Seeding density alters the role of soil moisture in structuring plant abundance during prairie restoration

Restoration practitioners often seek to reestablish native plant communities through seed sowing; however, the effectiveness can vary across species and soil resource gradients. We asked how seeding density and first‐year, plot‐level soil moisture affect the abundances of 11 tallgrass prairie species 4 years after seeding and whether species‐level differences were explained by wetland affinity or plant height, which we hypothesized would modulate competitive dynamics. We seeded plots at eight densities in each of seven sites spanning a landscape soil moisture gradient. Focal species abundances were generally greater in environments with lower soil moisture and increased with higher seeding density. For five species, higher seeding density consistently increased abundance regardless of soil moisture, pointing to dispersal limitation strongly constraining population sizes. However, for four other species, seeding density increased abundances only under the soil conditions where they were most limited (e.g., high or low soil moisture). For two species, seeding density did not increase abundance. Moreover, while a key goal in restoration is to use species' traits to guide decision‐making, we found mixed support for species' wetland affinity and maximum height predicting responses to seeding and soil moisture. Our results illustrate that higher seeding density is a useful strategy in grassland restoration, but its success and resulting cost‐effectiveness vary with underlying environmental conditions. This study illuminates some causes of variability in restoration outcomes and suggests ways to optimize seed mix design based on species tolerances and local soil conditions.