Rainforest zero-deforestation commitments could unintentionally encourage oil palm expansion in tropical grassy biomes and dry forests

Published in Ecology & Evolution
Rainforest zero-deforestation commitments could unintentionally encourage oil palm expansion in tropical grassy biomes and dry forests

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In recent years, many companies have committed to zero-deforestation supply chains, and an estimated two-thirds of palm oil is now traded under zero-deforestation commitments1. However, other tropical habitats, such as tropical grassy biomes (savannas, grasslands and shrublands) and dry forests (seasonally dry forests with a closed canopy) are less protected than tropical rainforests, yet could be suitable for oil palm expansion. These habitats support unique biodiversity and potentially high carbon stocks, so expansion of oil palm plantations in these areas could have unintended consequences, and undermine zero-deforestation commitments that are in place to avoid biodiversity loss and high carbon emissions.

In a new study funded by the University of York and Unilever, we show that over half of the total global area that is suitable for zero-deforestation oil palm expansion (i.e. expansion conducted in line with current zero-deforestation commitments) is in grassy biomes and dry forests. Within these biomes, we find that zero-deforestation commitments do not protect areas of high vertebrate richness from oil palm expansion. These potentially suitable locations for oil palm expansion overlap with the ranges of 10% of all threatened terrestrial vertebrate species, including the blue-throated macaw in Bolivia, giant pangolin in Congo, and Hellmich's Rocket Frog in Colombia.

Savanna in Gabon, a habitat where oil palm plantations have expanded recently
Savanna in Gabon, a habitat where new oil palm plantations have expanded recently. Photo credit: Kate Parr.

Our findings highlight the urgent need for new guidance to be developed to minimise the impact of agricultural expansion in tropical grassy biomes and dry forests. In our paper, we provide recommendations for the High Conservation Value-High Carbon Stock Approach to help identify valuable biodiversity in these biomes, extending current guidance for tropical rainforest and providing a framework to incorporate more detailed guidance for other habitats. For example, tropical grassy and dry forest biomes are often mis-identified as degraded habitat2, particularly because their vegetation structure can be superficially similar to degraded tropical rainforest (e.g. areas of grass with small shrubs, or forest with low canopy height and density). Areas of grassy biomes and dry forest that are suitable for oil palm expansion include both intact, ancient, high-biodiversity habitat, and highly-degraded habitat, because we were unable to distinguish these on a global scale. Therefore, we highlight the need for comprehensive definitions of degradation, and the development of indicators to identify degradation in each habitat type, recognising that some degraded areas could be the only remaining examples of some very rare habitats.

We recommend that companies extend their supply-chain commitments to ‘no conversion of natural habitat’, and the Roundtable on Sustainable Palm Oil (RSPO) includes this in its Principles and Criteria, in order to support comprehensive protection of biodiversity in all biomes.

To estimate areas climatically-suitable for oil palm expansion, we used species distribution modelling. Our suitability models were based on the climate (cumulative water deficit and minimum temperature of the coldest month) at locations of existing oil palm mills3. We ran models for both rainfed and irrigated oil palm expansion, using recent hydrological data on surplus available water (the difference between water supply and demand)4 to ‘irrigate’ locations where surplus rainfall is sufficient to remove a critical water deficit experienced by oil palm. For rainfed cultivation, we estimated that a total area of 170 Mha is suitable for zero-deforestation oil palm expansion under existing guidelines, representing potential for six-fold expansion of the current planted area of oil palm globally. Of these 170 Mha suitable for zero-deforestation expansion globally, >50% (95 Mha) are in tropical grassy biomes and dry forests. Furthermore, we found that an additional 85 Mha could be suitable for expansion in these biomes under irrigation. Our models of suitability for rainfed oil palm generally had good agreement with previous estimates5,6, while our methods are, to our knowledge, the first to incorporate surplus available water into suitability for irrigation.

Implications of zero-deforestation palm oil for tropical grassy and dry forest biodiversity is published in Nature Ecology and Evolution. The research was funded by Unilever and The University of York, and was a collaboration by scientists at the Universities of York, Liverpool, Oxford, the Potsdam Institute for Climate Impact Research, Unilever and BeZero Carbon. 

Read the paper at https://www.nature.com/articles/s41559-022-01941-6


  1. Haupt, F., Bakhtary, H., Schulte, I., Galt, H. & Streck, C. Progress on Corporate Commitments and their Implementation. (2018).
  2. Parr, C. L., Lehmann, C. E. R., Bond, W. J., Hoffmann, W. A. & Andersen, A. N. Tropical grassy biomes: Misunderstood, neglected, and under threat. Trends in Ecology and Evolution (2014). doi:10.1016/j.tree.2014.02.004
  3. World Resources Institute, Rainforest Alliance, Proforest & Daemeter. Universal Mill List. (2018). Available at: https://data.globalforestwatch.org/documents/gfw::universal-mill-list/about.
  4. Sutanudjaja, E. H. et al. PCR-GLOBWB 2: A 5 arcmin global hydrological and water resources model. Geosci. Model Dev. (2018). doi:10.5194/gmd-11-2429-2018
  5. Pirker, J., Mosnier, A., Kraxner, F., Havlík, P. & Obersteiner, M. What are the limits to oil palm expansion? Glob. Environ. Chang. 40, 73–81 (2016).
  6. FAO and IIASA. Global Agro-ecological Zones version 4 (GAEZ v4). (2021). Available at: http://www.fao.org/gaez/.

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