The occurrence and influence on durability of wood-decaying and blue staining fungi in bark-beetle infested Norway spruce wood

Abstract Outbreaks of bark beetles in recent years often leave large volumes of dead Norway spruce ( Picea abies L.) standing in the forest up to several years before harvest. This study evaluated how standing storage duration (0–4 years) in two German regions (Sauerland and Harz) influenced the development of rot and blue stain, water absorption and durability. The assessment for percentage coverage of fungi on dead trees showed increasing rot with prolonged standing storage, particularly in the uppermost and lowermost stem sections, whereas blue stain coverage did not increase over time. Microscopy did not indicate the occurrence of blue stain fungi to be correlated with the presence of basidiomycete wood-decay fungi, although some blue stained spruce specimens exhibited higher capillary water uptake (CWU) than non-stained wood, which could theoretically improve water availability for decay fungi. However, no significant difference was detected between the CWU of blue stained and non-blue stained groups overall. Durability and decay rate did not differ among trees of investigated standing stored durations in a durability test following EN 113-2 (2021). These results indicated that the blue staining of Norway spruce wood, while affecting water absorption, did not independently elevate the risk of basidiomycete decay. However, the longer the standing storage, the more likely for increased opportunities for mechanical damage, moisture ingress, and subsequent decay.