Genome size is a trait that can greatly influence phenotypic performance in ecological communities. Theory predicts that organismal demands for nutrients such as nitrogen (N) and phosphorus (P) increase with genome size because genetic material requires more of these resources than many other biomolecules. Thus, this demand may translate into stronger limitations in performance for individuals with larger genomes, as N and P availability are limiting in many natural systems. In plants, the impact of increased genome size may be particularly severe, as the availability of N and P often affect access to one another in many species, known as co-limitation. Additionally, water availability is known to impact plant access to N and P, which may suggest a possible three-way co-limitation. Though resource co-limitation is common for plants, it is unclear how genome size affects the strength of this co-limitation, and therefore competitive ability and success. These effects may be particularly important to understand during the establishment life-history phase, including seed germination and seedling growth, which is a demographic bottleneck for many plant species. Here, we studied big sagebrush (Artemisia tridentata, Asteraceae) as a model system to investigate the relationships between genome size, resource co-limitation (N, P, and water) and competitive ability during the establishment-life history phase. Using a greenhouse common garden experiment with a response surface design, we grew sagebrush seedlings paired in competition pots according to parental genome size variation under 8 factorial treatments of high or low N, P, and water supply. Analysis of height data across treatments found a significant interaction effect of competitor seedling parental genome size, P and water on focal plant height, where focal plant height decreased with greater parental genome size of the competitor only when there was low P and water availability. This suggests that competition was strongest under low P and water conditions and with competitor plants with larger parental genome sizes. Also, we found significant main effects for N and water supply, with seedling height increasing additively with both of these resources. These results suggest that big sagebrush is co-limited by N availability and water supply. We are currently collecting data for seedling biomass and foliar chemistry responses to the experimental factors, as well as direct measurements of seedling genome size to better understand the role of this factor. Our future analyses will consider the effects of genome size and resource availability on plant resource allocation and biomass.