Cover crop species alter tallgrass prairie community assembly.

Cover crops are increasingly being used in ecological restoration projects, and are hypothesized to facilitate establishment of sown species by reducing weed abundances without competing with the target mix. We tested these predictions and examined the role of cover crop species on later species composition and diversity using cover crop seed treatments. Treatments included a fall seeding of one annual (Raphanus sativus or Avena sativa), one biennial (Oenothera biennis), one perennial species (Elymus canadensis), two grass-forb species combinations, or nothing as a control. All plots received the same diverse tallgrass prairie seed mix in March of the following year. Plant communities were sampled through five growing seasons. We found that cover crop treatments influenced community assembly, and that cover crop species varied in their effectiveness at reducing weed abundances, with the perennial grass E. canadensis being especially effective at reducing weeds. After 5 years, treatments were neutral in their effects on species diversity. However, composition of establishing plants (not including the cover crop) differed significantly among cover crop treatments on all sampling dates, indicating that treatments had long-term effects. Plots containing the C3 grass E. canadensis had greater abundance of C3 grasses and forbs, and reduced C4 grass abundance compared to other treatments. Our results indicate that cover crop species differ in their effects on assembling tallgrass prairie communities and that niche modification (i.e., species altering the abiotic environment in a way that favors species that benefit from the alterations) occurs when the cover crop is a perennial grass.