Abstract Biotic and abiotic conditions have been found to strongly influence how biodiversity affects ecosystem functioning in forests. This context dependency of biodiversity‐productivity relationships in real‐world ecosystems may be shaped by the biogeographic context via deep‐time processes acting on the size and composition of the species pool such as dispersal limitation, environmental filtering, speciation and invasibility. However, the role of the biogeographic context in shaping multifaceted biodiversity and forest productivity relationships remains uncertain. Using data from the Spanish National Forest Inventory in climatically similar forests on the Canary Islands (637 plots) and mainland Spain (1434 plots), we investigate the extent to which above‐ground productivity is determined by ecological and evolutionary processes associated with the biogeographic context. We used structural equation models to test the drivers of above‐ground productivity in both contexts, that operate via multifaceted tree diversity (taxonomic, phylogenetic, and functional diversity) or alternative mechanisms, that is environmental conditions, non‐native species, and the number of trees. Our results show that mainland and island forests exhibit similar levels of productivity, yet island forests display overall lower multifaceted diversity. We found that the number of trees increased multifaceted diversity and also increased productivity directly and via their effects on phylogenetic diversity in both mainland and island forests. Further, non‐native species increased productivity in island forests. Synthesis: Our results suggest that multifaceted diversity, by capturing the diversity of evolutionary history, contributes to elucidating diversity‐productivity relationships in mainland and island forests that could not be detected by taxonomic diversity alone. By filling empty niches in island forests, we find that non‐native species are fundamentally altering ecosystem functioning on islands and thus, reveal how biogeographic context can shape biodiversity‐productivity relationships.