Plant species diversity and composition of restored tallgrass prairies

Abstract

Ecological restoration is vital to restore the plant diversity and pollinator habitat lost from grassland to agricultural land conversion. However, restorations can vary significantly from remnant tallgrass prairies in diversity and environmental characteristics. Research studying restoration outcomes is vital to understand this variation. We performed two studies describing the plant species diversity and composition in restored tallgrass prairies. First, we experimentally tested for soil pH legacy effects on restored prairie plant species diversity and composition. Fertilization, specifically with anhydrous ammonia (NH3) is a common land management practice in the US cropland that were formerly prairies. However, the repeated application of ammonium-based fertilizers causes soil acidification. Farmers can apply agricultural lime (usually calcium carbonate or other equivalent) to increase the soil pH. Lime is not always applied after fertilization which leaves a wide range of soil pH’s in farm fields. Soil pH has been found to be an important indicator for plant species diversity and composition, and these initial pH levels could cause a legacy effect on establishing prairies during restoration. We developed a split plot experiment to determine whether differing soil pH levels would show legacy effects on restored tallgrass prairie plant diversity and community composition. Elemental sulfur or calcium carbonate were added to soil to lower or raise the pH, respectively and a control treatment with no pH-altering amendment in subplots. Each subplot was set within a whole plot containing one of 11 grass forb seed mixture ratios, ranging from 0:100 to 100:0 in 10 percent increments. Lowered soil pH decreased species richness, Inverse Simpson’s Diversity Index, total biomass and total forb count compared to high and control soils. For total volunteer count, low soils were significantly higher than the high and control soils. A multivariate statistical test found that the plant community composition was different among the three treatments. Our results indicate that soil pH changes can be tied to legacy effects from past land uses by impacting biodiversity in restored prairie plant communities. Second, we sampled the plant species diversity and pollinator habitat of 18 CREP wetland buffer zones through the Des Moines Lobe in Iowa. Conservation programs across the United States have been instated to increase diversity and support ecological restoration. The Iowa CREP provides incentives for landowners to restore tile-drained areas to wetlands with tallgrass prairie buffer zones. Across these sites, the relative abundance of exotic and graminoid species were found to be significant predictors of both measures of site level γ – diversity. Both species richness and Simpson’s 1/D decreased as the relative abundance of exotic and graminoid species increased. The relative abundance of graminoid species was also found to be a significant predictor of milkweed stem density. Unexpectedly milkweed stem density (a measure of abundance) increased as the relative abundance of graminoid species also increased. Our results suggest that grass:forb ratios could be an important indicator of γ – diversity and milkweed abundance in restored tallgrass prairies. Collectively, this thesis will investigate the plant species diversity and composition in restored tallgrass prairies to better understand restoration outcomes and success.