Effects of restoration on Midwestern oak savanna biodiversity, structure, and oak regeneration

Abstract

<p>Over the last century, savannas throughout the world have been encroached by woody plants, altering over- and understory plant composition and distributions of understory resources. This dissertation investigates woody encroachment removal from Midwestern oak savannas, using a large-scale restoration experiment in Iowa. Four sites received a woody encroachment removal treatment (restoration) and four sites were retained as encroached controls. Chapter two details impacts on woody plant regeneration dynamics. Encroachment removal restored savanna canopy structure and overstory dominance by Querucs (oak) species; however, advanced regeneration was dominated by encroaching species within three years. I suggest that the encroached savannas represent an alternative stable state and that further management actions, potentially involving prescribed fire, will be necessary to maintain the savanna state.;Chapter three investigates spatiotemporal effects of encroachment removal on understory biodiversity. Restoration sites had increased alpha (within sample) Simpson's diversity and alpha and gamma (site-level) species richness relative to control sites, while gamma and beta (among-sample) Simpson's diversity, beta richness, and alpha species evenness were not affected. These changes were driven by widespread establishment of new species at the site-level (notably graminoids) and within-site proliferation of pre-existing species (predominantly graminoids and woody species). I highlight the utility of restoration experiments, like this one, for conducting research on multi-scale processes, such as species diversity.;Chapter four assesses development of understory resource and vegetation gradients. I found that encroachment removal restored light and soil moisture gradients and that these gradients were important for structuring post-restoration plant communities. The savannas in this study appear to be remarkably resilient to degradation, as important biophysical gradients were reestablished within years of restoration, even after decades of encroachment. These results are encouraging for future restoration at these sites and for woody encroachment removal efforts elsewhere.;Chapter five determines impacts of encroachment removal on patterns of Quercus alba (overstory dominant tree species) seedling success. Seedlings had greater survival and growth parameters in treatment sites, with generally better performance at further distances from trees. Thus, the mesic savannas in this study appear inherently unstable, as seedling recruitment is promoted in inter-canopy gaps. These results further support chapter two's conclusion that the encroached savannas represent an alternative stable state.</p>