It was calculated for newsprint, which is made in a totally different papermaking process from office and printing papers.
Paper is made from a mix of types of trees.
Some are hardwood, some are softwood. In addition, some are tall, some old, some wide, some young, some thin. We decided it was time to update these numbers, so Conservatree has tracked down some ways to make ballpark estimates more reliable than in the past. As a ballpark estimate, you can use .64 as the fiber estimate for coated papers compared to the entire weight of the sheet. If we assume that the groundwood process is about twice as efficient in using trees, then we can estimate that it takes about 12 trees to make a ton of groundwood and newsprint.