This post is also available in Indonesian here.
Drainage of peatlands in Indonesia to enable cultivation of plantation crops, including oil palm, has previously been linked to catastrophic fires and toxic haze, causing deaths, illness, and financial losses. In their latest research, Warren-Thomas and colleagues find out whether peat restoration initiatives affect trade-offs between biodiversity and oil palm yields on smallholder farms.
Tropical peatlands – reservoirs of carbon and biodiversity
Peat, a carbon rich soil formed from partly decomposed vegetation in permanently wet conditions, can form into deposits metres thick covering thousands of square kilometres in tropical environments. Peat swamp forests, full of specialised tree species that can cope with acidic soils and permanent inundation, grow on top of these peatland soils, adding their leaf litter to the slowly accumulating carbon stock beneath their roots.
In Indonesia, these forests also support endemic and threatened species of wildlife and plants, from orangutans and gibbons to microbes, making their protection key for biodiversity conservation. Demand for land has meant that efforts to clear forests and drain water from peatlands have taken place on an industrial scale, enabling the planting of Acacia, oil palm and other crops. However, this drainage means that the peat carbon, once kept safe from decomposition under water, is exposed to the air, breaking down to release carbon dioxide.
Emissions from decomposing peatland contribute to climate change, meaning peat conservation is critical for efforts to reduce global warming. Drained peat is also highly flammable: fires set to clear land can readily escape into the soil itself and burn for months, releasing toxic smoke in the process. During past ENSO events, when rainfall is low across Southeast Asia, peat fires and haze were severe enough to close airports, cause health issues, and deaths.
This means that there are efforts in Indonesia to retain water in peatlands, including by the Indonesian Peat Restoration Agency and introduction of new regulations on water table depths that must be followed by companies with peatland plantations. A recommended water table depth is -40cm below the soil surface.
How do peat restoration efforts affect peatland cultivation – and what’s the link to biodiversity?
Reducing peat drainage involves the blocking of drainage channels, raising water tables closer to the surface. This could affect the yields of oil palm, which could negatively impact livelihoods for smallholder farmers. Having wetter, boggier farms could also effect on-farm biodiversity, by altering how smallholders manage their oil palm plots.
It is already known that there are trade-offs between biodiversity value and yields in oil palm, relating to the intensity with which they are managed: for example, smallholdings with complex undergrowth tend to support more bird species.
Investigating bird diversity and oil palm yields in Jambi province, Sumatra
We investigated whether variation in water table depth on peatland oil palm farms affected yields, the understorey vegetation growing on the farms, and the bird species found living there, in 40 farms across a landscape in Jambi province, Sumatra. As we couldn’t do a before-after study of water table management, we found sites across the study landscape where water tables already differed due to differing degrees of drainage and canal management. We also investigated water table depth and bird species in a nearby area of peat forest, Sungai Buluh Peat Protection Forest, to find out how many bird species were shared between the oil palm and the forest, and how drainage was affecting water tables.
At each farm where farmers agreed to take part, we took peat cores using an auger and installed water table measuring pipes. We recorded multiple metrics of undergrowth complexity, and surveyed birds by conducting point counts. Finally, we conducted interviews with farmers to find out about their oil palm yields, as well as how they managed their farms.
Water table depths in smallholder oil palm are relatively shallow, and variation doesn’t affect yields
A key finding from the months of work spent digging, counting, measuring, blowing bubbles down tubes and talking to dozens of farmers who volunteered their time, was that water table depths didn’t appear to affect oil palm yields, or the diversity of birds found on farms. The water table depths in this landscape, as an annual mean average, ranged from -3 to -52 cm below the ground, but in the driest season dropped as low as -169 cm. We know that drainage can be much deeper in commercial plantations – but these results show that water tables can be much higher while supporting smallholder oil palm production.
Another key finding was that the bird species found in the oil palm farms were very different to those in forest. We found only half the number of bird species in total, and very different types of birds living in the two habitats.
Peat landscape restoration – looking to the future of forest conservation and peatland cultivation
There are big questions about the future of cultivation on drained peatlands. We know that when water is drained from peat, land subsides as the peat decomposes. Many peatlands are low-lying and coastal, so the land could eventually become permanently inundated with water and no longer suitable for cultivation. Forest recovery is therefore one option in the longer term but it would need both seeds and seed dispersers. Remaining forest fragments are critical sources of both.
Drainage in cultivated peatland can affect nearby forests, making them vulnerable to fires, as observed in Jambi during ENSO years. Keeping water tables higher to reduce fire risk can therefore protect remaining peat forest fragments – together with their carbon and biodiversity – for the future.
Our study shows that this should be possible without negatively impacting farmers’ oil palm yields, and offers new evidence to support sustainable peatland management in the long term. Management of tropical peatlands to protect livelihoods, biodiversity and peat carbon stores will continue to be a challenge for decades to come. The data collected for this study, including water table, rainfall and yield data, made freely available by this project, also offers a good opportunity to monitor outcomes in coming months for future research teams.
Read the full paper No evidence for trade-offs between bird diversity, yield and water table depth on oil palm smallholdings: Implications for tropical peatland landscape restoration in Journal of Applied Ecology