Heartwood Chemistry Predicts Natural Durability in Pinus nigra 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 preservatives. This study assessed the relationship between the fungitoxic acetone extractive content, in particular resin acids and stilbenes, and heartwood decay resistance among fifty-two Pinus nigra J. F. Arnold clones from a clonal seed orchard in Greece. Quantitative 1H-NMR spectroscopy was employed to determine total stilbenes (TSs) and total resin acids (TRAs) in heartwood samples, while decay resistance was evaluated through standardized weight loss tests using the brown-rot fungus Coniophora puteana (Schumach.) P. Karst. (1865) and the white-rot fungus Porodaedalea pini (Brot.) Murrill (1905). The heartwood exhibited exceptionally high extractive content (mean TAE = 304.15 mg gdhw−1), with resin acids (68.26%) predominating over stilbenes (22.31%). Regression analysis showed that the TAE and TRAs were the strongest predictors of decay resistance, explaining 33% of the variance, while stilbenes exhibited weaker and more variable associations. P. pini caused significantly higher mean weight loss (11.43%) than C. puteana (3.55%), indicating species-specific fungal aggressiveness. Among individual resin acids, abietic acids were the most influential contributors to decay resistance. The results demonstrate that resin acids have a dominant role over stilbenes in determining the natural durability of P. nigra (Black pine) heartwood and could serve as effective biochemical markers for selective breeding.