What determines the diversity and succession of lichens inhabiting post-bark beetle snags in the Western Carpathians?

The life strategy of Norway spruce allows the recovery of European spruce forests in a scenario of catastrophic disturbances caused by the European spruce bark beetle. However, little is known about how the development of this insect infestation has influenced the preservation of the ecological balance in these forests over the last decades. Based on the upper montane spruce forests in the Polish Western Carpathians, we decided to check what species of lichens are using the decaying wood of post-bark beetle snags and how the progressive changes in wood hardness and stand decomposition affect the process of species exchange. In 2018–2019, we investigated spruce snags on permanent monitoring plots in Gorce National Park, whose cause and time of death have been recorded since 1999, and earlier in 1992 and 1997. The study covered 374 post-bark beetle spruce snags at 76 sites. We found 84 species, including 77 lichens, 6 lichenicolous fungi and one non-lichenised fungus, 15 of which were exclusively wood-inhabiting species in Gorce range. Using generalised linear models, the wood age (A) and the scale of the forest stand breakdown phenomenon (B) were compared with the altitude (C), the aspects of hillside exposure (D) and the forest plant community (E) in the assessment of their effect on lichen species diversity and abundance. "A” was the most important of the tested factors, significantly and positively influencing both parameters, while “B–D” only weakly influenced lichen abundance. Five groups of wood age, significantly different in the lichen abundance and the composition of species were distinguished, and a characteristic combination of dominant species was determined for each of them. Based on the measurements of the wood hardness under the thalli using Shore’s method, the succession of species during the colonisation of the post-bark beetle snags was determined and four groups of species were selected, most frequent in the successive stages of wood decay process. The wood of spruces killed by the bark beetle is both an important substrate enabling the survival of obligately wood-inhabiting lichen species, as well as providing a habitat supporting the maintenance of epiphytes in the Carpathian forests. This study extends the knowledge about the specific requirements of lichens inhabiting spruce snags, as well as the pace and course of lichen succession on this substrate.