Plant Competitive Balance and Intransitivity Shape Ecosystem Multifunctionality in Grasslands Under Drought

Plant-plant interactions shape community composition and functional trait structure, key drivers of ecosystem functioning. Environmental conditions can modify these interactions, yet how they alter the competition-facilitation balance and its intransitivity (non-hierarchical interactions), and with what consequences for ecosystem functioning, remains unknown. To address this, we conducted a grassland mesocosm experiment manipulating drought and whole-soil inoculation. Drought increased both competition and intransitivity, while whole-soil inoculation decreased them. Reduced competition and higher intransitivity enhanced ecosystem multifunctionality by increasing plant productivity and soil hydraulic conductivity, with minimal effects on soil nutrient cycling. Dominant species shaped the competition-facilitation balance and intransitivity in opposite ways, which explains their contrasting effects on ecosystem functioning. These outcomes were partly independent of functional diversity (frequently interpreted analogous to resource-use complementarity) and species relative abundance effects, showing that the competition-facilitation balance and intransitivity capture aspects of plant-plant interaction structure not resolved by commonly used diversity-function metrics.