Recruitment limitation of early‐ and late‐flowering grassland forbs can be overcome with transplanting in prairie restorations

One important goal in prairie restorations is to have forb species that flower throughout the growing season (i.e. a range of flowering phenology). We conducted an experiment to test whether three early‐ and three late‐flowering forb species are seed or recruitment limited, if mowing can influence this limitation, and if forb additions will impact overall plant diversity in ongoing restorations.

The experiment was conducted at two restorations in northern Iowa and one in southern Minnesota, USA, using a split‐plot design. Whole plots tested the effect of mowing, and subplots tested the effect of target forbs being added as seeds or small transplants at varied richness levels (0, 1, 3 or 6 species added). Control plots received no forb additions. To compare establishment success, we estimated target species biomass, survivorship and per cent cover of target species across 3 years (2019–2021). Plant diversity measures (richness, inverse Simpson's diversity) were estimated within subplots to determine whether forb additions impacted diversity.

Biomass and relative abundance of early‐ and late‐flowering forbs was much higher when they were added as transplants than when added as seeds. Survivorship across life stages indicated that the transition from seed to adult had lower survivorship probabilities (0.6%) than the transition from transplant (juvenile) to adult (75%).

Plant diversity of these ongoing restorations was higher when target forbs were added compared to controls. Diversity was higher when target forbs were transplanted (8.4 species/plot and inverse Simpson's diversity of 4.0) than when they were seeded (7.6 species/plot and inverse Simpson's diversity of 3.8). Early‐flowering treatments increased diversity more than late‐flowering treatments, and diversity increased linearly with the number of forb species added.

Mowed and unmowed plots had similar levels of target species establishment and plant diversity.

Synthesis and applications: Our results indicate that early‐ and late‐flowering species are more recruitment limited than seed limited and that diversity is lower when these species are missing in restorations. We recommend transplanting a rich mix of early‐ and late‐flowering forbs to increase their biomass and ensure that flowering occurs throughout the growing season.