Why is the heartwood harder and more durable than the sapwood in most timber?

UV microspectrophotometric studies on the extractives in heartwood indicate that polyphenols and flavonoids constitute part of the cell wall matrix. Phenolic compounds contain an aromatic structure that has characteristic IR absorption bands around 1600 and 1500 cm-1; they are typical heartwood extractives which account for the high level of natural wood durability . Thus chemical connections formed between extractives and wood components, would restrict the softening of the wood biopolymers.