Lack of tree layer control on herb layer characteristics in a subtropical forest, China

Question: Knowledge of the interaction between understorey herb and overstorey tree layer diversity is mostly restricted to temperate forests. How do tree layer diversity and environmental variables affect herb layer attributes in subtropical forests and do these relationships change in the course of succession? Do abundance and diversity of woody saplings within the herb layer shift during succession? Location: Subtropical broad-leaved forests in southeast China (29 degrees 8'18 ''-29 degrees 17'29 '' N, 118 degrees 2'14 '' 118 degrees 11'12 '' E). Methods: A full inventory of the herb layer including all plants below 1-m height was done in 27 plots (10 x 10 m) from five successional stages (<20, <40, <60, <80 and >= 80 yr). We quantified the contribution of different life forms (herbaceous, woody and climber species) to herb layer diversity and productivity and analysed effects of environmental variables and tree layer diversity on these attributes. Results: Herb layer composition followed a successional gradient, as revealed by non-metric multidimensional scaling (NMDS), but diversity was not correlated to the successional gradient. There was no correlation of diversity across layers. Herb layer productivity was neither affected by tree layer diversity nor by herb layer diversity. Although abundance of woody species in the herb layer decreased significantly during succession, woody species contributed extraordinarily to herb layer species diversity in all successional stages. All environmental factors considered had little impact on herb layer attributes. Conclusions: The subtropical forest investigated displays an immense richness of woody species in the herb layer while herbaceous species are less prominent. Species composition of the herb layer shows a clear successional pattern, however, the presence or absence of certain species appears to be random.