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Questions: The maintenance of a diverse sapling pool is of particular importance for the regeneration and persistance of species-rich forest ecosystems. However, the mechanisms of co-existence of saplings have rarely been studied experimentally. Do species richness, species composition, species identity and stand density affect the co-existence, growth patterns and crown architecture of tree saplings? Location: Jiangxi Province, southeast China. Methods: In a field experiment, we manipulated the local neighbourhood of saplings of four early-successional subtropical species (Schima superba, Elaeocarpus decipiens, Quercus serrata and Castanea henryi) with regard to species richness (one, two and four species), species composition (monocultures, six-two-species combinations and one-four-species combination) and stand density (low, intermediate and high). We tested for treatment effects and impact of species identity on growth variables, biomass allocation, crown architectural traits and branch demography. Results: Species richness was a poor predictor of all response variables, but enhanced pruning and branch turnover. In contrast, species composition proved to be of great importance for growth, biomass allocation, crown architecture and branch demography. Local neighbourhood interactions of saplings were characterized by complementary or facilitative, as well as by competitive mechanisms. Intra-specific competition was higher than inter-specific competition for two species (C. henryi, Q. serrata) depending on the respective species combination. To a high degree, the competitive ability of species can be explained by species identity. Competition for light likely played a major role in our experiment, as evidenced by the strong response of crown architecture and branch demography to the manipulated predictor variables. Conclusions: Effects of species composition and species identity on growth rates and crown architecture variables of tree saplings point to niche separation as a mechanism of species co-existence, while effects of species richness were not yet prominent at the sapling life stage. In a field experiment, we manipulated the local neighbourhood of saplings of four species and surveyed tree sapling growth and crown architecture. Effects of species composition and species identity on growth rates and crown architectural variables point to niche separation as a mechanism of species coexistence, while effects of species richness were not yet prominent at the sapling life stage. © 2012 International Association for Vegetation Science.