Genecology studies in forest tree species focus on understanding how genetic differentiation improves populations’ fitness in their home environment. Yet, studies explaining interaction between adaptation to climate and intra-specific tree competition are scarce. This study hypothesizes that growth in populations of black walnut (Juglans nigra) adapted to differing climates are affected by competition differently. Results indicate that, for some growth traits, the effects were additive, and, for others, interaction was significant.