While coexistence in plant communities is frequently explained by effects of resource niche partitioning, the Janzen-Connell (J-C) hypothesis is an alternative approach that has been assumed as a major ecological mechanism explaining high species richness levels, in particular, in tropical forest ecosystems. Central components of the J-C hypothesis are non-competitive effects of density- and distance-dependence, thereby two drivers that contribute independently to species coexistence, but that are ultimately linked in the field. Here, we make use of the forest Biodiversity-Ecosystem Functioning project in subtropical China (BEF-China) to estimate density and distance dependence effects by means of a reciprocal tree seedling transplant experiment. Using monocultures of ten and seven tree species, respectively planted at two different sites, juveniles of all species were grown in their own (home) and in all other monocultures (away), thereby testing for distance effect, just as in three different levels of planting density, testing for density effects (see "Main Experiment: Seedling addition experiment - growth and biomass data" for further details) In addition, we repeated a similar set-up in a nearby common garden experiment, where we added a 'shadow' treatment to simulate different light conditions induced by the canopy layer. The general aim of the common garden experiment is to test for additional density dependent intraspecific competition for light in the absence of host-specific agents necessary for J-C effects. Density dependent patterns that are prevalent in the main experiment, while not being prevalent in the common garden experiment can be considered true J-C effects.

From the 11 different tree species used in the main experiment juvenile plants from 8 different tree species have been implemented in an additional common garden experiment established on a former agricultural used field nearby (former Pilot Experiment). Each species was planted at density levels of 1, 4, 9 and 25 individuals. Each species × density combination was grown in plots either established as 'shadow' or 'light'-treatment (3 replicates each). Each plot consisted of 1 species with 0.5 x 0.5 m2 in size (corresponding to the size of one splitplot of the main experiment). Plots established as 'shadow'-treatment have been surrounded by a container with 1.2 m in height wrapped in a shadow net. On the north facing site each shadow net was then opened up to 0.5 m in hight to provide a more suitable microclimate and prevent natural enemies exclusion. Each species X density X treatment combination was randomly allocated within the area of approximately 135 m2. The general aim of the common garden experiment is to test for density-dependent intraspecific competition effects as well as light and drought dependent mortality rates by excluding host-specific agents necessary for J-C effects.

Pilot experiment