Competition belowground is prevalent in both natural and managed landscapes. Competing individuals use unique species traits in order to acquire essential soil resources that vary temporally and spatially. Certain traits produce distinct phenotypes, such as shifts in root growth and architecture. These growth adjustments can take place along millimeter or meter-wide spatial scales. Other traits involve physiological mechanisms. Combined, these traits determine the competitive outcomes of interacting individuals. The sum of all species traits is likely to increase under inter-specific conditions due to the greater number of species, which in turn may alter competitive interactions compared to intra-specific conditions. As species assemble, either naturally or as a result of human intervention, it will be important to understand how belowground traits affect competitive interactions, which over time, affect the growth and productivity of tree-dominated landscapes.