Dominant species determine drought effects on grassland multifunctionality

Abstract Extreme drought often diminishes ecosystem multifunctionality in grasslands, but the potential mechanisms behind these responses are conflicting. The diversity hypothesis posits that diverse species promote grassland multifunctionality through compensatory interactions, while the mass ratio hypothesis suggests that the traits of dominant species control multifunctionality. It remains unknown whether either or both underpin multifunctionality responses to extreme drought. Here, we experimentally imposed an extreme 2‐year drought in two C 3 ‐dominated grasslands and analysed 11 individual grassland functions (e.g. productivity, soil and plant nutrient pools) along with 17 community diversity and trait metrics (e.g. species diversity, functional diversity, phylogenetic diversity and community‐weighted mean traits). Using structural equation modelling, we separated the effects of these community‐based mechanisms on the drought sensitivity of grassland multifunctionality. Experimental drought had both direct and indirect negative effects on multifunctionality. Diversity metrics did not influence multifunctionality. However, multifunctionality was positively associated with several community mean traits linked to resource acquisition (e.g. plant height and leaf nitrogen content) and negatively associated with traits related to resource conservation (e.g. leaf dry matter content and leaf carbon content). Drought shifted traits towards resource conservation, leading to reduced multifunctionality. Synthesis. These findings indicate that the mass ratio effect, rather than complementarity, drives grassland multifunctionality, highlighting the importance of community‐weighted traits in strongly dominated communities under drought. Together, these insights deepen our mechanistic understanding of drought effects on grassland multifunctionality and underpin strategies for sustainable grassland management and conservation.