Biodiversity enrichment with tree planting in oil palm plantations
The presence of native trees within oil palm plantations may have a positive impact on biodiversity, but may also be associated with economic losses. The trade-off between ecological benefits and economic losses, however, is least pronounced in intensively managed plantations, where an increase in tree density had the largest positive effect on the diversity of birds and the fewest repercussions on farmers’ revenues (Teuscher et al. 2015). To evaluate the ecological and socio-economic impact of tree planting in oil palm monoculture, the EFForTS project established a biodiversity enrichment experiment in 2013 (Teuscher et al. 2016). Six multi-purpose native tree species were planted, oil palms were partly thinned, and fertilizers and herbicides no longer applied.Survival rates and tree height varied widely among species. Shortly after planting, beneficial effects on bird and invertebrate communities could already be observed (Teuscher et al. 2016) with the number of planted tree species increasing the structural complexity of the vegetation and thereby presumably also other ecosystem functions (Zemp et al. 2019a). Notably, the yield loss associated with oil palm removal was compensated by per-palm yield gain, which was attributed to oil palm thinning, whereas tree planting had no effect on yield (Gérard et al. 2017). This positive effect, however, was restricted to the first years after planting. Trees grew less well in the proximity of oil palms, suggesting a trade-off between tree growth and palm oil production (Zemp et al. 2019b).
In order to support tree planting in oil palm plantations, smallholder oil palm farmers were identified as key actors due to their growing importance in the oil palm sector in Indonesia. Missing knowledge about how to manage trees in oil palm plantations and missing markets for high quality tree saplings appeared to hinder tree planting. These barriers can be overcome with the help of information and seedling provision. Both of these interventions increase the number of trees planted per hectare in smallholder plantations (Romero et al. 2019).