Tree-related microhabitats (TreMs) are crucial for biodiversity, supporting thousands of species. Most TreM research has focused on temperate forests, prompting us to evaluate the TreM typology in the boreal biome. We aimed to: (i) identify factors influencing TreM occurrence on living trees, (ii) assess redundancy and complementarity of TreM supply among Scots pine, birch, and aspen in Fennoscandia, (iii) compare TreM occurrence between Fennoscandian and Canadian boreal forests, and (iv) determine the minimum sample size for assessing TreM diversity in large-scale inventories. We inventoried 1515 trees in long unmanaged forests across Fennoscandia and compared results with a Canadian dataset. Findings show that Scots pine, birch, and aspen support distinct and complementary TreM assemblages, emphasizing the importance of mixed stands for biodiversity. Over larger geographic areas, local plot context-including climate, disturbance history, and biotic/abiotic factors-was the main driver of TreM occurrence. Unlike temperate forests, tree diameter was not a significant driver in boreal forests. The variability of TreM assemblages within the boreal biome underscores the need for large sample sizes to accurately assess TreM diversity. Sampling 1000 trees is sufficient to assess the seven TreM forms, while a larger sample is required to capture all fifteen TreM groups.