Increase in the moisture content in the cell walls causes swelling. Swelling of wood causes a noticeable increase in its dimensions. The swelling extent is dependent on the wood species, its structure, density and the drying conditions. The major parts of the wood affected by the swelling are cellulose and lignin, the cellular makeup of wood that makes it rigid. When these parts swell, they manifest on the surface of the wood.
A reduction in moisture content in cell walls causes shrinking, which is the exact opposite of swelling. When the wood begins to dry and consecutively lose moisture below the fiber saturation point, it begins to shrink. There are two types of shrinking: transverse shrinking where the wood shrinks from the pith outwards and longitudinal shrinking, where the shrinking takes place along the grain. Longitudinal shrinkage is not extensive as its values stand at 0.1 and 0.2 percent for most wood species.
Excess moisture causes wood to rot. Growth of fungi which thrives at moisture contents of 20 percent and above characterizes rotting. Two types of wood basically exist: one with brown coloration and scrappy appearance, and the other one with white or yellow coloration and a spongy look. Wood affected by rot is generally darker than the unaffected one.
Adverse effects of cupping and crowning causes excessive exposure of wood to high moisture levels. Cupping occurs when a block of wood forms a concave shape. In this scenario, the center of the wood is at a lower level than the edges. Disparity in terms of moisture content in several parts of the wood causes cupping. Crowning, on the other hand, occurs when the edges of the board of wood are at a lower level than the center and excessive moisture content at the center causes it.