How the relationship between density and shrinkage of wood depends on its microstructure

It has long been recognized that density is the dominant parameter in determining the shrinkage behavior of wood; denser wood generally shrinks more from the green to oven dry condition. There exists a more or less linear relationship: ~28 times the specific gravity gives the percentage volume shrinkage. There are of course not insignificant deviations, but overall this relationship holds. That such a relationship should exist is not at all intuitive; no acceptable explanations for this behavior appear in literature. A mechanical analysis of a cell model wherein the implications of the wall microstructure are taken into account shows that the nature of the microstructure is crucial for explaining the shrinkage behavior of wood with respect to its density. At the macro level, the fact that shrinkage in the tangential direction is significantly greater than shrinkage in the radial direction and that the ratio of these shrinkages is inversely proportional to the ratio of elastic moduli is also explained, and this relationship is utilized.