Seed limitation of plant population size and species richness in restored lowland tallgrass prairie

Understanding the ecological conditions under which seed addition increases the density of flowering stems in plant populations and the species richness of plant communities is central to designing effective and cost-efficient strategies for enhancing diversity of restored prairies. In February 2005, I initiated a factorial experiment to quantify the effects of 1) seed addition of 18 native plant species and 2) clipping to 20 cm followed by raking of overtopping vegetation on density of the 18 added species and on species richness of the plant community at two restored tallgrass prairies in south central Kansas. As of July 2006, both seed addition and clipping\raking of overtopping vegetation significantly increased plant species richness in plots. The magnitude of increase in plant species richness in response to seed addition was independent of whether over-topping vegetation was clipped\raked. As of October 2006, occurrence of 5 of the 18 species (Lespedeza capitata, Amorpha canescens, Salvia azurea, Monarda fistulosa and Helianthus maximilliani) was significantly greater in seed addition plots than in plots that did not receive seed. The interaction effect of seed addition X clipping\raking on the number of plots occupied did not reach statistical significance for any of the 18 species. In 2006, flowering stems of four species (Lespedeza capitata, Helianthus maximilliani, Rudbeckia hirta, Ratibida columnifera) were found in seed addition plots where those species did not occur before seed addition. No flowering stems of these four species were found in control plots. These preliminary results suggest that seed limitation of population size is relatively common and that the plant communities are unsaturated in these restored prairies at the western edge of the tallgrass region.