There is great interest in how plants interactions in different spacings change along spatial gradients in resource availability, biomass and energy production. In this study, we hypothesized that a positive impact of Acacia mearnsii when grown in reduced spacing, leads to a higher biomass and energy yield. The aim of this study was to evaluate how these spacings and their corresponding effects might influence the radiation use efficiency in biomass and potential energy yield in a short-rotation cycle of black waffle (Acacia mearnsii) in southern Brazil. A field experiment was conducted from September 2008 to September 2015 in the city of Frederico Westphalen, Brazil. The forest biomass was determined by destructive method. Also, the calorific value, radiation use efficiency, light extinction coefficient, solar radiation interception, leaf area index, biomass partitioning, carbon storage and potential energy yield were evaluated. Information generated in this study is relevant and provides information for companies interested in electricity generation from wattle biomass and forest producers thereby assisting in the planning of optimal spacing to be used for biomass production for energy. The highest biomass production and radiation use efficiency were obtained in the reduced tree spacing (2.0 x 1.0 m) which results in a higher amount of biomass for energy production. Therefore, the use of reduced tree spacing should be prioritized and recommended for future exploitation of forest energy plantations.