What is the most common type of wood used to make paper?

The most common wood used in paper making is the scrap from lumber production, and as to the species...whatever they harvest close to the paper mill is what they use. There are two broad categories of paper: groundwood and free sheet. Groundwood is the old-fashioned kind of paper, and there you really need softwood timber because they make it by soaking wood in water until the fibers come apart. Most groundwood ends up as cardboard. Free sheet is made by dissolving the fibers in chemicals. By the time the wood is sent into the sheet-forming process it's no longer wood but a kind of cellulosic glop, for lack of a better term. They're starting to really like poplar; it's as easy to work with as pine, and it grows so fast you can cut it for paper production in only seven years.