Shading and selection effect-mediated species mixing enhance the growth of native trees in dry tropical forests

Tropical dry forests remain vital to rural communities but are often degraded and require restoration. Biodiversity plays a crucial role in maintaining ecosystem functioning and resilience and in providing essential services in these ecosystems. In many cases, restoration involves planting monospecific plantations of robust exotic species; however, detailed ecological studies are required to understand how native species mixtures could become successful for restoration purposes. To address this knowledge gap, a tree diversity experiment (IDENT-Ethiopia) was conducted to examine the impact of species diversity and shading on the growth of nine native tree species in tropical dry forests. The experiment followed a block design with 270 plots, which included a gradient in native tree species richness (1-, 2-, and 4-species mixtures) and a second gradient based on the functional diversity of species traits, including plots of low, medium, and high functional diversity. A shading treatment (shaded and unshaded) was also replicated in triplicate. The stem volume growth of seedlings was measured 1 and 2 years after planting. The results revealed that seedling growth was significantly boosted by increased species richness and shading: stem volume growth increased by 50.9% in shaded environments and 30.5% in mixed plots. The study also demonstrated a positive diversity productivity relationship in 57% of all mixtures. Variance partitioning showed that this overyielding was a result of competitive dominance. In the shaded environment, these productive dominant species were Cordia africana, followed by Dodonaea angustifolia and Dovyalis abyssinica. Overall, the findings suggest that shading and species mixing are crucial factors for promoting seedling growth of native dryland species and ensuring the successful restoration of drylands.