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A board is a living product. During its life, moisture is taken on and is then lost, which does change the width of the board. Timber only dies when burnt.
Hey Verna,
Sorry to disagree but, the wood that is your lumber is dead, and has been for a long time, even when it was still part of the tree. In fact, the only living part is a thin layer of cells just under the cambium (essentially the last growth ring).
The reason wood shrinks is the same reason a sponge shrinks as it dries. The water evaporates, and the structure that takes up the volume of the wood collapses, or partially collapses. In living tissues, there are generally two types of water, cellular water, the medium that supports the cellular organelles, and bound water, which is water molecularly bound to the proteins and carbohydrates of the cells. In vascular plants such as trees, there is also the water carried by the xylem and phloem, the micro tubules that transport the water and nutrients up and down the tree. Most of that water is lost when the tree is cut (logging can be a very wet business).
The first water lost is the vascular water, followed by the cellular water, and finally the bound water.
Tom
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