Impacts of invasive Acacias on ion deposition in a coastal Bornean tropical heath forest

ABSTRACT

The uncontrolled invasive spread of exotic Acacias is a growing concern for many tropical ecosystems, particularly due to their negative impact upon nutrient cycling. Studies investigating the effects of Acacia invasion on nutrient cycling have focused on litterfall and litter decomposition, although its impact on ion deposition in tropical ecosystems is less understood. We quantified and compared the rates of ion deposition (Ca²⁺, Mg²⁺, NH₄⁺, NO₃⁻) between Acacia-invaded and non-invaded tropical heath (Kerangas) forest sites in Brunei Darussalam, Southeast Asia. Ion exchange resin (IER) sampler, IER ring and IER bags were used to determine ion depositions via throughfall, in soils and in stream water respectively. Throughfall in Acacia-invaded heath forest sites recorded significantly higher NH₄⁺ and Ca²⁺ depositions but lower Mg²⁺ than non-invaded, intact sites. For ion deposition in soil, all depths recorded significantly higher Ca²⁺ (except for 0 cm depth), Mg²⁺ and NH₄⁺ depositions, but significantly lower NO₃⁻ depositions, in Acacia-invaded sites. Ion deposition in stream water showed significant monthly variations, with Acacia-invaded sites recording significantly higher Mg²⁺ depositions in June, August and September 2015 but lower NH₄⁺ depositions in June, July and September, lower NO₃⁻ and Ca²⁺ depositions in June and September than non-invaded heath forest sites. Overall, these findings demonstrate that ion deposition via throughfall, in soil and in stream water were all affected by Acacia invasion, thus confirming the influence of alien invasive Acacias on nutrient cycling of tropical heath forests.

KEYWORDS:

• Acacia mangium
• invasive plants
• ion exchange resin
• nutrient cycling
• throughfall

Acknowledgments

This work was funded by the Brunei Research Council (UBD/BRC/11) and the UBD research grant (UBD/PNC2/2/RG/1(204)). MHI was funded by the UBD Graduate Research Scholarship. We thank the Forestry Department, Ministry of Primary Resources and Tourism, for use permit ([99]/JPH/UDN/17 PT.1), and the Brunei Darussalam Meteorological Department, Ministry of Transport and Infocommunications for providing weather data. We thank Miftachul Huda, Rungroj Chobhwan, Muhammad Najib, Muhamad Hafizul, Salwana Md Jaafar, Shamira Matali and Shaahidah Marjik for assistance with field work and laboratory analysis. We are grateful to the Coordinating Editor and two anonymous reviewers for critical feedback that have significantly improved our manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Geolocation information

This study was conducted in coastal tropical heath forests in Brunei Darussalam, Northwest Borneo (N 04°57.388, E 114°52.194).

Supplementary material

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Additional information

Funding

This work was funded by the Brunei Research Council (UBD/BRC/11) and Universiti Brunei Darussalam (UBD/PNC2/2/RG/1(204)).