Dominant species determine drought effects on grassland multifunctionality

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 two-year drought in two
C3-dominated grasslands and analyzed 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 modeling, 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.