Heartwood (or duramen) is wood that as a result of a naturally occurring chemical transformation has become more resistant to <answer>decay</answer>. Heartwood formation occurs spontaneously (it is a genetically programmed process). Once heartwood formation is complete, the heartwood is dead. Some uncertainty still exists as to whether heartwood is truly dead, as it can still chemically react to <answer>decay</answer> organisms, but only once.
What is heartwood naturally resistant to?
decay