Background/Question/Methods Native tallgrass prairies include many early and late-flowering forb species that are missing in restorations. Indeed, comparison of 93 restorations and five remnants in Iowa found that early-flowering forbs were less abundant in restorations, which may represent a resource gap for pollinators. We sought to determine why these gaps in early and late-season flowering exist in restorations by testing the following hypotheses related to forb establishment: 1) recruitment limitation is stronger than seed limitation in early and late-flowering forbs, 2) early-flowering forbs show stronger differences in recruitment limitation than late-flowering forbs, 3) mowing lessens the impact of recruitment limitation on early and late-flowering forbs, 4) the diversity of plant communities increases as more early and late-flowering forbs establish. We tested these hypotheses using a randomized split-plot design in three new restorations in Iowa and Minnesota, USA. In each, we conducted a mowing treatment, a transplant vs. seed addition treatment, and a forb addition treatment using three early-flowering species, three late-flowering species, or all six together. We collected data for two response variables of target species and overall plot community diversity. Also, flower counts and pollinator trapping were carried out throughout 2020. Results/Conclusions Survivorship of target forbs was over 100% higher in transplanted plots than in seeded plots, and aboveground biomass was higher in transplants than in seeded individuals (P < 0.0001). The interaction between the transplant vs. seed addition treatment and forb addition treatment was significant (P < 0.01) and suggests that early-flowering species benefited more from transplanting. The above results support recruitment limitation as a strong factor in preventing early and late-flowering forb establishment, especially among early-flowering species. Results did not indicate a significant impact of mowing on the recruitment of target forbs. A linear contrast found a significant trend of increasing diversity with increasing number of target species added in the forb addition treatment (P < 0.05). Therefore, the establishment of target species increased overall plant community diversity. This establishment is further reflected in higher diversity treatments having greater flowering niche breadth (P < 0.05). Mowing and transplanting also increased flowering niche breadth (P < 0.05), revealing a potential benefit to pollinators in terms of the presence of resources across the season. Pollinators were different in genera composition in the early, mid, and later parts of the season (P < 0.01), further emphasizing the need for targeted restoration of early and late-flowering forbs in grassland restorations.
