The environmental conditions like heat, sunlight, moisture and possible chemical reactions.
Living tree has bark to protect the its insides. The bark can usually weather the environmental conditions because the tree can renew it with the nutrients it gets from the ground. However, when a tree is cut down and stripped of its bark it loses the access to the bark’s protection and all the water, nutrients and energy it creates with its chlorophyll and dies.
The dead tree, cut into wooden planks, doesn’t have anything to protect it from the environmental conditions anymore, so now the heat (or cold), sunlight and moisture + possible chemicals get to react directly with the wood and slowly damage it. We see the damage as gray, because the wood loses basically all the chemical components of a living tree which leaves mainly cellulose behind.
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