- Researchers found that each hectare of rainforest converted to oil palm monoculture creates 174 tons of carbon emissions, most of which will find their way into the atmosphere and contribute to global climate change.
- After oil palm is harvested, the amount of biomass returned to the soil to feed living organisms underground can be 90 percent lower than in a functional, healthy rainforest. Since the soil in oil palm plantations is repeatedly cleared and treated with pesticides, very little natural litter like dead leaves and wood goes back into the ground.
- The research team said that their findings show that figures used by bodies like the IPCC and the RSPO to calculate the carbon cost of oil palm cultivation should be updated and that belowground carbon losses must be accounted for.
New research comparing the carbon costs of oil palm plantations with those of rubber production finds that while oil palm cultivation is the more efficient process in terms of land use, it’s also responsible for much higher emissions — and those losses are higher than the figure used by the Intergovernmental Panel on Climate Change (IPCC) and sustainable palm oil certification bodies.
Thomas Guillaume, a postdoctoral researcher at École Polytechnique Fédérale de Lausanne (EPFL), a research institute in Lausanne, Switzerland, led a team of researchers who analyzed the carbon costs and benefits of converting rainforests in Sumatra, Indonesia into oil palm plantations. The results of the study were detailed last month in the journal Nature Communications.
The researchers found that each hectare of rainforest converted to oil palm monoculture creates 174 tons of carbon emissions, most of which will find their way into the atmosphere and contribute to global climate change. Guillaume said that that figure — 174 tons of carbon per converted hectare — is roughly equivalent to 530 people flying from Geneva to New York in economy class.
“We made an exhaustive accounting of carbon stocks in rainforest sites and mature oil palm plantations,” Guillaume told Mongabay. “We measured aboveground biomass and dead biomass (dead wood, litter) as well as belowground stocks down to 50 centimeters: roots, dead roots, and soil organic carbon. The figure represents the difference between the sum of stocks in forests and in oil palm.”
This loss estimate is higher than the figures used by the IPCC and sustainable palm oil certification bodies like the Roundtable on Sustainable Palm Oil (RSPO) to quantify the amount of greenhouse gases emitted by oil palm operations. Guillaume said that the values used by the IPCC and certification bodies to calculate the carbon cost of oil palm cultivation “are quite old” and based on limited information because there were only a few studies available when their estimates were made.
There are two main reasons why the IPCC and RSPO estimates for carbon losses from oil palm cultivation are lower than what his team came up with, according to Guillaume:
“First, our oil palm biomass values as well as values from other recent studies are smaller than the default values used by IPCC and certification bodies. The second aspect is the belowground carbon stocks losses in roots, which can be consequent, and in soil organic carbon. Because of the uncertainty about roots and carbon loss in mineral soils (i.e. soils that are not peatland), IPCC preferred not to give values and so to neglect them. Similarly, certification bodies also do not account for soil carbon losses in mineral soil and not always for roots’ carbon losses.”
Guillaume and team also used more than two years’ worth of data collected by Germany’s University of Göttingen on soil and vegetation in central Sumatra in order to compare the impact of oil palm cultivation with both intensive and extensive rubber production practices.
“These two crops are important drivers of tropical land-use changes, especially in Sumatra, our study region,” Guillaume said. “In this region, rubber is cultivated in an extensive way (rubber trees are planted within a partly logged forest and cultivated without fertilizers or herbicide) and an intensive way (monoculture with herbicide and fertilizers but in lower amount than oil palm plantations). It gave use a gradient of land-use intensity and enabled us to assess trade-offs along this gradient between providing ecosystem services (e.g. harvested biomass) and regulating ecosystem services (e.g. carbon sequestration).”
The researchers found that intensive rubber farming is responsible for 159 tons of carbon loss, while extensive rubber production is associated with 116 tons. The difference between the carbon costs of growing oil palms versus rubber plants is largely due to the fact that oil palm plantations have a shorter rotation time, Guillaume and his co-authors note in the study. But they also found that growing oil palm is more efficient than either intensive or extensive rubber farming, as it results in less carbon loss relative to the amount of biomass harvested.
“Oil palm cultivation led to the highest ecosystem carbon losses but was also the most productive cultivation in terms of harvested biomass,” according to Guillaume. “Accordingly, less carbon was lost to produce the same amount of biomass when rainforests were converted to oil palm than to rubber plantations.”
But Guillaume is quick to point out that this finding does not negate the downsides of oil palm plantations in rainforests. After oil palm is harvested, the amount of biomass returned to the soil to feed living organisms underground can be 90 percent lower than in a functional, healthy rainforest. Since the soil in oil palm plantations is repeatedly cleared and treated with pesticides, very little natural litter like dead leaves and wood goes back into the ground.
“The quantity of biomass that humans take away in order to produce palm oil compared to the quantity left for the ecosystem sheds real doubt on the sustainability of this form of farming,” Guillaume said.
He added that his team’s findings show that figures used by bodies like the IPCC and the RSPO to calculate the carbon cost of oil palm cultivation should be updated and that belowground carbon losses must be accounted for. In the study, Guillaume and team make a number of other recommendations to lessen the environmental impacts of oil palm plantations, including management practices to mitigate soil degradation and erosion — for instance, planting cover crops and spreading dead oil palm fronds all over plantations. “In other terms, never keeping the soil bare,” Guillaume said.
Also, forests or previous oil palm biomass should not be burned to make way for a new planting: “If the wood is used for long-term construction or left on site to enable its integration in the soil, part of the carbon loss due to land-use change would not result in CO2 emissions. Furthermore, ashes and the nutrients they contain can be leached rapidly by heavy rainfalls. The risk is lower if the biomass that contain the nutrients is integrated in the soils.”
Other oil-palm-growing regions can suggest ways to improve cultivation practices, as well, according to the researchers. For instance, establishing oil palm plantations on savanna or pasture, as has been done in Colombia in areas that were once covered by forests, can lead to huge savings of carbon and other lessened environmental impacts, though the productivity of plantations on these types of lands can be slightly lower, according to Guillaume.
Meanwhile, many Cameroonian smallholder farmers don’t use any chemicals in their plantation, because they are too expensive to buy. This shows that oil palms don’t need fertilizers or herbicide to grow. Even though, again, productivity is lower using chemical-free methods, the resulting palm oil is organic and can perhaps fetch a higher price, even if it is not officially certified.
“More generally, we should pay much more attention to the soil. Not only to the loss of carbon sequestration but also to the loss of fertility induced by the decrease of soil organic matter,” Guillaume said. “I do not think it is reasonable to make political decisions based on a single study. However, there is a growing body of literature that is now available to make science-based decisions on this topic.”
• Guillaume, T., Kotowska, M. M., Hertel, D., Knohl, A., Krashevska, V., Murtilaksono, K., … & Kuzyakov, Y. (2018). Carbon costs and benefits of Indonesian rainforest conversion to plantations. Nature Communications, 9(1), 2388. doi:10.1038/s41467-018-04755-y
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