The establishment of a diverse plant community that is resistant to invasion is a major goal of restoration. The plant species planted in a restoration provide a template for diversity, but other factors at the planting stage such as seeding time and density may influence the realized diversity of the restored community. Post-planting disturbances such as herbivory also influence restored plant community diversity. In this study we investigated the effect of planting factors (seeding time and rate), and post-planting disturbance (vole herbivory), on the invasion of unplanted species into an experimental tallgrass prairie restoration. This study was conducted within twenty-four 14 by 14 m experimental tallgrass restoration plots at the Morton Arboretum in Lisle, Illinois. Half of the plots were seeded in the June and half in December 1997. Half of the plots were seeded at a low and half at a high rate (35 or 350 seeds m-2 per species of 18 tallgrass prairie species, respectively). These treatments were fully crossed. Voles were allowed access to half of each 14 by 14 m plot. In 2003, data on counts and cover of planted and unplanted species were collected, and these variables were used to assess the response of planted and unplanted species to restoration planting design and vole presence.
A MANOVA showed that seeding time and rate and vole presence were all important factors in determining the amount of unplanted and planted species in the restoration seven years after planting. Plots seeded in June to which voles had access were more invasible; there were more than two times as many unplanted individuals and three times greater cover of unplanted individuals (p<0.001 and p<0.01, respectively). The cover of planted species was 15% greater where voles were absent (p<0.01). Plots seeded at a lower rate were also more invasible; these plots had more than twice as many individuals and cover of unplanted species while higher density plantings resulted in two times more individuals and cover for planted species (p<0.001 for all response variables). These results show that planting factors and post-planting disturbance have long-lasting impacts on the ability of restored plant communities to resist invasion by unplanted species.