LAUSR

“What determines species diversity” was listed as one of the 25 highlighted and extremely challenging questions in the journal Science on its 125 anniversary (Pennisi, 2005). Why are there so many species in the world? How do these species coexist in one community? Ecologists and evolutionary biologists have struggled with these questions for centuries. A leading explanation, particularly in tropical forests, is Janzen-Connell hypothesis (Janzen, 1970; Connell, 1971), which suggests that local abundance of seeds and seedlings is negatively affected by conspecific neighbors through accumulation of host-specific natural enemies (conspecific negative density dependence, CNDD). Plant community diversity can be maintained when common species suffer stronger CNDD than rare species (‘rare species advantage’). Mounting literature in recent years has provided compelling evidence that soil fungal pathogens play a crucial role in limiting conspecific neighbors. However, few studies have paid attention to other functional groups of soil fungi beneath plants and interact with plant roots, such as symbiotic mutualists and decomposers, and the mechanical roles of different fungal functional groups to CNDD remain largely elusive across species and ecosystems. Inspired by previous studies on the CNDD, Chen et al. (2019) conducted a study in a 24 ha stem-mapping subtropical forest dynamics plot in China to uncover the underlying mechanisms. They used the long-term (2006-2014) tree seedling census data to investigate the conspecific neighborhood effects on seedling survival, and collected soil samples in plant rhizosphere from 322 individual trees belonging to 34 species to identify the soil fungal taxa using DNA sequencing. They found that seedlings experienced strong CNDD from both conspecific seedlings and adult neighbors, and tree mycorrhizal types influenced the strength of CNDD. Arbuscular mycorrhizal (AM) plants were more negatively affected by conspecific neighbors compared to ectomycorrhizal fungi (EcM) and ericoid mycorrhizal (ErM) plants, while EcM and heterospecific neighbors generally facilitated seedling survival. By combining the fungal accumulation rates in both diversity and density with the seedling survival data, they showed that pathogen accumulation was positively related to the interspecific variation in CNDD, whereas EcM fungi accumulation favored seedling survival. These results provide direct evidence that both mycorrhizal fungi and fungal pathogens mediate the strength of the CNDD. The findings are indicative of a conceptual shift towards the mechanical understanding of the complex interactions between plants and their enemies and allies. What we can learn from this work is at least threefold. First, long-term biodiversity monitoring is key to uncover the secret of species coexistence. Short-term monitoring in plant demography often ignores the impacts of environmental fluctuations over time, and cannot capture the temporal variations of local neighbor effects. Chen et al.’s work pro-