Natural substances, purified from naturally rot-resistant trees and responsible for natural durability, also known as natural "extractives", are another promising wood preservatives. Several compounds have been described to be responsible for natural durability, including different polyphenols, lignins lignans, such as gmelinol, plicatic acid), hinokitiol, α-cadinol and other sesquiterpenoids, flavonoids, such as mesquitol, and other substances. These compounds are mostly identified in the heartwood, although they are also present in minimal concentrations in the sapwood. Tannins, which have also shown to act as protectants, are present in the bark of trees. Treatment of timber with natural extractives, such as hinokitiol, tannins, and different tree extracts, has been studied and proposed to be another environmentally-friendly wood preservation method.