Species coexistence in restored grassland plant communities: Trait-based recruitment, niche-neutral assembly, and heterogeneous management

A core objective in the field of ecology is to understand mechanisms responsible for species coexistence. Recruitment is an essential process for coexistence in plant communities, and a greater understanding of recruitment dynamics could facilitate the restoration of degraded ecosystems. Despite the pressing need for restoration of the highly fragmented tallgrass prairie biome, few restorations approach the diversity of native remnant prairies. In this dissertation I examine multiple facets of recruitment in restored tallgrass prairies from a community ecology perspective, identifying multiple factors that influence the outcome of recruitment events with implications for enhancing the diversity of tallgrass restorations.;Despite heightened interest in stochastic dynamics among community ecologist, the influence of stochasticity on recruitment in restored plant communities has been poorly explored. In the first chapter, I hypothesized that increased propagule pressure would moderate stochastic recruitment and lead to greater predictability of sown species assemblages in grassland restorations, while also revealing divergent species compositions associated with prescribed disturbances. To examine these hypotheses, I conducted a field experiment in which a gradient of multi-species propagule pressure was applied to plots which had been subjected to prescribed disturbance regimes of varying intensities. I analyzed compositional dissimilarity among experimental plots to demonstrate that increased recruitment in response to propagule pressure and disturbance moderated stochastic structuring of sown plant communities. Furthermore, higher propagule pressure resulted in compositional divergence among disturbance treatments and increased the strength of affinities between species and a given treatment, indicating the presence of niche-based species-sorting dynamics.;A greater understanding of the relationship between plant traits and recruitment success in restored grasslands may facilitate the development of management practices that enhance the establishment of species which exhibit poor recruitment. In the second chapter, I compiled recruitment data for 190 native species from 54 experiments conducted throughout the tallgrass prairie region, and I quantified 10 traits for each species to examine the relationship between plant traits and recruitment. Analyses demonstrated that increased recruitment potential was associated with flowering during mid to late summer, long flowering duration, intermediate seed mass, intermediate height, and clonality. Recruitment potential is greatest when species exhibit multiple beneficial trait values, but can be severely limited by the expression of multiple disadvantageous traits.;The diversity of tallgrass prairie restorations is primarily constrained by the poor recruitment of many native forb species. Although multiple strategies have been proposed to enhance forb diversity in established species-poor restorations, the efficacy of these practices has not been rigorously compared with empirical data. In the third chapter, I conducted field experiments at three Midwestern research areas to examine the effects of multiple disturbance regimes on local species richness and the recruitment of native prairie forbs sown at each site. Native richness increased in response to sowing without disturbance, though disturbances increased light availability for arriving colonists and further enhanced recruitment. Analyses illustrated significant compositional differences among the native forb communities associated with disturbance treatments at each site, though no treatment resulted in decisively superior forb establishment. While the outcome of restorative interseeding practices is highly contingent upon localized site characteristics, my results indicate that the application of mosaic disturbance regimes within a single restoration site could maximize plant community diversity. In addition, heterogeneous management may concurrently enhance faunal diversity by providing the structural heterogeneity required to meet the diverse habitat needs of grassland birds.;These dissertation chapters collectively demonstrate that species diversity and composition in restored grasslands is regulated by many factors that influence recruitment. These factors include propagule pressure, the form and severity of disturbance regimes, local environmental conditions of restoration sites, and the aggregate of traits expressed by sown species.