This study evaluates the productivity of a harvester equipped with a double crane system for thinning with continuous felling and accumulation of whole small-diameter trees for bioenergy at different levels of automation. The simulations were performed using a discrete event simulation tool that has been developed recently and is specifically designed for simulations in forestry, incorporating spatial awareness of the simulated world. The study shows that introducing boom-corridor thinning with a semi-automatic double-crane system can significantly increase the productivity compared to conventional thinning and harvesting. For the specific harvester model used in this study, the modification that yielded the biggest productivity increase was automating the release and placement of the harvested trees. Studies on the effects of implementing automation for other forest machine operations could be analyzed using a similar approach.