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

This study presents the first long-term analysis forest dynamics in lowland Bornean heath forests in Brunei Darussalam, based on two 0.96 ha permanent forest plots, established in 1992 at Bukit Sawat Forest Reserve and Badas Forest Reserve. The plots were re-censused in 2022, during which all trees with stem diameter ≥ 5 cm were tagged, identified and measured for their stem diameter and basal area. Over the 30-year period, mean stem abundance and mean tree density significantly decreased, while mean stem diameter and mean basal area significantly increased, at both heath forest sites. At Bukit Sawat, mean species richness and diversity indices were significantly higher in 1992 than in 2022, but no significant temporal dfferences were observed at Badas. No significant differences between sites were detected in mean annual diameter growth rate, mortality and recruitment rates. Notably, both plots exhibited significant shifts in tree community composition over the 30-year period, coupled with a 24% increase in total species richness at Bukit Sawat and a 4% decrease at Badas. Despite their locations in protected areas, droughts events and anthropogenic disturbance over the past three decades likely contributed to observed changes in forest structure, species diversity and community composition. These findings provide important insights into the long-term dynamics of threatened Bornean heath forests and underscore the need for continued monitoring to guide conservation and management strategies.