Wood does indeed expand and contract a lot due to changes in humidity. The expansion and contraction is far greater in certain directions and will vary accordin

Wood does indeed expand and contract a lot due to changes in humidity. The expansion and contraction is far greater in certain directions and will vary according to species and how the grain is orientated in a particular board. Laminiating strips or sheets in addition to the strength from the glue (typically in plywoods this is done with perpendiclar grain directions) can counter this. For a solid wood panel door for example you really want a very stable hardwood species for the frame and for the panels to be fitted without glue into recesses with room for them to expand and contract.