Natural decay resistance of wood is mainly restricted in the heartwood and based on wood microstructure and chemical composition. The genetic variation in the durability of Scots pine (Pinus sylvestris L.) wood against decay and the relationship between resistance and chemical composition was studied. The laboratory tests of 6 and 8 weeks were carried out with a brown rot fungus Coniophora puteana (Schum. ex Fr) Karst (strain Bam EBW 15). The increment core samples of sapwood and juvenile heartwood, obtained from about 30-year-old half-sib progenies, were decayed using a modification of the standardised EN 113 method. The average density of the sapwood was 391 mg/cm3 (one stand) and that of heartwood samples 337 and 376 mg/cm3 (two stands). The average mass loss of the sapwood samples was 114 mg/cm3 (6 weeks, stand 1) and that of the heartwood samples 80 mg/cm3 (6 weeks, stand 1) and 123 mg/cm3 (8 weeks, stand 2). The variation among heartwood samples was high. The additive genetic component was small in stand 1 but quite large in stand 2, which resulted in a low narrow sense heritability h2 in stand 1 (0.07) but high in stand 2 (0.37). New samples were taken from the most resistant and susceptible trees for chemical analyses. The concentration of resin acids was higher in the group of decay resistant trees than in the group of decay susceptible trees. About 90 % of the resin acids in the heartwood were of the abietane type, abietic acid being the most abundant.

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