Celtis tenuifolia is a threatened shrub species in southern Ontario which co-occurs with Celtis occidentalis and Celtis laevigata across the eastern United States. Although it is quite common further south, only six disjunct sites in Ontario currently exist, and these fragmented populations are susceptible to extinction due to habitat loss caused by quarrying operations, sand pit expansion, and beetle infestations. It is difficult for conservation managers to distinguish the three related Celtis species morphologically, and taxonomists have questioned the species limits in this group. Using microsatellite markers and flow cytometry, we assessed the relationship between genetic diversity, ploidy, and morphology in this group. We have determined that C. tenuifolia is an apomictic triploid with hybrid origins, and flow cytometry provides a reliable tool for distinguishing it from the sexual diploids C. occidentalis and C. laevigata. Furthermore, the same C. tenuifolia genotypes occur in a variety of ecological settings, demonstrating remarkable phenotypic plasticity. The results of these assessments will provide direction to the conservation management of Celtis tenuifolia, specifically providing field workers with reliable identification of the plant, as well as resolving concerns around hybridization and the risks of inbreeding and outbreeding depression. I would first like to thank my supervisor, Tyler Smith, for his support, guidance and patience throughout this project. In addition, I would like to thank my co-supervisor, Root Gorelick, and my committee members Joseph Bennett and Julian Starr for sharing their advice, perspective, and encouragement, Song Wang and Tracey James for their lab work and instruction,