The role of extractives in naturally durable wood species

• Nine wood species were evaluated for durability in fungal soil bottle and termite bioassays.
• Extracted and unextracted blocks were used to examine the influence of extractives.
• Results found extracted blocks were generally significantly less durable than unextracted controls.
• Honey Mesquite and Black Locust still exhibited significant durability once all extractives were removed.
• The relative important of extractive content are discussed.

There are numerous examples of wood species that naturally exhibit enhanced performance and longevity in outside exposure independent of preservative treatment. Wood extractives are largely considered to be the contributing factor when evaluating and predicting the performance of a naturally durable wood species. However, little test methodology exists that focuses on the extent of the role of extractives in wood durability. In this study, eight candidate naturally durable wood species plus a non-durable control were evaluated in laboratory soil block tests for resistance to termite attack and decay by three brown-rot and three white-rot decay fungi. Chemically extracted test blocks were compared to unextracted controls. Extracted durable species were also compared to non-durable controls. Results showed nearly all of the wood species exhibited higher weight loss due to termite or fungi when extractives were removed and extracted samples had weight losses that were comparable to the non-durable controls.