Assessing grassland restoration success: relative roles of seed additions and native ungulate activities

1. Grassland restorations often lack rare forb and grass species that are found in intact grasslands. The possible reasons for low diversity include seed limitation, microsite limitation and a combination of both. Native ungulates may create microsites for seedling establishment in tallgrass prairie restorations by grazing dominant species or through trampling activities, but this has never been tested in developing prairies. 2. We experimentally tested for seed and microsite limitation in the largest tallgrass prairie restoration in the USA by adding rare forb and grass seeds in two trials inside and outside native ungulate exclosures. We measured seedling emergence because this stage is crucial in recruiting species into a community. We also measured light, water and standing crop biomass to test whether resource availability could help to explain seedling emergence rates. 3. Ungulates increased light availability for each sampling time and also increased above-ground net primary productivity (ANPP) during summer. 4. Seedling emergence of rare prairie forbs and grasses was consistently greater when we added seeds. 5. Seedling emergence was conditionally greater with a combination of seed additions and grazing, but grazing alone was unable to increase emergence. 6. When ungulates increased seedling enhancement, the mechanism was partially associated with increased water and light availability. 7. Exotic and cosmopolitan weed seedling emergence was not affected by grazing. 8. Synthesis and applications. These results suggest that tallgrass prairie restorations are primarily seed limited and that grazing alone may not be able to increase seedling emergence of rare species without the addition of seeds. Therefore, adding seeds to grassland restorations may increase seedling emergence of rare species, and mimicking effects of grazing may increase emergence when seeds are added.