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1. Variation in species pools can affect plant diversity, but it remains unclear whether the magnitude of the response varies because of resource availability, community invasibility or other environmental factors, and whether colonization along environmental gradients reflects niche-based species sorting or neutral processes. 2. We hypothesized that unimodal diversity “productivity patterns in grasslands are dependent on species pools, with peak richness occurring at intermediate productivity due to species sorting associated with species traits. We used a seed-addition experiment to test the influence of immigration on plant species richness across multiple grasslands (old fields), each of which encompassed a broad range in productivity. We then tested whether species sorting occurs during colonization, if this varies with site productivity, and whether sorting patterns are associated with species traits 1 and 4 years after seed addition. 3. Augmentation of species pools increased species richness across fields with the greatest increase in colonization in all fields at sites with intermediate productivity. The increased diversity following species pool augmentation during colonization (year 1) was associated with seed characteristics, seedling growth rates and allocation, and water use, suggesting that colonization and seedling establishment were the result of species sorting. 4. Although there were some changes in trait “environmental relationships between the first and fourth growing season, patterns that were observed in the first year were still apparent in the fourth year, highlighting the persistent importance of species sorting and traits that affect colonization on species distribution and diversity patterns in grasslands. 5. Synthesis. ‚Our experimental results suggest that species richness and richness “productivity relationships in successional grasslands are at least partially determined by species sorting and trait “environmental relationships occurring during colonization.