By: Bill Kleiman – Director Nachusa Grasslands, Elizabeth Bach – Research Scientist Nachusa Grasslands, Elizabeth Becker – Doctoral Student SDSU / UC Davis Joint Doctoral Program in Ecology

Restoring any ecosystem is extremely difficult and getting it right on the “first try” nears impossibility. For tallgrass prairies, attempts to leap from an abandoned agricultural landscape to a highly diverse prairie are often thwarted by non-native species and an over-abundance of warm season grasses like big bluestem.  How then, can we go from an established prairie with moderate forb diversity to a high diversity prairie?

Over a decade ago, Bill Kleiman, the Project Director of Nachusa Grasslands, asked this question and wanted to know if adding seed to an already restored prairie (i.e., overseeding) would increase diversity. He established treatment plots across three sites at Nachusa in 2009 and 2012 that varied in seed inputs and disturbance intensities to test this. The full details of this experiment can be found here, but the abridged version of the treatments is below:

• Control plots: no manipulations
• Seed: overseeding only
• Low intensity disturbance: harrowing and overseeding
• Intermediate intensity disturbance: harrowing, overseeding, and additional applications of Poast grass herbicide during the growing season
• High intensity disturbance: Disking, harrowing, and overseeding

In 2023, species composition and abundance data were collected in each of the treatment plots (photo 1). Data collection consisted of identifying all species and their percent cover in three 1m x 1m quadrats that was then averaged across each treatment plot.

Photo 1: Species composition and abundance data collection in a treatment plot.

When compared to control plots, it was found that the high intensity disturbance treatments increased native species richness by over 40% and native species diversity by 15%. The low intensity disturbance plots also increased native species richness and diversity by 20% and 12% respectively, when compared to controls. In high intensity plots, we found an average of 23.2 native species averaged across all plots compared to only 16.4 native species in the control plots. In the low intensity plots the average number of native species found was 21.6 native species. This data can be seen in the graph below.

Figure 1. Average native species richness across each treatment. The number of native species is indicated at the top of each box. The stars on the graph indicate the amount of significance compared

In short, we found that overseeding paired with topsoil disturbance is key to increasing plant species richness and diversity in established prairie plantings. Overseeding alone did not seem to result in significant increases in richness or diversity over time in this instance. This could be because this was a singular overseeding event and multiple rounds of overseeding may be needed for it to successfully increase native species richness and diversity.  

Disturbance is likely important in this scenario because it disrupted the roots of some established species and reduced competition above- and belowground for newly sown species. Even the low intensity disturbance, which may not have fully disrupted the rooting structures of established species, was likely important for forming microsites in the soil which allowed some seeds to successfully germinate and establish over time.

Contrarily, we found that additional herbicide application may negate the positive impacts of disturbance, inhibiting native species richness and diversity. This is likely due to non-target effects of the herbicide though it is unclear what was driving these non-target effects in this experiment.

Finally, as Bill noted in previous posts, it is difficult to distinguish these treatment differences by just walking through the prairie. Since the treatments can be resource intensive, mangers should weigh these differences with respect to their specific goals before implementation.

If you would like to read the full research article for this published work, please contact Elizabeth Becker (ebecker(at)sdsu.edu) and she can send you copy. Thanks for reading!