Population structure of Armillaria species in managed Norway spruce stands in the Alps

The preferentially saprotrophic basidiomycete Armillaria cepistipes and the pathogenic Armillaria ostoyae occur sympatrically in many European forests. The spatial population structure of both Armillaria species was investigated in three 1-ha plots established in managed Norway spruce (Picea abies ) forests in the Swiss Alps (1400 m asl). A total of 740 Armillaria isolates, 296 from rhizomorphs in the soil and 444 from the stumps, were recovered and identified to species and genets (somatic incompatibility groups). The incidence of A. cepistipes and A. ostoyae varied greatly among the plots. Two to six genets of each Armillaria species were identified and mapped in each plot. Genets of the same species overlapped rarely and only on the borders. Large spatial overlaps between A. cepistipes and A. ostoyae genets were observed in the plot with the highest incidence of fragmented genets. In five subplots (c. 0.1 ha) established along interspecific boundaries, overlaps were found in all three plots. Our study suggests a strong intraspecific competition in both Armillaria species. Evidence for competition between the two species, as indicated by spatial mutual exclusion, was only observed on parts of the investigated area.