Shading and species diversity act as safety nets for seedling survival and vitality of native trees in dryland forests: Implications for restoration

Restoring degraded dryland requires a diverse mix of trees and shrubs with suitable characteristics. Unfortunately, many restoration projects have failed in tropical drylands due to monoculture plantations or a lack of understanding regarding specific species requirements like shading. Therefore, the central question is whether species diversity and shading can improve seedling survival and vitality in dryland conditions. We conducted a tree diversity experiment in northern Ethiopia as part of the International Diversity Experiment Network with Trees (IDENT). We used 17,280 seedlings from 9 native dry Afromontane tree and shrub species, distributed across 270 plots in three replication blocks. Each block contains a shading treatment (shaded versus non-shaded) with three levels of species richness: one-, two- and four-species mixtures with varying levels of functional diversity (FD) categorized as low, medium, and high. Survival and vitality data of the seedlings were analyzed using generalized linear mixed models (GLMMs) and linear mixed-effects models (LMMs). After two growing seasons, the overall seedling survival rate was 84%. We found significantly higher seedling survival, vitality, and chlorophyll content (SPAD value) in shaded conditions, irrespective of the tree diversity level. However, increasing species richness did improve seedling vitality in non-shaded conditions, but its impact diminished in shaded situations. The findings from this study suggest that careful selection of tree species, appropriate shading, and optimal levels of species diversity can greatly enhance the survival and performance of seedlings. This is particularly important for the successful restoration of dryland forests.