Aggregating Species at Seeding May Increase Initial Diversity during Grassland Reconstruction

Because the distances over which plants interact can be relatively short, the extent to which individuals interact with conspecific and heterospecific neighbors during the initial phases of grassland reconstruction may affect local species diversity and invasibility. To determine whether aggregating conspecific individuals at seeding (while controlling seed density) can be used as a technique to improve tallgrass prairie establishment, we used a greenhouse experiment with four native tallgrass prairie species (functional diversity controlled) seeded at three evenness levels into potting soil. We randomly assigned species to one (random) or a group of four (aggregated) fixed locations in an 8 × 8 grid (16 cm × 16 cm with 2 cm spacing) and subsequently seeded half of the communities with the non-native cool-season grass intermediate wheatgrass (Thinopyrum intermedium). After the equivalent of one growing season (four months), aggregated communities were more diverse and had a marginally greater proportion of legumes than random communities. Initial species pattern did not affect community invasibility, but invaded communities were less diverse and more dominated by the native cool-season grasses. Our results suggest that some tallgrass prairie species may benefit from initial conspecific aggregation and confirm that interactions that determine diversity, but not necessarily invasibility, during grassland establishment occur over short (cm-scale) distances. Aggregating seeds of conspecific species within the grassland reconstruction process may be used as a technique to improve diversity in grassland reconstruction sites and future projects need to consider whether these initial responses can be replicated and maintained within field-scale projects.