This is the VOA Special English Agriculture Report.

Farmers, especially in developing countries, are often criticized for down forests. But a new study suggests that many farmers recognize the value in keeping trees.

Researchers using images found at least ten percent tree cover on more than one billion hectares of farmland. That is almost half the farmland in the world.

The World Agroforestry Center in Kenya led the study. The findings were reported last week in Nairobi at the second World Congress of Agroforestry.

Earlier estimates were much lower but . The authors of the new study say it may still underestimate the true extent worldwide.

The study found the most tree cover in South America. Next comes Africa south of the Sahara, followed by Southeast Asia. North Africa and West Asia have the least.

The study found that climate conditions could not explain the amount of tree cover in different areas. Nor could the size of nearby populations, meaning people and trees can live together.

There are areas with few trees but also few people, and areas with many trees and many people. The findings suggest that things like land rights, or government policies can influence tree planting and protection.

Dennis Garrity heads the World Agroforestry Center. He says farmers are acting on their own to and plant trees. The problem, he says, is that policy makers and planners have been slow to recognize this and to support such efforts.

The satellite images may not show what the farmers are using the trees for, but trees provide nuts, , wood and other products. They provide windbreaks and shade from the sun. They also help prevent soil loss and protect water supplies. Even under drought conditions, trees can often provide food and a way to earn until the next growing season.

The important thing, says one expert, is to find the right tree for the right place for the right use.

Some trees act as natural fertilizers. They take nitrogen out of the air and put it in the soil. Scientists at the Agroforestry Center say the use of fertilizer trees can the need for chemical nitrogen by up to three-fourths. And they say it can double or triple crop production.

Trees also capture carbon dioxide, a gas linked to climate change.

Wangari Maathai is a Kenyan environmentalist and Nobel Peace Prize winner. She says the study shows that trees are to agricultural production everywhere.

And that's the VOA Special English Agriculture Report, written by Jerilyn Watson. I'm Jim Tedder.