Forest Habitat and Substrate Interactions Drive True Slime Mould Diversity Across Poland

True slime mould assemblages respond acutely to microhabitat structure, which may constitute potential indicators of forest dynamics; however, large-scale syntheses integrating habitat scale and substrate specificity remain exceedingly scarce. By collating 3085 occurrence records into eight ecologically coherent habitats and ten substrate guilds, we quantified richness, entropy, turnover and indicator strength via rarefaction, Chao1/ACE, Shannon–Simpson indices, β-diversity partitioning, NMDS, PERMANOVA and IndValg analysis. Broadleaved deciduous forests accounted for 37.9% of observations and hosted the most taxa, while lignicolous samples in both deciduous and bog–mire contexts dominated species counts; open grasslands were compositionally depauperate. Species replacement, not nestedness, structured assemblages (βSIM/βSOR≈0.82), and habitat plus substrate explained two-thirds of variance. Indicator analysis isolated six habitat-diagnostic genera (notably Cribraria, Hemitrichia and Licea) and, at species resolution, highlighted Diderma niveum, Fuligo septica and Ceratiomyxa fruticulosa as high-fidelity bioindicators of montane grassland, bog–mire and broadleaved forest conditions, respectively. Taken together, our findings lay the groundwork for employing true slime moulds to identify habitat types and assess their ecological condition, while underscoring the conservation value of dead wood retention and structural heterogeneity. The benchmarked indicator set we provide enables rapid assessments and establishes a temporal baseline for tracking climate- and management-driven change in Central European Eumycetozoa diversity.