A biodiversity enrichment experiment was established as part collaborative research center EFForTS [Ecological and socio-economic functions of tropical lowland rainforest transformation systems (Sumatra, Indonesia)] funded by the German Research Foundation. Tree islands were planted in a monoculture oil palm plantation in December 2013. The aim of the enrichment experiment is to analyse trade-offs between ecological and economic functions and to contribute to the development of ecologically improved management concepts in oil palm landscapes.

The experimental site seen from an Unmanned Aerial Vehicle (Watit Khokthong, 2015). Squares depict experimental plots and are drawn to scale. The plot ID is shown at the top-right side of each plot.

Design
The experiment consists of 52 tree islands planted in a monoculture oil-palm landscape, with a minimum distance of 85 m between them. The islands, i.e., plots, vary in tree species composition and diversity following a random partitions design. The experiment comprises four partitions that differ in plot size (5 m x 5 m, 10 m x 10 m, 20 m x 20 m, 40 m x 40 m). Each partition is divided into five blocks, one per tree diversity level (0, 1, 2, 3, and 6 species). Within each block, each species was randomly drawn from the species pool without replacement and thus selected exactly once Before planting, approximately 40 % of the oil palms on the plots were removed to enhance the light availability. Trees were planted with a 2-m grid spacing, and the distance between trees belonging to the same species was maximized. We planted six trees on the 5 m x 5 m plots, 25 trees on the 10 m x 10 m plots, 100 trees on the 20 m x 20 m plots and 400 on the 40 m x 40 m plots.

The random partition design with P for the four partitions that differ in plot size, Q the five blocks per partition, R the diversity level of the block, M the plots, and A-F the species (A Parkia speciosa, Fabaceae; B Archidendron pauciflorum, Fabaceae; C Durio zibethinus, Malvaceae; D Dyera polyphylla, Apocynaceae; E Peronema canescens, Lamiaceae; F Shorea leprosula, Dipterocarpaceae). Within a block, each species was randomly drawn from the species pool without replacement; no repetition of the species composition was allowed between plots. Trees were planted on plots with R = 1/2/3/6, but not on plots with R = 0/ctrl. There are four control plots (R ctrl, M 53-56) of the same size (10 m x 10 m), which are managed as usual. A special experimental management (stop of herbicide/pesticide/fertilizer application and stop of weeding 2 years after establishment) is applied on plots M = 1-52. The actual spatial arrangement of the plots in the plantation was random; plots were not aggregated according to partition, block, or diversity level.

Site characteristics

Research
Three main questions are investigated: (1) Are gap enrichment plantings a suitable measure for biodiversity enrichment in oil palm plantations? (2) Do tree islands act as nuclei for colonization of flora and fauna? (3) What are the socio-economic and ecological trade-offs?

Within a few years after planting, tree diversity and plot size enhanced tree growth and stand structural complexity; it is, however, still much lower than that of forests and the long-term temporal trajectories are unclear. Native tree species spontaneously colonized the experimental plots but also invasive plant species thrived. Overall, the experimental effects on the biodiversity of birds, plants and other taxa were mixed. After an initial increase in oil palm yield, we are beginning to see a decline that coincides with the incipient competition between trees and palms.

For Phase 3 (2020-2023), we work on four work packages (1) tree dynamics and yield, (2) plant successional trajectories, (3) scenarios and upscaling, and (4) integration and synthesis. Research will focus on the role of invasive plants and landscape configuration as potential drivers of ecological disservices, recruitment limitations and restoration success, as well as on determinants of performance and function of trees. Other studies addressing the changes in the environment, biodiversity & ecological functions, and economic functions will be integrated and led towards a synthesis. We will test for the role of direct and indirect effects of biodiversity restoration.

Extra information
Send an e-mail to the contact persons, explore the website of the experiment, go on a interactive field tour (here or here) or have a look at the publications.
theses

• Khokthong W (2019) Drone-based assessments of crowns, canopy cover and land use types in and around an oil palm agroforestry experiment. PhD Thesis, University Of Göttingen.
• Gérard A (2016) Experimental biodiversity enrichment in an oil-palm plantation. PhD thesis, University Of Göttingen.
• Teusher M (2015) Ecological impacts of biodiversity enrichment in oil palm plantations. PhD Thesis, University Of Göttingen.

• Azhar A, Tawakkal MI, Sari A, Rizali A, Tarigan SD, Nazarreta R, Buchori D (2022) Tree diversity enhance species richness of beneficial insect in experimental biodiversity enrichment in oil palm plantation. International Journal of Oil Palm 5(2): 39–49 - https://doi.org/10.35876/ijop.v5i2.82
• Darras KFA, Yusti E, Huang JC, Zemp D, Kartono AP, Wanger TC (2021) Bat point counts: A novel sampling method shines light on flying bat communities. Ecology and Evolution 11(23) 17179–17190 - https://doi.org/10.1002/ece3.8356
• Korol Y, Khokthong W, Zemp DC, Irawan B, Kreft H, Hölscher D (2021) Scattered trees in an oil palm landscape: Density, size and distribution. Global Ecology and Conservation e01688 - https://doi.org/10.1016/j.gecco.2021.e01688
• Messier C, Bauhus J, Sousa-Silva R, Auge H, Baeten L, Barsoum N, Bruelheide H, Caldwell B, Cavender-Bares J, Dhiedt E, Eisenhauer N, Ganade G, Gravel D, Guillemot J, Hall JS, Hector A, Hérault B, Jactel H, Koricheva J, Kreft H, Mereu S, Muys B, Nock CA, Paquette A, Parker JD, Perring MP, Ponette Q, Potvin C, Reich PB, Scherer-Lorenzen M, Schnabel F, Verheyen K, Weih M, Wollni M, Zemp DC (2021) For the sake of resilience and multifunctionality, let's diversify planted forests! Conservation Letters e12829 - https://doi.org/10.1111/conl.12829
• Somenguem LD, Röll A, Ellsäßer F, Ehbrecht M, Irawan B, Hölscher D, Knohl A, Kreft H, Siahaan EJ, Sundawati L, Stiegler C, Zemp DC (2021) Microclimate and land surface temperature in a biodiversity enriched oil palm plantation. Forest Ecology and Management 497(119480) - https://doi.org/10.1016/j.foreco.2021.119480
• Tarigan S, Buchori D, Siregar IZ, Azhar A, Ullyta A, Tjoa A, Edy N (2021) Agroforestry inside oil palm plantation for enhancing biodiversity-based ecosystem functions. IOP Conference Series: Earth and Environmental Science 694(1): 012058 - https://doi.org/10.1088/1755-1315/694/1/012058
• Ballauff J, Zemp DC, Schneider D, Irawan B, Daniel R, Polle A (2020) Legacy Effects Overshadow Tree Diversity Effects on Soil Fungal Communities in Oil Palm-Enrichment Plantations. Microorganisms 8:1577 - https://www.mdpi.com/2076-2607/8/10/1577
• Merten J, Stiegler C, Hennings N, Purnama, ES, Röll A, Agusta H, Dippold MA, Fehrmann L, Gunawan D, Hölscher D, Knohl A, Kückes J, Otten F, Zemp DC, Faust H (2020) Flooding and land use change in Jambi Province, Sumatra: integrating local knowledge and scientific inquiry. Ecology and Society 25(3): 14 - https://doi.org/10.5751/ES-11678-250314
• Khokthong W, Zemp DC, Irawan B, Sundawati L, Kreft H, Hölscher D (2019) Drone-based assessment of canopy cover for analyzing tree mortality in an oil palm agroforest. Frontiers in Forests and Global Change 2: 1–10 - https://doi.org/10.3389/ffgc.2019.00012
• Seidel D, Annighöfer P, Stiers M, Zemp CD, Burkardt K, Ehbrecht M, Willim K, Kreft H, Hölscher D, Ammer C (2019) How a measure of tree structural complexity relates to architectural benefit‐to‐cost ratio, light availability, and growth of trees. Ecology and Evolution 9(12): 7134–7142 - https://doi.org/10.1002/ece3.5281
• Zemp DC, Gérard, A, Hölscher D, Ammer C, Irawan B, Sundawati L, Teuscher M, Kreft H (2019) Tree performance in a biodiversity enrichment experiment in an oil palm landscape. Journal of Applied Ecology 56: 10 - doi: https://doi.org/10.1111/1365-2664.13460
• Zemp DC, Ehbrecht M, Seidel D, Ammer C, Craven D, Erkelenz J, Irawan B, Sundawati L, Hölscher D, Kreft H (2019) Mixed-species tree plantings enhance structural complexity in oil palm plantations. Agriculture, Ecosystems and Environment, 283: 106564 - doi: https://doi.org/10.1016/j.agee.2019.06.003
• Grossman JJ, Vanhellemont M, Barsoum N, Bauhus J, Bruelheide H, Castagneyrol B, Cavender-Bares J, Eisenhauer N, Ferlian O, Gravel D, Hector A, Jactel H, Kreft H, Mereu S, Messier C, Muys B, Nock C, Paquette A, Parker J, Perring MP, Ponette Q, Reich PB, Schuldt A, Staab M, Weih M, Zemp DC, Scherer-Lorenzen M, Verheyen K (2018) Synthesis and future research directions linking tree diversity to growth, survival, and damage in a global network of tree diversity experiments. Environmental and Experimental Botany - doi: 10.1016/j.envexpbot.2017.12.015
• Gérard A, Wollni M, Höscher D, Irawan B, Sundawati L, Teuscher M, Kreft H (2017) Oil-palm yields in diversified plantations: Initial results from a biodiversity enrichment experiment in Sumatra, Indonesia. Agriculture, Ecosystems and Environment 240: 253-260 - doi: http://doi.org/10.1016/j.agee.2017.02.026
• Teuscher M, Gérard A, Brose U, Buchori D, Clough Y, Ehrbrecht M, Hölscher D, Irawan B, Sundawati L, Wollni M, Kreft H (2016) Experimental biodiversity enrichment in oil-palm-dominated landscapes in Indonesia. Frontiers in Plant Science 7: 1538 - doi: 10.3389/fpls.2016.01538




View of a 6-species plot from the ground (January 2020) and from a drone (November 2016).