Foliar Trait and Nutrient Differences Between Invasive Acacia and Native Species in Disturbed Coastal Kerangas Forests of Brunei Darussalam

Abstract

Invasive Acacia species are fast-growing exotic trees that were introduced to Brunei Darussalam in the 1990s for plantation forestry and land rehabilitation. Over time, several Acacia species have spread beyond planted areas and are now invading disturbed coastal Kerangas (tropical heath) forests, raising concerns about their impacts on native vegetation adapted to nutrient-poor conditions. However, information on the functional traits associated with Acacia invasion in Brunei’s Kerangas ecosystems remains limited. This study investigated differences in selected foliar traits and foliar nutrient concentrations among three invasive Acacia species (Acacia auriculiformis, Acacia holosericea and Acacia mangium) and six co-occurring native species (Buchanania arborescens, Calophyllum soulattri, Dillenia suffruticosa, Melastoma malabathricum, Ploiarium elegans and Symplocos polyandra,), which represent a range of functional strategies present in disturbed Kerangas habitats. Measured foliar traits included leaf thickness, specific leaf area (SLA), leaf dry matter content (LDMC) and green leaf tissue pH, while foliar nutrients assessed were leaf nitrogen content (LNC) and leaf phosphorus content (LPC). Results showed that invasive Acacia species, particularly Acacia auriculiformis and Acacia mangium, exhibited significantly higher SLA, thinner leaves and higher foliar nutrient concentrations compared to native Kerangas species. These traits indicate a resource-acquisitive strategy associated with rapid growth and competitive ability under disturbed conditions. In contrast, native Kerangas species displayed lower SLA, higher LDMC and greater investment in leaf structural traits, reflecting conservative resource-use strategies consistent with adaptation to nutrient-poor environments. Dillenia suffruticosa exhibited trait characteristics similar to invasive Acacia species, while Melastoma malabathricum showed mixed strategies. Other native species were associated with more conservative trait syndromes. Overall, the findings indicate that differences in foliar traits and nutrient allocation strategies are likely to contribute to the invasion success of Acacia species in coastal Kerangas forests, with potential implications for the persistence of Brunei’s increasingly rare Kerangas ecosystems.