water in the cell cavities is called free water. As the name implies, the free water is relatively accessible, and an accessible source of water is necessary for decay fungi to start growing. Therefore, decay can generally only commence if the moisture content of the wood is above fiber saturation. The fiber saturation point is also the limit for wood shrinkage. Wood shrinks or swells as its moisture content changes, but only when water is taken up or given off from the cell walls. Any change in water content in the cell cavity will have no effect on the dimension of the wood. Therefore, wood...shrinks and swells (only) when it changes (to) moisture content below the point of fiber saturation.