Farmers, especially in developing countries, are often criticized for cutting down forests. But a new study suggests that many farmers recognize the value of keeping trees. Researchers using satellite 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. Earlier estimates were much lower but incomplete. 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 alone 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, markets or government policies can influence tree planting and protection. Dennis Garrity, who heads the World Agroforestry Center, says farmers are acting on their own to protect 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, fruit, wood and other products. They also help prevent soil loss and protect water supplies. Even under drought(干旱)conditions, trees can often provide food and a way to earn money until the next growing season. Some trees act as natural fertilizers. They take nitrogen(氮气)out of the air and put it in the soil. Scientists at the Center say the use of fertilizer trees can re-duce the need for chemical nitrogen by up to three-fourths. Trees also capture carbon dioxide, a gas linked to climate change. 小题1:Through the study, the researchers found that .
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