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Members of the Phialocephala fortinii s.l. – Acephala applanata species complex (PAC) are ubiquitous endophytes forming complex communities in roots of conifers and ericaceous shrubs across the Northern hemisphere. Two kinds of disturbances (clear-cutting and drought) and their effects on the resident PAC community, as well as on the introduction of alien strain 7_45_5, were investigated using mesocosms with natural Norway spruce ( Picea abies ) regeneration, both under controlled climate chamber conditions and natural conditions in the forest. This is the first record of successful strain introduction into a well-established forest ecosystem. Introduction was more successful when planting inoculated living spruce saplings, compared to inoculation using autoclaved colonized roots. 7_45_5 was less assertive in the forest, where Phialocephala subalpina clearly dominated. Clear-cutting favored A. applanata and simultaneously reduced the overall frequency of PAC. Drought only had a significant influence on 7_45_5, which was more abundant in dry than moist, non-clear-cut plots. To conclude, disturbances and arrival of foreign strains can alter resident PAC communities significantly.

Regeneration of Picea abies in high-elevation mountain forests often depends on the presence of coarse woody debris (CWD), as logs provide sites with more favorable conditions for spruce regeneration compared to the forest floor. However, there is little quantitative knowledge on the factors that are conducive to or hindering spruce establishment on CWD. We examined spruce regeneration on CWD by sampling 303 plots (50 cm × 50 cm each) on 56 downed logs in a virgin forest in the Swiss Alps. Variables describing microsite conditions were measured, and fungi were isolated from wood samples. To investigate the relationship between the ecological factors and establishment success, two models were fitted with seedling and sapling density as response variables, respectively. Besides log diameter, the models identified different ecological factors as significant for seedling and sapling establishment, i.e., regeneration depends on different factors in different development stages. Seedling density depended on the type of rot, log inclination, and decay stage. Sapling density depended mainly on light availability, cover by bark and moss, the time of tree fall, and the distance between the log surface and the forest floor. A total of 22 polypore fungi were isolated from the wood samples, four of them being threatened species. White- and brown-rot fungi were found in all decay stages. The visual assessment of the type of rot in the field corresponded in only 15% of cases to the type of rot caused by the isolated fungi; hence caution is needed when making field assessments of rot types.

Dark septate endophytes of the Phialocephala fortinii s.l. - Acephala applanata species complex (PAC) are presumed to be the most abundant root colonizing endophytes of conifers across the Northern hemisphere. To test the competitiveness of different PAC strains, PAC-free Picea abies saplings were inoculated with five different PAC strains by planting them in pre-colonized substrates. Saplings were left to grow for six weeks and then transplanted crosswise into a substrate colonized by one of the other four strains for a further two weeks. PAC were isolated and genotyped using microsatellite markers. The power of colonization, i.e. the ability of colonizing roots already colonized by another PAC strain, and the power of retention, i.e. the ability of a resident strain of not being suppressed by an invading PAC strain, were calculated for each strain in every combination. The experiment was run twice under two different climatic conditions. Our results show that PAC strains differ (1) in their ability to colonize PAC-free, non-sterile roots, (2) in resistance against being suppressed by another PAC strain and (3) in their ability to invade roots already colonized by another PAC strain. In addition, both the PAC-PAC and the PAC-host interactions depend on the climatic conditions.