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Janzen-Connell effects in a forest BEF experiment: Strong distance-dependent seedling establishment of multiple species.

The Janzen-Connell (JC) hypothesis is a major ecological explanation for high species richness, in particular in tropical forest ecosystems. Central components of the JC hypothesis are non-competitive effects of distance- and density-dependence, two drivers that contribute independently to species coexistence, but which are ultimately linked in the field. However, while numerous studies provide evidence for either distance- or density-dependent effects based on observational data, experimental testing of simultaneous and interactive effects of distance and density has rarely been conducted yet, especially in a comprehensive multispecies approach. Here, we make use of the forest Biodiversity-Ecosystem Functioning project BEF-China to estimate distance- and density-dependent effects in a reciprocal tree seedling transplant experiment of eleven tree species. We deployed 13,490 juveniles of all eleven species in their own (home) and in all foreign monocultures (away) as well as at three different levels of planting density, thereby testing for distance and density effects, respectively. In addition, to quantify the amount as to which density effects were brought about by potential additional effects of intraspecific competition, we set up a Common Garden Experiment with different levels of planting density, where an additional 'shadow' treatment controlled for effects of canopy shading. While the 'away' and 'high-density' treatments significantly impaired the performance and productivity of seedlings, leaf damage and survival was exclusively affected by either the home/away- or the density-treatment, respectively. Negative density-dependent effects on leaf damage were less pronounced in the 'home' treatment, showing that the effects were not additive. In addition, results obtained in the Common Garden Experiment showed that negative effects of high density may be also brought about by intraspecific competition as an alternative density-responsive mechanism and less by true JC effects. Overall, our results provide strong support on a multi-species basis for the influence of host-specific effects already operating in early stages of a forest plantation. However, they also emphasize the need to appropriately account for potential additional density-responsive mechanisms such as intraspecific competition or micro-environmental conditions when addressing the role of JC effects for species coexistence. This article is protected by copyright. All rights reserved.