COS 90-5 - Effects of forb seeding density on community structure and development over the first 8 years of a prairie restoration

Abstract: In tallgrass prairie restorations, the amount of seed to add is an important consideration given the need to balance practical concerns with the goal of establishing a diverse community of native species. Forb diversity is often particularly challenging to restore, and typical seeding densities used in restoration may be insufficient for the establishment and persistence of many forb species. Forb seeding density could also have effects on community structure beyond forb establishment, potentially increasing spatial heterogeneity in species composition by increasing species sorting across a heterogeneous environment, or decreasing it by decreasing stochasticity of establishment.

We ask 1) Does increased forb seeding density increase sown forb establishment, and, as a result, the overall diversity and floristic quality of the restoration? 2) Does increasing forb seeding density improve invasion resistance? 3) How does increased seeding density affect compositional heterogeneity within and between plots 4) How do the above, and the community in general, change over time? Our study took place at Free State Prairie, an experimental restoration established in eastern Kansas in 2014. All eighteen 13.7x16.3m plots were sown with equal densities of grass seed (~8894.839 g PLS/ha). Densities of forb seed depended on treatment, six plots each were sown with either no forb seed, a typical density used in restoration (~2648.5 g PLS/ha), or twice this typical density (~5297.14 g PLS/ha). Our analyses included only treatments where forbs were seeded.

Based on cover data collected annually from 2014-2021, increasing forb seeding density increased the cover and richness of sown forbs, and thus floristic quality. Increased seeding density also improved resistance to invasion by some nonsown species, decreasing nonsown richness. Due to this decrease and a decrease in sown grass richness, increased seeding density did not increase overall diversity. Compositional heterogeneity within and between plots fluctuated year-to-year, but there was no consistent effect of time or seeding density. Most effects of forb seeding density and time were independent of each other. Community composition changed over time, with larger differences between initial years than between later years. These temporal changes were characterized by a decrease in nonsown cover and an increase in sown cover. Our results suggest that increased forb seeding density can be beneficial to restoration, increasing sown forb establishment and floristic quality, but these effects tended to be small in magnitude compared to the changes in the community over time, and considering the level of increase in seeding density.