Doors stick in damp weather, and chairs pull apart when it's dry. Wood is a marvelous material in many ways, but it does have its problems under changing humidity conditions. The original function of wood, of course, was to hold up trees, not houses. Wood that is supporting a living, healthy tree is not subject to large, uncontrolled humidity changes. The inner part of the trunk is shielded from fluctuations by the outer part, which is carrying moisture between the leaves and the roots of the tree.
Food travels in the living phloem tissue just under the bark.
Water and minerals are carried through the xylem, the living but no longer growing outer part of the woody trunk. When the tree dies or is cut, it dries until its moisture content is in balance with the humidity of the surrounding air. As water is removed from the xylem tissue, it shrinks. But the shrinkage is not uniform. The cells of the xylem have the dual job of transporting fluids along the tree and resisting bending.