Post-fire impacts on tree diversity in coastal heath forests of Brunei Darussalam

The adverse impacts of fires on the diversity of native plants in forest ecosystems are well documented. Tree diversity was studied in coastal heath (Kerangas) forests of Brunei Darussalam, Northwest Borneo after fire events. Eight 20 x 20 m plots were set up in once-burnt (3 plots), twice-burnt (3 plots) and unburnt heath forests (2 plots). All trees (live and dead) with stem diameters of 1 cm and above were censused, and measures of percentage canopy openness and soil variables (pH, gravimetric water and organic matter content, and concentrations of total nitrogen and phosphorus) were determined. Soil pH, gravimetric water content and total nitrogen and phosphorus concentrations, and canopy openness were significantly higher in the burnt plots. Stem diameters were significantly higher in the unburnt heath forests, with mostly Acacia species recording smaller stem sizes (<10 cm dbh). Tree species richness and diversity were significantly lower in the once-burnt plots than in unburnt, intact heath forest plots indicating substantial loss of native tree diversity in fire-affected habitats. Tree community compositions of the once-burnt plots were dominated by invasive Acacia mangium. This study highlights the negative impact of fire on native tree diversity, soil properties and canopy openness of these coastal Kerangas forests, and further provides preliminary evidence that fire events have facilitated the successful establishment of invasive Acacia species.