Degradation of tropical forest is considered as one of the greatest threats to biodiversity conservation. Such losses are mainly due to agricultural expansion and overharvesting of economically important trees. To enhance scientific understanding, this paper investigates the relationship between tree diversity and productivity in 58 mixed species plantations in protected tropical areas as a case study. We measured overall diversity using species richness and productivity using stand basal area and mean annual increment of stand basal area. Our research shows a negative relationship between tree diversity and productivity. These findings suggest that mixed species plantations should not be implemented if the main purpose is to consider the productivity of forest stands.