EFFECTS OF CLEAR-CUTTING, THINNING, AND WOOD MOISTURE-CONTENT ON THE SUSCEPTIBILITY OF NORWAY SPRUCE STUMPS TO HETEROBASIDION-ANNOSUM

The effects of clear-cutting and thinning as well as heartwood and sap wood moisture content on spore infection by Heterobasidion annosum (Fr .) Bref. were investigated in summer cuttings in southern and central Sweden. At five sites, 20-100 stumps in clear-cut, thinned, and precom mercially thinned stands of Norway spruce (Picea abies (L.) Karat.):we re created and left for natural spore infection. An additional 20 stum ps per stump type and site were artificially inoculated with conidiosp ores of H. annosum. The probability of natural infection was 0.73 and 0.53 for stumps in thinned and clear-cut stands, respectively. Almost all (95%) of the artificially inoculated stumps in thinned and clear-c ut stands became colonized, and no differences due to stump type were found. The probability of infection of stumps in precommercially thinn ed stands was lower than for the other stump types of both naturally a nd artificially inoculated stumps. The proportion of colonized sapwood was reduced with increasing moisture content. Stump colonization seem ed unaffected by temperature, even though stump temperatures exceeded 40 degrees C for 2 h at one site.