<p>Converting monocultures into mixed-species plantations is a key strategy for enhancing biodiversity and ecosystem resilience. Although the Paraná Atlantic Forest has been heavily deforested for agriculture and afforestation with exotic species, some saw-timber plantations utilize the sun-demanding native Gymnosperm <i>Araucaria angustifolia</i>. These monospecific stands provide an opportunity to test whether non-pioneer native tree species can develop under their canopy and thus convert monospecific stands into mixed stands. This study evaluated the establishment of five native broadleaf tree species with different coverage requirements that were planted in young and intermediate araucaria stands in Misiones, Argentina, South America. The interplanted species are valuable timber trees currently harvested only from rainforests. We assessed seedling survival and growth in relation to araucaria competition, stand type, and understory herbaceous competition. Mortality and height growth differed between species and stand types: the shade-demanding grew less than the sun-demanding seedlings, and growth was higher in young stands. Tree competition decreased growth differentially in each species and stand type. Herbaceous vegetation decreased the growth of sun demanding seedlings more than araucaria competition did, but it had no effect on shade-demanding seedlings. These findings underscore the importance of considering both overstory and understory dynamics when designing mixed-species plantations. Considering species-specific competition responses can improve reforestation strategies and lead to more functional and resilient forest ecosystems. Planting high-value rainforest timber under araucaria plantations is an effective way to increase landowners’ incomes and reduce logging pressure on the few remaining remnants of native forest.</p>