This study investigates the impact of arrival order and seed traits on plant community assembly and biodiversity in a grassland ecosystem in northern Sweden. A field experiment was conducted using 20 different grassland species, with germination pre-tests in the greenhouse and sowing experiments in the field. Seed mass and germination speed were selected seed traits to provide insights into the competitive ability and early life history strategies of the sown species. In the field study, four arrival order treatments with both slow- and fast-germinating species were performed on 70 m2 plots. After three years, the aboveground biomass of the established species was harvested and analyzed, revealing that species arriving second exhibited significantly lower biomass compared to those arriving first. Our results thereby suggest that arrival order has a significant impact on the productivity of species in grassland ecosystems. Priority effect strength (PES) was assessed using two different equations, with results indicating no significant relationship between PES and seed mass or germination speed. Biodiversity analysis revealed that plots with simultaneous late arrival had significantly lower Shannon diversity indices compared to arrival earlier in the season. These findings suggest that arrival order significantly influences plant productivity, while the effect on biodiversity in grassland ecosystems is less clear. Further, seed traits such as mass and germination speed may not play a significant role in determining the strength of priority effects during community assembly.