There is an increase in awareness of the carbon emissions resulting from peatland drainage and peat fires. There is in general however a lack of recognition of the link between carbon loss and peatland subsidence, i.e. lowering of the soil surface.
This paper presents the case study of the Rajang Delta in Sarawak, Malaysia where peatland subsidence will cause flooding, rendering 50% and 67% of the land unsuitable for palm oil cultivation after 25 and 50 years respectively. This is 3 to 4 times the size of Singapore.
What is soil subsidence?
Subsidence is the lowering of the soil surface as a result of physical compression of the peat and loss of carbon due to oxidation and erosion. Peat soils comprise of 10% accumulated organic materials and 90% water. When drained, the peat oxidizes and all peat above the drainage level will eventually be lost.
In the tropics the process of subsidence after drainage is very fast due to high temperatures. Flood risks are also more severe because of high precipitation. Continuous peatland subsidence may eventually result in increased flooding and possibly in production loss in agriculture and silviculture, as shown in studies in South-east Asia and elsewhere (Hooijer et al. 2012).
- Water table close to surface
- Peat accumulation from vegetation over thousands of years
- Water tables lowered
- Peat surface subsidence and CO2 emission starts
- Decomposition of dry peat CO2 emission
- High fine risk in dry peat: CO2 emisson
- Peat surface subsidence due to decomposition and shrinkage
- Most peat carbon above drainage limit released to the atmosphere within decades,
- unless conservation / mitigation measures are taken1
What can be done to mitigate further subsidence and resulting flooding in the tropics and instead create sustainable peat landscapes?
Radical change in land use planning and gradual phasing out of current unsustainable land-uses on peat
In South-east Asia the land conversion of peat swamp forests started relatively recent, mostly for palm oil and pulp plantations and other drainage dependent agricultural land-uses. In 2010, 20% of the peatlands in South-east Asia were occupied by palm oil and pulp plantations and only 34% remained under (mostly degraded) forest cover. Extrapolating past trends, the total area covered by plantations could double or even triple by 2020 (Miettinen et al. 2012).
If current business as usual land-use planning for development of drainage-based plantations on peat is not radically changed and if current unsustainable land uses on peatlands will not be phased out, large-scale subsidence and flooding of lowland peatlands will become inevitable with tremendous social, ecological and economic consequences.
Recommendations for sustainable peat landscapes
- No (new) drainage based plantations on peat. There is a need for a stronger Moratorium on conversion of peatlands and other forests in Indonesia and a similar moratorium should be considered for Malaysia;
- Issuance of a Government Regulation limiting drainage use in forest and plantation concessions in peatlands;
- Conservation of all remaining tropical peat swamp forests in view of their valuable ecosystem services (including carbon storage, water regulation, biodiversity);
- As an interim measure towards permanent solutions, existing agriculture and plantations should minimise drainage, bringing the water level up, curb the application of fertilizers and maintain a permanent soil cover to reduce the rate of peat soil degradation and enhance peat soil protection;
- In the mid and long term, degraded peatlands should be rewetted and rehabilitated, either to natural habitat or to alternative land-uses like paludiculture (cultivation of commercially interesting crops on rewetted peatlands). Indigenous peatland species like Tengkawan (Ilipe Nut), Jelutung (Asian latex) and Sago (starch) can yield an income for local people and provide opportunities for global marketing;
- Plantation companies will eventually have to abandon their peat-based plantations or change their production systems: they can either move to suitable mineral soils areas, or must switch to production systems that require no drainage;
- Implementation of environmental and social safeguards in peatland development, restoration and conservation.