 Repressive measures against small farmers destroy peoples' livelihoods David Kaimowitz |
The UN's Food and Agriculture Organization (FAO) and the Center for International Forestry Research (Cifor) say the evidence shows no link.
Loss of forest cover does play a role in smaller floods and in the loss of fertile topsoil, it says.
It accuses Asian governments of blaming floods on small-scale loggers and farmers to deflect criticism.
Widespread but wrong
The belief that deforestation causes major floods and increases the damage which they do appears to be widespread.
China's catastrophic floods of 1998, when the Yangtse and Yellow rivers broke their banks, were linked by Chinese officials to deforestation; the environmental group WWF and the Red Cross also drew a causal connection.
 The incidence of major floods has not increased despite deforestation |
But the FAO/Cifor report cites evidence from Bangladesh, Nepal, South Africa, Thailand and the US showing that the frequency and extent of major floods has not changed over the last century or two, despite drastic reductions in forest cover.
"I think the belief comes about because forests do help to reduce floods in small areas, and so people assume it must also apply to severe floods in large areas," said Cifor's director-general David Kaimowitz.
"But our sense is that in general the conclusions of scientific studies indicate that changes in land use and land use cover have only a minor role in large-scale flooding events," he told the BBC News website.
Natural barrier
On a smaller scale, forests act like sponges, soaking up excess water.
Water can spread out and be absorbed in surrounding forest soil; but when all the woodland is inundated, the ground simply does not have enough capacity.
Reports of the recent floods caused by Hurricane Katrina and Tropical Storm Stan in north and central America have suggested that vegetation formed a natural barrier to flood water.
Research has shown that during last December's Asian tsunami, coastal zones with intact mangrove swamps fared better than other areas.
"Those studies come mainly from Sri Lanka," observed Dr Kaimowitz, "and the tsunami had to go all the way across the ocean to get there.
"I have been in Aceh where you had waves 20m high - no mangrove is going to stop that."
The report does acknowledge, however, that forests can safeguard natural resources by binding soil and preventing it from being washed away.
This has been a particular factor in Haiti, which has lost more than 90% of its original forest cover, tying into a cycle of poverty and environmental degradation which sees the poorest people in the Americas scrambling to exploit what land is left, often unsustainably, which then leads to further soil loss during the next flooding event.
Blame game
Professor Edmund Penning-Rowsell, head of the Flood Hazard Research Centre at Middlesex University in London, UK, believes the FAO/Cifor conclusions are broadly correct.
"There is an effect from forests," he told the BBC News website, "but it disappears as you get to larger and larger scales.
"People tend to up-scale their ideas from personal experience; but you can't scale up from local floods to something like the Ganges.
"Most floods are caused by people building in the wrong places; and Bangladesh for example is almost entirely built on a flood plain, which is very fertile but does carry a big risk."
 |  Most floods are caused by people building in the wrong places Edmund Penning-Rowsell |
FAO/Cifor say that Asian governments in particular have curbed small-scale logging and land clearance by local people without justification.
"You have to be very careful before taking repressive measures against small farmers on hillsides or small-scale logging activities, because you're destroying peoples' livelihoods," said David Kaimowitz.
"The most extreme case was the logging bans established in regions of China following the floods of 1998 which put more than a million people out of work, when it's almost certain that logging played very little role in the floods.
"Sometimes one gets the impression that governments don't want to consider whether they should have warned people, evacuated people, made sure they're not so poor that they have to live in vulnerable areas."
Floods on the rise?
These are issues which are unlikely to disappear soon, as the incidence of floods with major loss of life appears to be rising rather than falling.
As the FAO/Cifor report makes clear, this is partly due to the growing global population and the consequent expansion of human settlements into areas which had once been marginal. As a result, each flood claims more lives than it would have done a century ago.
Also, it says, human diversion of watercourses has changed the pattern of floods, often moving a problem from upstream to downstream areas.
But Edmund Penning-Rowsell believes that there may be another factor - that the true frequency of floods is beginning to increase.
"It does appear that large floods are becoming more frequent," he said.
"It might take us 100 years to find out for sure - but there does seem to be a tendency in that direction."
