Almost half of the world's farmland has at least 10 per cent tree cover, according to a study indicating that farmers are far less destructive to carbon-storing forests than previously believed.
"The area revealed in this study is twice the size of the Amazon, and shows that farmers are protecting and planting trees spontaneously," Dennis Garrity, director general of the World Agroforestry Centre in Nairobi, said in a statement.
The centre's report, based on satellite images and the first to estimate tree cover on the world's farms, showed tree canopies exceeded 10 per cent on farmland of 10 million square kilometres - 46 per cent of all agricultural land and an area the size of Canada or China.
By one yardstick used by the UN Food and Agricultural Organisation, a "forest" is an area in which tree canopies cover at least 10 per cent of an area. The definition excludes, however, farmland or urban areas.
The report said that farmers keep or plant trees for uses such as production of fruit, nuts, medicines, fuel, building materials, gums or resins. Trees also provide shade for crops, work as windbreaks, boundary markers or to help avert erosion.
And trees are often hardier than crops or livestock so can be a back-up for farmers on marginal land in hard times.
Previous estimates of the area of farmland used in agroforestry had ranged up to only about three million square kilometres.
Farms are often portrayed as enemies of forests - homes to a wide diversity of animals and plants. Forests are also giant stores of heat-trapping carbon dioxide, the main greenhouses gas.
"We're pleasantly surprised - it quantifies an under-appreciated resource," said Tony Simons, deputy director general of the World Agroforestry Centre.
The report found that trees were integral to agricultural landscapes in all parts of the world, with the exceptions of arid North Africa and West Asia.
Mr Simons said the report indicated a new front for fighting climate change. Farmers would do more to preserve trees if they got credits under a new UN climate pact due to be agreed at a meeting in Copenhagen in December.
Negotiators are looking at ways to slow deforestation in developing nations - deforestation accounts for 20 per cent of all emissions of greenhouse gases from human sources.
"This study offers convincing evidence that farms and forests are in no way mutually exclusive," said Wangari Maathai, a Kenyan environmentalist who won the 2004 Nobel Peace Prize for a tree-planting campaign across Africa.
At the end of the last Ice Age about 10,000 years ago, natural forests covered about 70 per cent of the world's land area. They now cover only about 26 per cent.
Net deforestation rates, according to the FAO, slowed to 7.3 million hectares per year from 2000-05, an area the size of Sierra Leone or Panama, from 8.9 million in 1990-2000.