Tree performance in a biodiversity enrichment experiment in an oil palm landscape

1. Large‐scale conversion of tropical forests into oil palm monocultures has led
to dramatic losses of biodiversity and ecosystem functions. While ecological
restoration is urgently needed in many oil palm landscapes, there is a lack of
scientific knowledge of sustainable management strategies.
2. We established experimental tree islands of varying sizes (25 m2 to 1,600 m2)
and diversity levels (1, 2, 3 and 6 species) in an oil palm plantation in Sumatra,
Indonesia. Six native multi‐purpose tree species including Archidendron
pauciflorum (Jengkol), Durio zibethinus (Durian), Parkia speciosa (Petai), Shorea
leprosula (Meranti), Peronema canescens (Sungkai) and Dyera polyphylla (Jelutung)
were planted between living and felled oil palms. Here, we analyse the controlling
factors of tree growth and survival during the first 4 years at the level of local
neighbourhood and tree island.
3. We found a significant effect of diversity levels on tree productivity, that is, basal
area was higher in multi‐species than in single‐species tree islands. This overyielding
was attributed to enhanced tree growth, while mortality had no effect. In the local
neighbourhood, tree species richness had a positive effect on tree growth during
the first year only, indicating that selection and dominance of well‐performing
species at high level of diversity are most likely driving overyielding.
4. Trees grew better away from living oil palms, suggesting tree‐palm competition.
Proximity to felled oil palms increased growth especially during the first years,
during which the planted trees might have benefited from the additional available
space and resources. Despite positive edge effects from the conventional oil palm
management in the surrounding, tree island size had an overall positive effect on
tree growth.
5. Synthesis and applications. We planted native trees in an oil palm landscape
following a tree island pattern. The establishment success differed widely among
the six planted species. The selection of particular species is a decisive factor to
foster a positive relationship between diversity and tree growth. Planting larger
tree islands (e.g. 1,600 square metres) is a better option to enhance tree growth,
but tree‐palm competition implies a trade‐off between tree growth and palm oil
production in the tree islands.