Different aspects of diversity, such as species richness and structural diversity, have been shown to enhance forest ecosystem functions, including biomass production. However, whether diversity-productivity relationships change over time or with climatic conditions remains uncertain. We analyzed above-ground woody biomass increment (AWBI) derived from annual inventory data and micro-densitometry on stem disks from 'Sardinilla' in Panama, one of the oldest tropical tree diversity experiments. We investigated AWBI in five tree species growing in monospecific and species-rich neighborhoods. We hypothesized that a) species and structural diversity would increase AWBI, with these effects strengthening over time, b) species diversity effects on AWBI would be mediated by structural diversity, and c) overyielding in diverse neighborhoods would persist under drought. We observed higher AWBI in species-rich compared to monospecific neighborhoods despite slightly decreasing wood density. The strong complementarity effects in mixtures increased over time, indicating progressive strengthening of diversity effects. Species diversity strongly effected AWBI by directly enhancing productivity and indirectly, via increasing structural diversity. Structural diversity had a direct positive effect on AWBI, but this effect weakened with tree age. Overyielding in species-rich neighborhoods persisted or even increased under extremely dry conditions likely due to complementary water use among species. Our results corroborate that mixed planted forests are more productive and have a greater ability to maintain their performance under stressful conditions compared to monocultures. Forest management aiming at maximizing carbon sequestration in plantations should include fast-growing species with high wood density and promote not only tree species richness but also structural diversity.