So wood not only dries and shrinks when exposed to low humidities, but it also regains moisture and swells when exposed to higher humidities. So, changes in RH cause shrinkage or swelling. In general, the denser the wood, the more it will shrink and swell. As the amount of shrinkage is directly related to checking, the denser woods are harder to dry. Wood shrinks the greatest amount in the tangential direction, as mentioned earlier.
Longitudinal shrinkage is usually negligible.
However, there can be appreciable longitudinal shrinkage in wood in the juvenile core or in tension wood. As an example of why longitudinal shrinkage causes warp, consider a quartersawn piece of lumber with one edge having juvenile wood and the remainder of the piece having mature wood. The juvenile wood edge will shrink lengthwise, while the rest of the piece will shrink very little lengthwise. This difference results in crook toward the pith. The same scenario, but with the shrinkage differences being between faces rather than edges, is a major reason why lumber bows.