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Type of Wood
The type of wood plays a huge role in the yield when converting it to pulp. Wood is made up primarily of cellulose fibers and lignin. Lignin binds the fibers together. The various pulping processes work to break down the lignin in order to get a heavily fibrous pulp, which makes the best paper. The lignin in a tree is what makes it the wood hard. The woods that produce the highest yields when being pulped are those with the longest and most dense fibers. Wood chips from softwoods grown in northern climates consistently produce the highest pulping yields. (See Reference 3)
Age of Wood
Once you strip the bark away from a tree, you'll find two types of wood inside. The dark-colored wood in the center of the tree is called the heartwood, which is dormant, and wood chips from this produce the highest yield when pulped. The outer, lighter-colored wood is called the sapwood. This is the wood through which fluids move up from the roots into the tree to produce growth in the tree. The fibers in the sapwood are not fully mature and will produce a lower yield than the heartwood. No tree, regardless of its quality, is considered pulp material until it is 10 years old. Trees will not produce the highest pulp yield they are capable of until they are past 20 years old. The older the tree, the greater the heartwood and, thus, the greater the yield. (See Reference 4)
Condition of Wood
The same type of tree, grown in different settings and climates, will produce sometimes dramatically different yields. A close proximity to an ample water supply such as a lake or river will promote a tree's growth, but the raw wood will have a greater water content in it. The water content of a tree can exceed 10 percent, all of which is lost during pulping. It is important to keep the wood chips of a uniform size to produce both the highest yield and highest quality from the wood used. (See Reference 5)
Pulping Processes
The single biggest factor in the yield you get from any set of wood chips is the type of process that is used in pulping. A process can be completely mechanical, completely chemical or any combination in between. Mechanical processes tend to damage fibers and leave a lot of damaged lignin intact in the final pulp. Chemical processes can be used that will completely eliminate lignin and leave fibers with little damage. That type of chemical process produces some of the highest quality paper, but with low yields. High-yield processes often use bleaching to provide brightness to paper that, otherwise, needs not be of the highest quality, such as newsprint and notebook paper.