Seed mixes of intermediate grass:forb ratio create vegetation of greater ecosystem service provision in restorations

Restoring pollination services using forb-dense seed mixes is seen as a possible way to mitigate global pollinator decline. Research is needed on whether this management factor impacts other ecosystem services that could be influenced by greater emphasis on forbs than grasses in seed mixes. We tested grass:forb seeding ratios varying from 0:100 to 100:0 for their effects on multiple ecosystem services in a prairie restoration. We hypothesized that intermediate grass:forb seeding ratios, which have greater plant diversity, would result in greater supporting, regulating, and aesthetic services. We sowed 11 grass:forb seeding ratios and measured ecosystem service indicators (plant richness, plant productivity, animal-pollinated flower production, volunteer biomass, pollinator visitation, and butterfly visitation, an indicator of aesthetic services) over four growing seasons at a restoration in Iowa, USA. There were significant quadratic relationships between grass:forb seeding ratio and all of the above indicators except net primary productivity, which was not significantly related to grass:forb seeding ratio, suggesting no trade-offs among these ecosystem services at intermediate ratios. We further showed that greater service provision was related to increases of various seed mixture diversity indices. A structural equation model showed seed mix diversity has positive effects on invasion resistance and animal-pollinated flower production that indirectly enhance pollination and aesthetics. Volunteer biomass was suppressed with greater seed mix diversity. Overall, these results indicate that prairie restorations with intermediate grass:forb seeding ratios will have the optimal provision of ecosystem services, especially those that generate aesthetic appeal to people.