Competitive interactions are considered to be a central force maintaining local diversity and controlling the structure of plant communities. The results of several investigations indicate that the outcome of plant competition can be influenced by biotic and abiotic factors. However, those studies usually evaluate single factors. Since biotic and abiotic factors affect plant competition at the same time, it is important to understand the nature of their inter-relationships. In this dissertation I assessed the effects of ultraviolet (UV) radiation and mycorrhizal colonization on plant competition. The work presented in chapter two evaluates how arbuscular mycorrhizal (AM) symbiosis and UV radiation affect vegetative growth, morphology and allocation in seedlings of Populus deltoides (eastern cottonwood), Salix nigra (black willow), and Betula nigra (river birch), three shade intolerant tree species. The results showed that UV radiation induced changes in leaf morphology and biomass allocation and that AM colonization resulted in the reduction of plant biomass. This work demonstrated that ambient levels of UV radiation can reduce the rate of AM colonization and suggested that, in some species, AM colonization may help to overcome the detrimental effects of UV radiation stress. Chapter three evaluates the effects of competition and UV radiation on early vegetative growth in seedlings of P. deltoides. The results showed that UV radiation affected growth and morphology of P. deltoides seedlings when seedlings were grown alone, but not when seedlings were grown in competition. Chapter four builds on the findings from chapters two and three to evaluate the individual effects and interactions of UV radiation and mycorrhizal symbiosis on competition between seedlings. The results found in chapter four showed that UV radiation induced changes in leaf morphology and biomass allocation in all three species, but had no significant effects on growth. The results confirmed that ambient levels of UV radiation diminish the rate of AM fungi colonization. However, neither AM colonization nor UV radiation affected the outcome of plant competition. These findings indicated that the high degree of total competitive stress masked the effects of the UV radiation stress and AM colonization on plant growth. The results indicate that competition and stress factors are not always interactive.