Mycobiota in trunks of dying spruce trees in the 'Puszcza Bialowieska' Promotional Forest Complex and its ecological function

The European spruce bark beetle (Ips typographus) outbreak observed in the ‘Puszcza Białowieska’ Promotional Forest Complex since 2012 resulted in a total volume of 1.4 million m3 of dead trees in multiple spruce stands. The study aimed to assess the species composition of fungi occurring in spruce trunks at early stages of European spruce bark beetle attack. All the samples used in laboratory analyses were collected in 2016 and 2017 in Białowieża, Browsk, and Hajnówka forest districts. A total of 60 dying spruces selected at random were sampled providing 120 samples. Surface−sterilised samples were used in fungal isolations using malt extract agar medium. The resulting colonies were identified morphologically, additional molecular identification was carried out for representatives of morphotypes using ITS rDNA fragment. In addition to these analyses, the frequently occurring basidiomycete fungi on recently died spruce trees were identified based on the morphology of fruiting bodies. One of the most common symptoms observed on trunks of spruces at the early stages of dieback was the occurrence of sapstains starting from bark beetle galleries. In some trunks the stained wood sections were separated by rusty−red stripes. A total of 51 fungal species including 40 ascomycetes and 11 basidiomycetes were isolated from dying spruce trunks. The number of ascomycete species included 16 ophiostomatoid fungi (members of Ophiostomatales and Microascales). The frequently occurring species in this group were: Endoconidiophora polonica (=Ceratocystis polonica) (17.5%), Grosmannia penicillata (20.8%), Ophiostoma brunneolum (9.2%), O. piceae (5.8%) and O. abieticola (4.2%). Among other ascomycetes only three species were frequently isolated: Neonectria fuckeliana (15.0%), Clonostachys rosea (13.3%) and Cosmospora viridescens (9.2%). The rusty−red coloured areas harboured primarily Amylostereum areolatum and Stereum sanguinolentum. The most frequently detected basidiomycete fungus on trunks at the initial stages of dieback was Cylindrobasidium sp. (=Gloeocystidium ipidophilum), whereas Fomitopsis pinicola was the most frequent species on spruce trunks dead for some years, either standing or wind−snapped. We emphasise the potential role of the detected fungi in forest ecosystems of the Białowieża Forest, both as potential tree pathogens and decomposers of spruce debris remaining in the stand