Dominant Grasses, Not Subordinate Forbs, Sustain Ecosystem Multifunctionality Across Spatial Scales

Anthropogenic nitrogen (N) deposition and shifts in plant functional composition can alter grassland biodiversity and ecosystem functioning, yet their interactive effects on multifunctionality across spatial scales remain poorly understood. Using an eight-year field experiment that combined plant functional group removal with N addition in a subalpine meadow, we tested how N addition and the removal of dominant grasses or subordinate forbs affected biodiversity and multifunctionality from empirical α-scale plots to derived β- and γ-scale estimates based on four-plot assemblages. We found that grass removal reduced α- and γ-multifunctionality, whereas forb removal did not affect multifunctionality across scales under both ambient and N addition conditions. The decline in γ-multifunctionality after grass removal was associated mainly with reduced α-component multifunctionality. Grass removal increased plant α-diversity and β-diversity under ambient conditions, whereas forb removal reduced both α- and β-diversity, especially under N addition. However, neither plant nor soil α-diversity predicted α-multifunctionality and changes in plant β-diversity were not accompanied by comparable changes in β-multifunctionality, indicating that, within the range of diversity changes observed here, multifunctionality was more strongly tied to grass-associated functions than to plant diversity. N addition modified plant diversity responses to functional group removal but did not change multifunctionality responses. Our multiscale framework clarifies how dominant functional groups and biodiversity components contribute differently to ecosystem multifunctionality under nutrient enrichment, underscoring the need for integrated conservation approaches that safeguard both dominant and subordinate species.