Water availability modulates the effects of weed community functional structure on wheat performance

Weeds constrain crop productivity but support farmland biodiversity, providing important agroecosystem services. Understanding how weed community structure influences crop performance is essential for developing sustainable weed management strategies. We conducted a greenhouse experiment with winter wheat, exploring how various components of weed community structure – weed biomass, taxonomic diversity, functional diversity, and dominant traits interact with water availability to affect crop biomass and grain yield. Weed communities were assembled from 17 species (grasses, legumes, and forbs) across three richness levels (1, 2, or 4 species) and grown under two watering regimes (well-watered and drought). For each weed community, we quantified community-weighted means (CWMs) and functional dispersion (FDis) for six traits: leaf area, specific leaf area, leaf dry matter content (LDMC), plant height, flowering onset, and seed weight. Aboveground biomass of weeds and wheat, and crop yield, were recorded at harvest. Data were analysed using generalised linear mixed-effects models. Wheat biomass and yield were significantly higher under well-watered conditions, while increasing weed biomass reduced crop performance. Weed diversity effects depended on water availability: species richness, functional group richness, and multi-trait FDis were positively associated with crop performance under drought but not under well-watered conditions. Trait-based metrics explained more variation in crop biomass and yield than richness-based indices. CWMs of traits significantly influenced wheat performance, with LDMC, flowering onset, and height showing strong negative effects on yield. FDis of LDMC was positively associated with crop performance under drought, suggesting that functional complementarity may buffer stress under resource limitation. Combined CWM+FDis models provided the highest explanatory power, accounting for 70% of variation in crop biomass and 62% in yield. If these controlled-environment findings extend to field conditions, they underscore the importance of trait diversity and composition in mediating crop-weed interactions and the potential for trait-informed weed management under variable climatic conditions.