A tree is a living, growing being, and when you cut one down for lumber, its natural processes continue for a short time. The cells hold and transfer water until it finally evaporates. The cells then contract and the wood shrinks and warps. The amount of deformity a plank of wood sustains depends on what kind of wood it is, which part of the tree it came from and how it was cut.
Why Wood Warps
Trees transfer water from the roots to the leaves primarily through the bark and outside of the trunk. The wood from this part of the tree is younger and more water-laden, consequently shrinking more when it dries out. A board that contains sapwood from the outside of the trunk and heartwood from the middle tends to warp in the direction of the sapwood as it dries. The direction of warping depends on the configuration of the grain, the width and thickness of the board, and overall density of the wood.
Flat-Sawn Vs. Quarter-Sawn Wood
Millers can make boards from a tree by cutting flat pieces from the entire trunk it or dividing the trunk lengthwise into quarters before cutting boards from it. Boards that are flat-sawn using the former method are more likely to have a mixture of heartwood and sapwood across the grain. Because sapwood shrinks more, it is more likely to warp. Boards that are quarter-sawn using the the latter method, however, are more stable because the grain is symmetrical. The wood dries out evenly and tends not to warp.
Wood with a close grain has less of a tendency to warp than open grain. The closer the grain, the less space between the wood fibers, reducing the possibility of shrinking. Hardwoods generally have a closer grain than softwoods, and the densest hardwood species grow in the tropics. Many tropical species also contain natural oils that remain in the grain after the wood has dried. This prevents shrinkage. Because it is so hard, wood from these species is difficult to mill and work. Most of the species have been over-harvested, so the wood is rare and expensive.
The Most Stable Wood
The tendency for a board to warp depends on the species of wood and how the board was cut from the tree. The most stable board is cut from a hardwood tree with dense grain in a way to minimize shrinkage rates across the grain. Wide boards, even dense hardwood ones, are prone to warpage if they come from the outside of the tree. On the other hand, even a softwood board may resist warping if it is primarily heartwood. Because they have a close, symmetrical grain, quarter-sawn hardwood boards are usually the most stable.