It reveals that on more than one billion hectares -- which make up 46 percent of the world's farmlands and are home to more than half a billion people -- tree cover exceeds 10 percent. That includes 3.2 million sq km in South America, 1.9 million in sub-Saharan Africa and 1.3 million in Southeast Asia.

Scientists from the World Agroforestry Centre (WAC) using detailed satellite imagery have reported the discovery.

This is the first study to quantify the extent to which trees are a vital part of agricultural production in all regions of the world.

"The area revealed in this study is twice the size of the Amazon, and shows that farmers are protecting and planting trees spontaneously," WAC Director General Dennis Garrity said.

"Trees are providing farmers with everything from carbon sequestration, to nuts and fruits, to windbreaks and erosion control, to fuel for heating and timber for housing," he added.

"Unless such practices are brought to scale in farming communities worldwide, we will not benefit from the full value trees can bring to livelihoods and landscapes," Garrity added.

"If planted systematically on farms, trees could improve the resiliency of farmers by providing them with food and income," said Tony Simons, deputy director general at the WAC.

"Whilst Western Europe has some 250 native tree species and North America has a larger set of 600 trees species -- the developing tropics has a staggering 50,000 tree species to manage and utilise," said Simons, according to a WAC release.

"For example, when crops and livestock fail, trees often withstand drought conditions and allow people to hold over until the next season."