Contrasting environmental drivers of tree community variation within heath forests in Brunei Darussalam, Borneo

Understanding how abiotic factors influence Bornean tropical tree communities and diversity is a key aspect in elucidating the mechanisms of species co-existence and habitat preferences in these biodiverse forests. We focused on investigating forest structure, tree diversity and community composition of lowland Bornean heath forests in Brunei Darussalam, within two 0.96 ha permanent forest plots at Bukit Sawat Forest Reserve and Badas Forest Reserve. All trees with stem diameter ≥ 5 cm were tagged, identified and measured for their stem diameter and basal area. Soil physiochemical properties (pH, gravimetric water content and concentrations of total carbon, nitrogen, phosphorus, potassium, calcium and magnesium in topsoil) and environmental factors (relative humidity, canopy openness, litter depth and topographic variables of elevation, slope and aspect) were quantified as potential drivers of tree community differences. A total of 2,368 trees were recorded, representing 229 tree species in 211 genera and 58 families. Significant between-site differences in forest structure and tree community compositions were detected, despite limited differences in environmental and soil properties. Tree community composition at Bukit Sawat appeared to be influenced by topographic variables, while those at Badas were influenced by canopy openness. Our results showed that small-scale soil and environmental variation appeared to shape the local tree communities at Bukit Sawat and Badas. We recorded numerous Bornean endemic and tree species of high conservation values. We thus highlight the necessity of conducting long-term research on the forest dynamics of Bornean heath forests to effectively manage these high conservation value habitats which are currently experiencing changes driven by disturbances.