Cascading trade-offs in the tree species–soil–root–fungi nexus: Species-specific responses of Populus tomentosa and Pinus tabuliformis across urban green spaces

Urbanization profoundly alters belowground ecological processes, yet how root traits and multiple fungal guilds co-vary along urban green space gradients remains poorly understood. Here, we investigated two common tree species in Beijing—Populus tomentosa (deciduous, acquisitive) and Pinus tabuliformis (evergreen, conservative)—across four urban green space types (parks, residential greens, road shelterbelts, and suburban forests). By integrating fine-root traits, soil properties, and three fungal guilds (rhizosphere fungi, root endophytic fungi, and ectomycorrhizal [EcM] fungi), we tested cascading trade-off hypotheses within a multi-level framework. Our results reveal a cascading hierarchy, whereby tree species and green space type first modulate soil properties, followed by fine-root traits, which then collectively determine the composition and diversity of fungal communities. EcM diversity in P. tomentosa was primarily filtered by specific root length (SRL), whereas EcM diversity in P. tabuliformis responded directly to soil pH and organic carbon. Fine-root traits generally followed the root economics spectrum (RES) but exhibited pronounced species-specific plasticity. Notably, P. totomentosa showed a positive correlation between SRL and root tissue density, which deviates from the classical RES trade‑off and indicates an enhanced “Do‑it‑yourself” nutrient acquisition strategy under urban soil heterogeneity. In contrast, P. tabuliformis adopted an “Outsourcing” strategy (thicker roots, higher mycorrhizal dependence), yet shifted dominant EcM genera across green space types. Furthermore, in relatively nutrient‑poor urban soils, P. tomentosa exhibited a Gadgil effect (EcM and endophytic fungi co‑varying similarly, opposite to rhizosphere fungi), while P. tabuliformis showed the reverse pattern. Stochastic processes dominated fungal community assembly overall, except for root endophytic fungi of P. tabuliformis, which were strongly limited by dispersal. Collectively, our findings extend the RES framework to urban ecosystems, reveal species-specific cascading trade-offs in the root–soil–fungal nexus, and provide a mechanistic basis for managing belowground functions in urban forests.