How much carbon dioxide is emitted from smallholders’ farms on peatland?
Smallholders clear forests on peat swamps for their farms, emitting considerable amounts of greenhouse gases. But how much? Scientists from the World Agroforestry Centre went for a walk to find out.
Walking on Indonesian peat that has been drained is like walking on a thick, bouncy, rubber mat. There are some 20.6 million hectares of peatland in Indonesia, nearly half the global total. That’s a little more than the area of Belarus and a little less than Ghana.
If peat swamps are not drained, they’re mostly water with a suspended layer of dead leaves, branches and occasional tree trunks in early stages of decomposition. Despite seeming a perhaps inhospitable environment for plants, peat-swamp forests are typically very dense, with tree and understorey plants specially adapted to the watery, soft conditions. If you do try and take a walk through such a swamp, when your feet go beyond the roots of the trees they sink deeply before finding a branch or trunk that holds. These deeper bits of trees could be very old indeed.
Carbon accumulation in tropical peat, compared to other types of forest, occurs not because of high plant production, but rather owing to slow decomposition of roots and wood under anaerobic conditions. Southeast Asian peat swamps can contain up to 10,000 years of litter accumulation in peat domes more than 10 metres thick at their core. When forests growing on peat are cut and the swamp drained, carbon emissions in the early years after drainage range 100–200 tonnes a year, which is similar to the belowground biomass of the forest that preceded it.
Indonesia has experienced the world’s highest land-based carbon emissions over the past decades owing to a combination of deforestation mainly for agricultural purposes, drainage of peatlands and out-of-control burning to clear land. The country has also, however, been an early champion of climate-change mitigation measures in the forest and peatland sectors, and of an integrated policy environment for combining adaptation and mitigation.
When developing land-use policies, the specific issue of tropical peatlands and the fires and haze caused by their conversion to agricultural uses has been heavily emphasized, focusing mostly on large-scale plantations of oil palm and pulpwood. For smallholdings, there has been a lack of reliable data about emissions from peat. Smallholders’ patchwork farms have less intensive drainage, usually dug by hand, than large-scale plantations that use heavy machinery to dig wider and deeper drainage canals.
To learn more about emissions from smallholdings, Ni’matul Khasanah and Meine van Noordwijk led a team of scientists from the World Agroforestry Centre (ICRAF) Indonesia office who measured peat subsidence and changes in density and carbon content over 2.5 years in Tanjung Jabung Barat District, Jambi Province, Sumatra. Around 200,000 hectares or 40% of the district is peat from which the forests were cleared from the 1970s onwards, mainly for smallholdings of various tree crops. The team measured remnant logged-over forest and four dominant land-use types managed by smallholders that differed in time after drainage: 1) rubber agroforestry (more than 30 years); 2) mixed coconut and coffee (more than 40 years); 3) mixed betel nut and coffee (more than 20 years); and 4) a new oil-palm plantation (one year-old).
The team found that the recently established oil-palm plantation had the highest rate of peat subsidence, at 4.7 cm a year, emitting 121 tonnes of carbon dioxide (CO2) a year. This was significantly lower than emissions from large-scale oil-palm and pulpwood plantations, at about 178 tonnes CO2. Not surprisingly, the remnant forest had the lowest emissions but, probably owing to the modified drainage in the surrounding landscape, it still had a rate of subsidence of 1.8 cm, emitting 40 tonnes of CO2 annually. The other land-use types subsided by 2–3 cm, emitting 70–85 tonnes of CO2. These figures were higher than the default values (averages from various documents) suggested by the Intergovernmental Panel on Climate Change: 51 tonnes for smallholdings, 55 tonnes for commercial plantations and 10 tonnes for disturbed secondary forests.
As well as studying how much peat subsided and how much carbon was emitted, the team also examined other factors, such as the effect of fertilizers on emission rates. They found that fertilizer application had a minimal effect on the rates of peat subsidence and emissions. Under fully controlled experimental conditions, fertilizer application had been shown to increase the decomposition rate.
They concluded that much more specific attention should be given to smallholders managing peat if emissions were to be reduced. In particular, how smallholders managed water needed to be improved. Drainage was uncontrolled through open canals, which affected the adjacent remnant swamp forests, shifting them from carbon sinks to carbon sources. Effective solutions would require more detailed work with smallholders and the government agencies associated with peat management.
Khasanah N, van Noordwijk M. 2018. Subsidence and carbon dioxide emissions in a smallholder peatland mosaic in Sumatra, Indonesia. Mitigation and Adaptation Strategies for Global Change (2018):1–17. https://doi.org/10.1007/s11027-018-9803-2
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