Why do diverse species live together and what's the mystery behind it? This is the core question of community ecology. Earlier studies found that the more individuals of a tree species living closely together, the poorer recruitment and survival of the species. The intra-specific inhibition provides space for other species to colonize, thereby promoting species co-existence. But what exactly causes the "self-inhibition" phenomenon?  The Janzen-Connell Hypothesis (hereinafter JC hypothesis) proposed that the more conspecific neighbors, the more likely they are to attract and gather the herbivorous specialists of pathogens, insects or other animals, thereby negatively affecting seedlings of the plant species. Studies have revealed the importance of pathogens and insect herbivores to plant population dynamics, but manipulation experiments at community scale are still rare, especially in temperate forests with less-diversity.

In view of this, Dr. JIA Shihong, research scientist WANG Xugao, together with their collaborator Robert Bagchi at the University of Connecticut and their colleagues from the Natural Forest Ecology Group of the Institute of Applied Ecology, Chinese Academy of Sciences, established a manipulation experiment in a broad-leaved Korean pine forest in Changbai Mountain in 2014. They investigated the effects of large animals, insects and pathogens on seedlings of trees by fencing and spraying fungicides and pesticides.

The researchers found that the application of fungicides at community scale significantly reduced the recruitment and survival of seedlings near conspecific adults; Pesticides reduced seedling recruitment, but not their survival; The setup of fences had no effects on seedling recruitment or survival. They concluded that pathogenic fungi and insects inhibit seedling recruitment at high conspecific density (i.e., through conspecific density dependence mechanism), and consequently provide living space for other species and promote species coexistence. In addition, the authors found that fungicide treatments mainly affected seedling recruitment near conspecific adults with ectomycorrhiza and shade-tolerant traits, while insects were mainly responsible for the seedling recruitment around adults of non-shade-tolerant species.

The results of this field manipulation experiment at community scale demonstrated that pathogenic fungi and herbivorous insects are key drivers affecting plant species coexistence in the temperate forest, and that the strength of the two drivers depends on life history strategies of tree species. This study provides empirical evidence for understanding tree species coexistence mechanism in temperate forests, and also a theoretical support for forest management and biodiversity protection.

This study has been online published in Nature Communications (DOI: 10.1038/s41467-019-14140-y) under the title "Tree species traits affect which natural enemies drive the Janzen-Connell effect in a temperate forest." This study was financially supported by the Priority Science & Technology Programme of the Chinese Academy of Sciences and the National Natural Science Foundation of China.