Heartwood Chemistry Predicts Natural Durability in <em>Pinus nigra</em> Clones: The Critical Role of Resin Acid over Stilbenes in Decay Resistance

The natural durability of wood, determined primarily by its chemistry, meets the growing demand for environmentally sustainable alternatives to toxic wood preserva-tives. This study investigated the relationship between heartwood chemical composi-tion and decay resistance among fifty-two (52) Pinus nigra Arn. clones from a clonal seed orchard in Greece. Quantitative 1H-NMR spectroscopy was employed to determine total acetone extractives (TAE), total stilbenes (TS), and total resin acids (TRA) in heartwood samples, while decay resistance was evaluated through standardized weight loss tests using Coniophora puteana (Schumach.) P. Karst. (1865) and Porodaedalea pini (Brot.) Murrill (1905). The heartwood exhibited exceptionally high extractive content (mean TAE = 304.15 mg/gdhw), with resin acids (68.26%) predominating over stilbenes (22.31%). Re-gression analysis showed that TAE and TRA were the strongest predictors of decay re-sistance, explaining 33% of the variance, while stilbenes exhibited weaker and more variable associations. Porodaedalea pini caused significantly higher mean weight loss (11.43%) than C. puteana (3.55%), indicating species-specific fungal aggressiveness. Among individual resin acids, abietic acid were the most influential contributors to decay resistance. The results demonstrate that resin acids have dominant role over stilbenes in determining the natural durability of P. nigra heartwood and could serve as effective biochemical markers for selective breeding.