Facilitation and competition are recognized as important interactions structuring plant communities. This dissertation investigates the relationship of Lespedeza cuneata and Heterotheca subaxillaris in an old field in South Carolina. Heterotheca, an annual plant, is often found associated with Lespedeza, a perennial, nitrogen-fixing legume. The relationship between the two species was explored through a pattern analysis and by modeling the dependency of the positions of Heterotheca on the density of Lespedeza. Results of a Ripleys K analysis revealed that the two species were clustered, which suggested a potential positive interaction. Further modeling indicated that the intensity of Heterotheca was maximized at moderate densities of Lespedeza. Heights and biomasses of Heterotheca showed a slight yet significant increase as Lespedeza density increased. Mortality also had a slight increase with increasing Lespedeza density. It is thought that Lespedeza may affect its environment in terms of local resources. Soil nutrients, soil moisture, canopy openness, and soil temperature were measured in plots of varying Lespedeza density. Increasing density of Lespedeza led to decreased canopy openness and temperatures, and increased levels of NO3- and NH4+, although results of a net nitrogen mineralization study found no significant differences between Lespedeza density. High densities of Lespedeza led to higher soil moisture immediately following rain, but lower temperatures thereafter; moderate densities led to higher soil moisture at the 20 cm depth. Germination of Heterotheca seeds was not affected by the presence of Lespedeza soil or litter. The growth of Heterotheca responded positively to increases in nitrogen, and while growth was hindered by shade, reproductive output was not negatively affected by shade. These results suggest that there are complex interactions between Heterotheca and Lespedeza. At high densities, Lespedeza likely competes with Heterotheca for light resources, but the benefits of Lespedeza to soil nutrients and moisture at moderate densities may contribute to the success of Heterotheca.