Small and large-scale farmers in Riau province, Sumatra, have been blamed for the recent choking smoke smothering Singapore and parts of Malaysia. But scientists in Indonesia have added a third category of 'mid-level entrepreneurs'. These entrepreneurs buy unregulated access to land for oil palm and clear it by burning, seemingly unrestrained by government.

Scientists at the World Agroforestry Centre, who have been studying land conversion in Sumatra, say they have identified this third group of local land investors who operate outside the government system, making them potentially more difficult to regulate.

These people acquire land under informal rules at village level, effectively sidestepping the Government land-use system. They bring in their own labour to clear the land for oil palm, regardless of the land's formal government status and in the absence of any permits to do so.

Policies and policing need to be adjusted to deal with the newly identified group if the annual fires and subsequent haze are to be reduced. Holding plantation companies accountable for the fires within their boundaries would help reduce the problem but not extinguish it.

About half of the fire 'hot spots' in Riau province are on land with legal permits for large-scale operations (industrial timber, oil palm and logging). The rest occur as part of illegal activities, in areas which have been slated for conservation or non-production.

These hot spots are mostly concentrated in three districts within Riau province. Some neighbouring districts with similar conditions have so far avoided the problem this year, which suggests that lessons might be learnt about governance.

The fire-haze episode straddling the Strait of Malacca in June 2013 has reignited a decades-long debate about responsibility. In the current debate, finger pointing still alternates between the small- and large-scale agricultural operators. The latter include companies with headquarters in Singapore and Malaysia, where the undesirable haze accompanies the financial returns on their investments.

Before 1998, the blame for starting the fires was put exclusively on smallholders' 'shifting cultivation' techniques, with large-scale plantations and development projects protected from any criticism by the government.

But the 1997/8 fires in Sumatra and the change of regime in Indonesia threw new light onto the debate and it became evident that burning was the cheapest option widely used by all farmers, whether on a small or large scale or on peat or mineral soils.

Ekadinata E, van Noordwijk M, Budidarsono S, Dewi S. 2013. Hotspots in Riau, haze in Singapore: the June 2013 event analyzed. ASB Policybrief 33. Nairobi: ASB Partnership for the Tropical Forest Margins.