For the past two decades in the Czech Republic, Norway spruce (Picea abies) trees have been weakened by drought and subsequent attack by Armillaria root disease and bark beetles (Ips typographus, I. duplicatus, and Pityogenes chalcographus). We determined whether thinning of trees decreased mortality at 40 forest locations in the Czech Republic. The majority of the locations experienced long-term drought stress and very low groundwater levels. We also identified the species of Armillaria associated with the spruce decline and determined the reliability of visual detection of Armillaria infection.
The mortality of trees at the 40 locations increased nearly linearly during the 5?years of the study. Sites were established in 2012, thinned or not thinned in October 2012, and were assessed in September and October in each year from 2012 to 2016. Thinning, with an intervention intensity of 30–40%, did not alter tree mortality at the locations. The proportion of dead trees was significantly greater in older than in younger stands in the last 3?years of the study.
Among the trees that were suspected of being attacked by Armillaria based on visual inspection by foresters, 53% were confirmed (supported by statistical significance) to be infected by Armillaria spp. after morphological and genetic assessment. Among the trees with confirmed infections, 56% were infected by Armillaria ostoyae, 32% were infected by A. cepistipes, and 1% were infected by A. gallica (the rest could not be identified). Resinosis was positively associated with the occurrence of A. ostoyae. This species was more common on loamy flat locations, whereas A. cepistipes was more common on stony, sloped locations. The occurrence of Armillaria was negatively associated with increasing pH.
Although our results indicate that spruce decline is strongly associated with the presence of Armillaria and that visual detection of Armillaria is reliable, management options for reducing Armillaria infection and damage appear to be highly limited. Forest managers should begin preparing to convert stands of Norway spruce to stands of other tree species that are better adapted to the local conditions and that are less susceptible to attack by Armillaria species.