Differential Impacts of Acacia Invasion on Nutrient Fluxes in Two Distinct Bornean Lowland Tropical Rain Forests

Abstract

Invasive Acacia species can alter nutrient cycling processes in forest ecosystems, particularly affecting total litterfall production and litter decomposition patterns. This study examined the effects of exotic Acacia mangium Willd. on total litterfall production, nutrient concentrations in leaf litterfall fractions, leaf litter decomposition, and nutrient release in lowland heath (HF) and mixed dipterocarp forests (MDF) in Brunei Darussalam, Borneo. Above-ground litterfall traps were installed in HF and MDF with and without invasive Acacia present, representing four habitat types in total, and monthly collections were conducted for 12 months. Litter decomposition bags were deployed to determine the rates of decomposition and nutrient release. Habitats invaded by Acacia exhibited higher total litterfall production, increased leaf litter concentrations of nitrogen, potassium, and calcium, and increased addition of all nutrients measured in litter (nitrogen, phosphorus, potassium, calcium, and magnesium, especially in the Acacia-invaded mixed dipterocarp forest (AMDF) and nitrogen and potassium in Acacia-invaded heath forest (AHF)), reduced nitrogen and potassium use efficiencies in AHF, and reduced stand-level nitrogen and calcium use efficiencies in AMDF. Litter decomposition rates and nutrient release were lower in AMDF than in the three other habitats. The significantly higher total litterfall production coupled with higher nutrient addition in the two Acacia-invaded habitats is expected to progressively increase the abilities of these habitats to produce large quantities of nutrient-rich litter and will likely eventually lead to an enrichment of nutrients in the soil, thus facilitating further invasion by Acacia, particularly in the MDF.

Review: Diversity, structure, and community composition of Bornean heath forest with a focus on Brunei Darussalam

“…The low soil pH of heath forests can hinder organic matter decomposition, which further slows down nutrient cycling and affects forest productivity (Xu et al 2013;Sellan et al 2020). Heath forests often experience closed nutrient cycling due to the presence of deep organic layers (Moran et al 2000;Jaafar et al 2022b). The infertility of sandy heath soils has been linked to low pH, water shortage, and nutrient deficiency, particularly, in nitrogen and phosphorus (Whitmore 1984;Proctor 1999;Atikah 2014;Din et al 2015).…”

“…ex G.Don (Quong-Vuong et al 2018;Islam et al 2019;Ibrahim et al 2022). A. mangium (Figure 5C), in particular, is an urgent threat to Brunei's coastal heath forests, because it affects native tree species diversity (Suhaili et al 2015;Tuah et al 2020) and modifies soil and leaf litter properties (Yusoff et al 2019;Ibrahim et al 2021;Jaafar et al 2022a), ion deposition (Ibrahim et al 2022), and nutrient cycling of invaded heath forests (Jaafar et al 2022b). It is shade-intolerant and performs well in disturbed areas with high light exposure, and its successful invasion is partly attributed to its ability to form an extensive soil seed bank in the areas it occupies (Suhaili et al 2015).…”