Post-fire restoration of degraded sagebrush ecosystems over large areas of the Great Basin is challenging, in part due to unpredictable outcomes. Low rates of restoration success are attributed to increasing frequencies of wildfires, biological invasions, and climate variability. Quantifying restoration outcomes by accounting for sources of biotic and abiotic variability will improve restoration as a predictive science. One source of biotic variability is neighbor interactions, which can regulate demographic parameters of coexisting species and are an important determinant of community structure, ecosystem functions, and population dynamics. Our objective was to quantify how intraspecific variability in big sagebrush, Artemisia tridentata, including three subspecies and two ploidy levels, is related to subspecies’ reaction to conspecific neighbor presence. Neighbor interactions can alter population growth rate via competition or facilitation depending on specific environmental conditions. Using a longterm common garden experiment, we developed spatially-explicit hierarchical models to quantify the effects of size-structured crowding on plant growth and survival. We found that neighbor interactions can vary significantly over time and space, and tend to be more pronounced under wetter and cooler climate conditions. We further tested if water availability, one of the major limiting factors in arid ecosystems, can underlie competitive interactions in a common garden, including density dependence. We used a deuteriumtracer experiment to quantify belowground zone of influence and crowding effect on plant water uptake. The results suggest that intraspecific variability in lateral root extent v may be linked to subspecies identity and ploidy level. We did not find strong evidence that neighbor presence and size can alter water uptake from a shallow soil horizon, potentially suggesting size-independent partitioning of water resources between neighboring plants. We further hypothesize that variability in root architecture may reflect an axis for ecohydrological niche segregation contributing to the process of plant coexistence and evolution in heterogeneous landscapes. Our study complements previous knowledge of belowground processes in big sagebrush populations, including patterns of resource acquisition, and indicates promising avenues for further research of the ecology and evolution of this species. The results highlight how local plant-plant interactions can be a source of variation in common garden experiments, which are used to evaluate adaptive capacity and seed transfer zone development for A. tridentata populations. Potential applications of our work include planting density recommendations for big sagebrush in applied and experimental contexts, and provide mechanistic understanding of intraspecific diversification and ecological tradeoffs related to local adaptations.