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Plant composition is controlled by a combination of environmental, biotic, historical and management factors. Although there has been much focus on restoring grassland diversity, it is unclear which factors and/or interactions of factors are constraining diversity in grasslands and the relative influences of different factors. We measured soil fertility, soil texture, grazing intensity, fire frequency and plant cover in 694 plots, located within 33 remnant and restored fields in managed grasslands in the central Great Plains. Using univariate (general linear model) and multivariate (PERMANOVA) analyses, we identified significant factors and their relative contributions to plant richness, evenness, floristic quality index (FQI) and composition.
We found that species richness declines with nitrogen across all fields; however, remnant fields have higher species richness for any level of soil nitrogen. Remnant fields also have significantly more soil nitrogen than restored fields. Increased grazing intensity correlates with increased richness. Conversely, evenness and FQI are only affected by burn frequency. We found species composition is equitably controlled by environment, management and restoration status, explaining over one-third of the total variation. Soil nitrogen has the largest effect on composition but it is not exponentially greater than soil texture, grazing intensity, fire frequency and restoration status.
We performed an indicator species analysis to identify species associated with each environmental and management factor. Indicator species analysis reveals that the differences in environment and management maintain high beta diversity; the extremes of every factor maintain plant communities with similar floristic quality indices (FQI) and proportions of native/exotic species. Our results reinforce the premise that a complexity of drivers control ecosystems; no single management factor or environmental factor controls plant composition. Maintaining a diversity of management intensities and regimes helps sustain plant diversity across a variable landscape.
Advisors: Jean Knops and Chad Brassil