The role of tree species richness and identity for herb layer characteristics and next generation tree species establishment in a subtropical forest in China

In recent years, research on Biodiversity ecosystem functioning (BEF) relationships has extended its scope towards to long-lived tree diversity experiments, with their large contribution to ecosystem functions at the global scale. The herbaceous understorey contributes an impressive part to the overall diversity in forests and plays an important role for tree regeneration as well as for ecosystem functioning in general. Vice versa, trees in the overstorey control understorey herb and shrub growth such as through competition for resources. However, despite its importance, our understanding of the interactions between the herbaceous understorey and tree layer diversity is still limited. In the context of the Biodiversity-Ecosystem Functioning project in subtropical China (BEF-China), this thesis investigates the role of tree species richness and identity for herb layer characteristics and next generation tree species establishment. This thesis combines the outcome of three different experimental approaches, including one study on the effect of tree species richness and identity on herb layer composition, richness and productivity and how these relationships across strata change with abiotic environmental conditions and competition intensity. The second study focuses on the temporal aspect and studies the effect of tree species richness and identity on understorey productivity over a period of three years. The third study examines the influence of Janzen-Connell effects on next generation tree species establishment. Results obtained in all three different approaches support light transmittance as major limiting factor at the current state of forest development. While we did encounter additional effects for tree species identities, a similar effect with variations in tree species richness could not be found yet. Moreover, we were able to demonstrate conspecific negative distance-dependent seedling establishment and intraspecific competition being already effective at this early stage of forest development. While the subsequent interplay of the different identified and prospective mechanisms remains to be tested, this thesis adds to our understanding of the current and later phases of forest development and highlights the need to account for confounding effects.