The role of herb layer characteristics and productivity for forest ecosystem functioning

点击翻译按钮获取中文摘要

While ecosystem functioning can be affected by tree or herb layer diversity in isolation, the structural complexity of forests allows for additional niche complementarity, competition and facilitation across strata. In the first phases of BEF-China, we studied top-down and bottomup effects between herb and tree layer diversity in subtropical forests. We found ecosystem functions like herb layer productivity to be influenced by environmental variables, but also initial control of tree diversity on herb layer invasibility.In the third phase, we now address the mechanisms behind tree layer x herb layer interactions. We continue to quantify herb layer characteristics and their feedback effects on the trees in the Main Experiment. In addition, we will take into account effects of herb layer x tree diversity interactions on tree recruitment and address the role of the herb layer for the nutrient balance of all plots.As major objectives, we will test 1.a) for increasing effects of tree diversity with diminishing influence of the Ecoscape on the herb layer and 1.b) for effects of functional group presence in the herb layer on tree layer x productivity relationships, 2.) for Janzen-Connell effects by studying woody recruitment in the Main Experiment, and 3.) for important quantitative contributions of the herb layer to the whole CNP balance of the ecosystem.In a first work package removal of woody weeds will be done in the context of the BEFmod experiment to study bottom-up effects on the trees. In a second work package, we aim to predict future stand development in the Main Experiment after weeding stop by studying recruitment of woody species. We will quantify the amount of woody recruits in the herb layer for early reproducing tree species of the Main Experiment. By means of seed addition experiments, we will test for Janzen-Connell effects as alternative hypothesis why monocultures might underperform in productivity. We address the roles of negative distance and density dependence vs. environmental filtering for woody recruits and will test for positive diversity effects along a tree diversity gradient that show reduced Janzen-Connell effects. In a third work package, our subproject will quantify herb layer leaf nutrient concentrations (P and N, Ca, Mg, K) with NIRS and calculate herb layer contents on a plot basis as a measure of resource acquisition following P fertilization. In cooperation with SP5, these analyses will show how nutrient contents and stocks in the herb layer respond to tree diversity, the Ecoscape and nutrient limitation, and thus, will contribute an important term to the overall nutrient balance in the Main Experiment.Our subproject will help identifying interaction effects of stratum level diversity and thus help deciding about the extent as to which complementarity, competition and facilitation between herb and tree layer change in up growing subtropical forests.