Congruent functional patterns conceal contrasting environmental drivers of aboveground and belowground community assembly across Europe

Above- and belowground plant organs interface with fundamentally different environments and have evolved under distinct selective regimes. This raises the possibility of divergent community-level responses along environmental gradients between these two compartments, an unresolved question with important implications for predicting ecosystem responses to global change. Using 29,223 European vegetation plots, 2,981 aboveground and an unprecedented 612 belowground species traits, we quantified functional variability and composition for both compartments and assessed their associations with climate, geo-edaphic features, human pressure, and vegetation structure. Spatial patterns of functional structure were broadly similar between compartments, but environmental control was asymmetric: while functional composition was comparably well explained in both compartments (R² up to 66% aboveground, 59% belowground), environmental drivers explained far more functional variability aboveground (34–37%) than belowground (5–10%). Climate was the dominant driver overall, but soil properties and terrain roughness were equally or more important belowground, particularly for fine-root diversity and economics. Environmental relationships were consistently stronger in forests than in open vegetation, suggesting that vegetation context modulates environmental filtering across compartments. These findings reveal that spatial congruence can conceal contrasting assembly processes, meaning that predictions of ecosystem responses to global change based on aboveground functional patterns alone will systematically misrepresent belowground dynamics.