Stem Abundance, Basal Area and Species Richness of Intact and Degraded Kerapah Forest Plots in Lumut, Brunei Darussalam

Abstract

Kerapah forests are a rare form of permanently waterlogged tropical heath forest that develop on highly acidic, nutrient-poor soils and support a distinctive flora adapted to harsh edaphic conditions. In Brunei Darussalam, these forests are increasingly threatened by deforestation and degradation, yet quantitative information on their forest structure and species composition remains limited. This study presents baseline structural and floristic data by comparing stem abundance, basal area, and species richness between intact Kerapah forest stands and areas that were completely cleared in 2012 and allowed to regenerate naturally for five years prior to sampling in Lumut, Brunei Darussalam. Six plots (three intact and three cleared), each measuring 15 × 50 m, were established, and all trees with diameter at breast height (DBH) ≥ 6 cm were measured and identified to species level. Forest structural attributes, diameter class distribution, and species composition were analysed and compared between forest conditions. Intact forest plots exhibited substantially higher stem abundance, basal area, and species richness per plot than cleared forest plots (101.3 ± 14.3 vs. 2 stems, 41.9 ± 7.7 vs. 0.08 m² ha⁻¹, and 27.7 ± 1.9 vs. 2 species, respectively). Diameter class distribution in intact forests was characterised by a high proportion of pole-sized and young adult trees, indicating ongoing regeneration, whereas cleared plots contained only pole-sized individuals, reflecting severe structural simplification and limited recovery. The intact plots supported multiple tree species listed on the IUCN Red List, including Critically Endangered (Gonystylus bancanus), Endangered (Dryobalanops rappa, Rubroshorea pachyphylla), and Vulnerable species (Lithocarpus andersonii, Madhuca curtisii, Rubroshorea albida), underscoring the high conservation importance of intact Kerapah forests. In contrast, cleared plots lacked threatened species and characteristic dominants, possibly because 5 years was too short to observe any meaningful regeneration. This study provides an important ecological baseline to inform evidence-based rehabilitation, conservation planning, and long-term management of Kerapah forests in Brunei Darussalam.