Testing the efficacy of riparian restoration strategies in oil palm

Michael Pashkevich, Sarah Helen Luke, William A. Foster and Edgar Clive Turner, Department of Zoology, University of Cambridge, Cambridge, United Kingdom, Anak Agung Ketut Aryawan, Andreas Dwi Advento, Dwi Nugroho Adhy, Adindha Surya Anugraha, Jassica Prajna Dewi, Edi Edi, Mohammad Naim, Pujianto Pujianto, Dedi Purnomo, Syafrisar Putra, Soeprapto Soeprapto, Suhardi Suhardi, Ribka Sionita Tarigan, Resti Wahyuningsih, Whendy Whendy and Jean-Pierre Caliman, Sinar Mas Agro Resources and Technology Research Institute, Pekanbaru, Indonesia, Julia Drewer, UK Centre for Ecology & Hydrology, Edinburgh, United Kingdom, David J. X. Tan, Department of Biology, University of New Mexico, Albuquerque, Eleanor M Slade, Nanyang Technological University, Singapore, Singapore

Background/Question/Methods
Oil palm is a tropical crop that is grown to produce palm oil: the world’s most traded vegetable oil. Oil palm is often grown in areas that were once rainforest, and has therefore substantially altered tropical ecosystems. Ecological restoration is a promising strategy to reduce the negative impacts of some of these changes, and improve levels of biodiversity and functioning within oil palm systems. As many taxa provide ecosystem services that support crop growth, restoration may also benefit the productivity of plantations. However, experiments that test the efficacy of restoration strategies within tropical agricultural systems are rare.

In this talk, we introduce and present early findings from the Riparian Ecosystem Restoration in Tropical Agriculture (RERTA) Project: an experiment that is testing the efficacy of riparian restoration strategies in replanted oil palm plantations (Sumatra, Indonesia). RERTA is implementing three restoration treatments (one passive, and two active) and one no-restoration control treatment across long-term monitoring plots surrounding two rivers. We have collected data on biodiversity, ecosystem processes, and crop productivity both before and after oil palm replanting and implementation of treatments. Data were collected within riparian areas, just outside riparian areas, and deep within the oil palm landscape. We present early findings (collected within two years of treatment implementation) from our surveys of flying invertebrates, and spiders living in the oil palm canopy and on the ground. We focus on these groups because they can provide important pollination and pest control services within oil palm plantations.

Results/Conclusions
Early findings indicate that restoration of riparian areas within oil palm plantations is possible, and can be done fairly rapidly. For instance, although the experiment is less than two years old, many planted trees in our active restoration treatments are > 7 metres high, and these riparian areas are beginning to resemble forests. Our terrestrial invertebrate surveys indicate that biodiversity is already benefitting from the restoration treatments. However, these benefits vary between different orders (flying invertebrates) and species (spiders), and also between different microhabitats within the plantation. We conclude that restoration of riparian areas is a tractable means of increasing terrestrial biodiversity within oil palm landscapes, but benefits are not distributed equally across taxonomic groups and microhabitats. As our restoration treatments are still young, we expect that their ecological benefits will increase over time. Our findings have implications for helping to guide future restoration initiatives in established oil palm plantations, and to inform sustainability certification criteria.