Fine root (≤ 2 mm in diameter) biomass/necromass (B/N) ratio, representing many dynamic key root parameters, can serve as a powerful measure of root vitality. Based on a global synthesis of fine root biomass and necromass in forest ecosystems, we describe a framework for recognizing responses of B/N ratio to biotic (e.g. mycorrhizal type) and abiotic (e.g. latitudinal region) characteristics. Arbuscular mycorrhizas (AM) and ectomycorrhizas (ECM) forests had similar average B/N ratios (3.28 vs. 3.23). AM forest B/N ratio decreased with increasing altitude, stand density, tree age and soil carbon/nitrogen ratio (C/N), but increased with soil pH. In contrast, ECM forest B/N ratio increased with increasing mean annual precipitation (MAP), altitude, stand density, but decreased with tree age. The average B/N ratio was higher in temperate forests (4.39) than in tropical (2.97) and boreal forests (2.40). The B/N ratio was relatively stable in temperate forests irrespective of changes in biotic and abiotic factors. In tropical forest, the B/N ratio was sensitive to mean annual temperature, altitude, soil C/N ratio and pH, whereas in boreal forests it was more sensitive to MAP, stand density and tree age. The late‐successional forest B/N ratio was closely aligned with biotic and abiotic factors. Our analysis revealed that the relationships of B/N ratio with climate, topography, edaphic and stand characteristics were dependent on mycorrhizal types and latitudinal regions. These findings provide a basis for large‐scale prediction of fine root dynamics and for better understanding of belowground processes of global forest ecosystems in a changing world.