Impact of land-use intensity, productivity, and aboveground richness on seed rain in temperate grasslands

Seed rain, the amount of seeds reaching an area via primary or secondary dispersal, affects the regeneration of plant communities and shapes future community composition. In agricultural grasslands, the composition and density of seed rain are affected by management, but major drivers of seed rain quality and quantity along land-use gradients are poorly understood. We studied how land-use intensity (LUI), its components (i.e. fertilization, mowing, and grazing) and aboveground vegetation productivity and richness influence seed rain. We collected the seed rain over a five months in 142 grasslands and identified emerging seedlings. Our analysis revealed that seed rain of grasses and forbs reacted differently to land-use intensity and vegetation richness, whereas LUI effects on grass seed density resulted in changes of total seed density. Seed rain richness increased with vegetation richness, but decreased at higher LUI levels. This is reflected by decreases in total seed mass and the abundance of stress tolerators and an increase in ruderals in seed rain with increasing LUI. Among LUI components, fertilization most strongly affected seed rain, with negative effects manifesting at intermediate fertilization intensities. Mowing once a year increased seed rain density, whereas it decreased at higher mowing frequencies. Grazing reduced seed density and richness by reducing grass seed density, while forb seed density remained unaffected. Higher LUI and productivity increased total seed density but reduced taxonomic and functional diversity in seed rain and negatively impacted grassland regeneration.