet no long-term study of their dynamics had ever been attempted until now.

The research team, working with the Institute for Biodiversity and Environmental Research at Universiti Brunei Darussalam, revisited two permanent plots originally established in 1992 as part of a long-term forest dynamics monitoring programme. During the 2022 re-census, every living tree with a stem diameter at breast height of at least 5 centimetres was tagged, identified and measured using standard protocols of the Center for Tropical Forest Science. Species identifications were confirmed with assistance from the Brunei National Herbarium and voucher specimens were collected for verification. The two sites, though similar in size and monitoring history, differ markedly in environmental conditions. Bukit Sawat features gently uneven terrain interspersed with poorly drained, waterlogged patches supporting a mix of heath and peat swamp forest species, while Badas sits atop a peat dome and is dominated by typical heath forest taxa, particularly the conifer Agathis borneensis.

The results paint a picture of forests that are thinning from within. At both sites, the total abundance of trees decreased substantially over the 30-year period, from 1,613 to 1,317 trees at Bukit Sawat and from 1,341 to 1,052 trees at Badas. Mean stem abundance and mean tree density were significantly higher in 1992 than in 2022, while mean stem diameter and mean basal area were significantly higher in 2022. In other words, fewer trees are now occupying the same area, but the survivors are larger. The size class distributions followed reverse J-shaped patterns in both censuses, with small trees under 10 centimetres in diameter comprising more than half of all stems. Notably, the abundance of large trees with diameters of 40 centimetres or more increased at both sites, particularly at Bukit Sawat, reflecting the natural growth of smaller trees into larger size classes over the decades.

When it came to species diversity, the two sites told different stories. Bukit Sawat recorded a 24 percent increase in total species richness, from 168 species in 1992 to 208 species in 2022, yet mean species richness and diversity indices such as Shannon’s Index and the Inverse Simpson’s Index were significantly higher in the 1992 census than in 2022. Badas, by contrast, recorded a 4 percent decrease in total species richness, from 113 to 108 species, with no significant temporal differences in mean diversity indices detected. Species turnover was evident at both sites, with 83 species newly recorded at Bukit Sawat and 36 newly recorded at Badas in 2022, while 53 species at Bukit Sawat and 46 species at Badas recorded in 1992 were absent three decades later. The most species-rich families remained unchanged, with Dipterocarpaceae leading at Bukit Sawat and Myrtaceae at Badas.

Perhaps the most striking finding was the significant shift in tree community composition at both plots over the 30-year period. Non-metric multidimensional scaling ordinations revealed clear differences in community composition between the two census periods at each site, and PERMANOVA results confirmed that these shifts were statistically significant, with R-squared values of 0.0629 at Bukit Sawat and 0.0614 at Badas. The dominant species also declined in abundance. At Bukit Sawat, Dipterocarpus borneensis remained the most abundant species but dropped from 99 to 80 individuals. At Badas, Syzygium bankense declined from 285 to 194 individuals and Agathis borneensis fell from 217 to 151. These directional changes suggest that the forests are not merely fluctuating but are undergoing genuine community reassembly.

What drove these changes? The study period encompassed several severe El Niño events, including the devastating 1997-1998 and 2015-2016 droughts that swept across Borneo. The researchers suggest that these drought events may have had the greatest impact on tree dynamics, species richness and community composition at the two sites. Heath forests are generally considered more vulnerable to prolonged drought and associated fire events than other forest types, and multiple fire events were reported in the surrounding areas of both plots during the 1997-1998 El Niño drought. Droughts are known to significantly alter forest structure, species composition and tree dynamics in Bornean tropical forests, and the accumulated effects of repeated drought stress over three decades likely helped shape the observed shifts.

Human disturbance also left its mark, despite both plots being located within protected areas. At Bukit Sawat, the team recorded signs of increased disturbance in the plot’s immediate vicinity, including the presence of the invasive Acacia mangium and several Macaranga species typical of secondary habitats at the plot edges. The Bukit Sawat plot lies only about 100 metres from the Labi Road, where Acacia mangium is commonly observed along the roadsides, and this species has been recorded as invasive in Brunei’s heath forests. At Badas, evidence of peat degradation resulting in peat subsidence and significant lowering of the peat groundwater level had been reported from the surrounding peat dome, causing peat and small fires. The plot sits less than one kilometre from the Badas pipeline road. Edge effects, which are well known to increase tree mortality and damage at forest boundaries, may also have contributed to the higher stem losses observed in sub-plots at the plot perimeters.

Interestingly, when the researchers compared growth, recruitment and mortality rates between the two sites, they found no significant differences. Mean annual diameter growth rates were 0.10 centimetres per year at Bukit Sawat and 0.18 centimetres per year at Badas, while mean annual mortality rates were 2.56 percent and 2.80 percent and mean annual recruitment rates were 1.83 percent and 1.94 percent respectively. These figures are comparable to ranges reported for other tropical forests and fall within the pre-drought mortality ranges recorded in lowland heath forests in Sabah. The similarity in demographic rates across the two contrasting sites suggests that the fundamental processes of tree turnover operate at comparable paces in Bornean heath forests, even as community composition diverges.

The authors acknowledge that the disturbance-related factors they postulate, including drought, fire, edge effects, Acacia invasion and peat degradation, were not directly quantified and could not be statistically tested as causal effects. They recommend that future research in these permanent plots incorporate long-term climatic and hydrological monitoring alongside the quantification of disturbance causes, frequency and intensity, to help elucidate the mechanisms underlying temporal changes in heath forest structure and composition. As climate change intensifies the threats facing tropical forests worldwide, this study stands as a crucial first step in understanding the long-term dynamics of Bornean heath forests and their responses to a changing environment, underscoring the need for continued monitoring to guide conservation and management strategies for these unique and threatened ecosystems.

Subject of Research: Long-term forest dynamics and tree community change in Bornean heath forests over 30 years

Article Title: Long-term dynamics of tropical heath forests in Brunei Darussalam: forest structure, tree demography and community reassembly over 30 years

Article References: Mohd Ikbal, I. S., Md Jaafar, S., Ahmad, N., & Sukri, R. (2026). Long-term dynamics of tropical heath forests in Brunei Darussalam: forest structure, tree demography and community reassembly over 30 years. Biodiversity Data Journal, 14, Article e194507. https://doi.org/10.3897/bdj.14.e194507

Image Credits: AI Generated

DOI: 10.3897/BDJ.14.e194507

Keywords: heath forests, Brunei Darussalam, Borneo, forest dynamics, tree demography, El Niño drought, community composition, conservation, Long-term, dynamics, tropical, heath