Exotic species are assumed to alter ecosystem functioning. However, little is known of the relationships within vertically structured plant communities such as forests, where tree saplings interact with herbaceous species, especially in the early phases of succession. This relationship was tested in a common garden experiment which assessed the impacts on tree saplings and herbaceous species following nutrient addition and the introduction of exotic herb species. We studied the relationships between tree sapling diversity, herb-layer productivity and invasibility in an early-successional forest ecosystem. A full factorial design was applied, within which species com- position was crossed with nutrient and exotic seed-addition treatments. The seed-addition treatment included mixtures of seeds from eight exotic herb species, and herb community attributes were assessed after a four month growing season.

We applied fertilizer and exotic seed-addition treatments in a factorial design to randomly chosen plots. Eight exotic herbaceous species, selected according to their abundance in the Jiangxi and nearby Zhejiang provinces (seeds collected in 2008), were used for the seed-addition. Each exotic species was represented by 150 seeds per seed mixture applied to each plot. Fertilizer was applied shortly before seed-addition and plots received 13 g·N·m2·yr–1 added as NPK (17-13-15) standard fertilizer. At the end of May 2009, after weeding of the plots, the seed mixtures were sown into the bare ground at the centre of each plot (0.6 m × 0.6 m) along with four tree saplings, representing the respective tree diversity level. The remaining plot area acted as a buffer zone with the same treatment combination (tree diversity × fertilizer) but without seed-addition. At the end of September 2009, the total number of individuals (abundance) and the number of reproducing individuals of each herb species were counted. The height of each species was assessed by averaging the height of five randomly selected individuals. These values were used to calculate the abundance-weighted mean height of the herb-layer. In addition, herb-layer aboveground biomass was determined by harvesting each species separately. The harvested plant material was oven-dried (70°C 48 h) and weighed.

Pilot Experiment, Xingangshan, Jiangxi Province, China 29°06'N 117°55'E

Final harvest of the SP11 Pilot experimental plots was conducted in September 2009

Exotic herbaceous species were Ageratum conyzoides, Amaranthus spinosus, Chenopodium ambrosioides, Crassocephalum crepidioides, Oenothera biennis, Phytolacca americana, Sesbania cannabina and Solidago canadensis. Tree species used: Elaeocarpus decipiens, Schima superba, Quercus serrata, Castanea henryi

The variables of interest were invasibility measures like cover and abundance (cover, abundance) of introduced exotic species. In addition, fitness variables like number of flowering or fruiting individuals (ind_with_flow, ind_with_fruits) verify the success of the exotic species and the resident native species. On the community level the productivity of the herb-layer can be assessed by summing up biomass of each species (total_BM) and then can be analysed as a function of the treatments applied (seed_add, fert).