Going Out On A Limb With A Tree-Person Ratio | NPR

Trees, both leafy and otherwise, reflect sunshine in very particular patterns, making it possible for satellites to map and computers to count strips of land where trees are. Biologists can then sample those places - forests, suburbs, city parks, even city streets - assume a tree density, multiply by acre or hectare, and calculate that. In 2005, there were 400,246,300,201 (more or less) trees on our globe. ...

Punching the figures into her calculator, she figured that the world supports 61 trees per person.