Mechanisms promoting tree species co-existence: Experimental evidence with saplings of subtropical forest ecosystems of China

Abstract

Questions: The maintainence of a diverse sapling pool is of particular importance for the regeneration and persistance of species-rich forest ecosystems. However, the mechanisms of coexistence of saplings have rarely been studied experimentally. We thus ask: Do species richness, species composition, species identity and stand density have effects on the coexistence, 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 the four earlysuccessional subtropical species (Schima superba, Elaeocarpus decipiens, Quercus serrata and Castanea henryi) with regard to species richness (1, 2 and 4 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 the 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 characterised by complementary or facilitative as well as by competitive mechanisms. Intraspecific competition was higher than interspecific 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 architectural variables of tree saplings point to niche separation as a mechanism of species coexistence, while effects of species richness were not yet prominent at the sapling life stage.