Habitat effects emerge in late-stage decomposition: Fungal communities in European beech deadwood of Wolin National Park

Deadwood-inhabiting fungal communities are strongly structured by tree species and decomposition stage. However, the extent to which local habitat conditions differentiate these communities across the decomposition gradient remains poorly understood. In Wolin National Park (northern Poland), we investigated fungal communities inhabiting European beech deadwood across three decomposition classes and three beech forest habitat types (acidic beech forest, EU habitat type 9110; rich beech forest, 9130; orchid beech forest, 9150). Fungal communities were identified using ITS2 metabarcoding and analyzed for taxonomic and trophic diversity, as well as community assembly. Fungal diversity increased along the decomposition gradient, and all decomposition classes supported distinct fungal communities. Late-stage deadwood was enriched in fungi associated with non-woody substrates (e.g., multi-trophic saprotrophs and mycorrhizal fungi). Fungal communities in decomposition class I were predominantly more similar than expected by chance, but this pattern weakened along the decomposition gradient. Communities became increasingly dissimilar in class III, indicating a shift toward greater community differentiation during decomposition. Although the decomposition stage remained the primary driver of fungal community composition, habitat type became an important driver in decomposition class III (R 2 =0.13), where fungal communities differed between acidic and orchid beech forests. Seemingly, as decomposition progresses, deadwood incorporates more fungi originating from habitat-specific source pools in surrounding soils and vegetation. Our results show that habitat-dependent differentiation of fungal communities emerges only during late-stage decomposition, after substantial substrate transformation. These findings highlight the importance of retaining late-stage deadwood across diverse habitat types to maximize the conservation of fungal biodiversity.