Quantification of total phenolic compounds in the heartwood of Picea abies using the Folin–Ciocalteu methods showed that concentrations of phenolic compounds in the heart‐ and sapwood of Picea abies varied, depending on the decay class to which boles were allocated. In the heartwood, mean phenolic concentration was 3.28 mg/g, compared with 2.44 mg/g in sapwood. Experiments using different concentrations of phenolic compounds showed that Heterobasidion parviporum is more sensitive to the effects of phenolic compounds found in sapwood, and less sensitive to phenolic compounds found in heartwood. The high concentration of phenolic compounds extracted from spruce stimulated the decomposition of wood by Armillaria ostoyae. This demonstrates the great ability of A. ostoyae to metabolize phenolic compounds. This is confirmed by natural conditions, where A. ostoyae attacks mainly sapwood and H. parviporum attacks heartwood of P. abies. In heartwood, the average antioxidant activity was approximately 10% higher than in the sapwood. The extraction process and presence of amino acids and reducing sugars might influence on the results. The results suggest that timber quality monitoring in spruce stands could be partly based on measurements of phenol compound concentrations in sapwood, as the technique enables prediction of the potential impact of decay fungi early in succession.