Networks of friends and foes and the fate of tree seedlings

Why are there so many different tree species in most tropical rainforests? Ecologists have increasingly turned to plant–soil feedback (PSF), the mutual interaction between plants and the soil biotic community, where plants modify, and are modified by, that soil biotic community. In the 1970s two ecologists, Daniel Janzen and Joseph Connell, independently proposed a mechanism to explain the persistence of species‐rich communities through such feedbacks. The Janzen–Connell effect is based on conspecific negative density dependence (CNDD), implying larger juvenile mortality (seeds, seedlings, saplings) in the immediate vicinity of conspecific adult organisms. As a consequence of CNDD other species would be preferentially recruited under their canopy. Fungal pathogens are likely the major agent in causing seedling CNDD (Song & Corlett, 2021). Roots of tropical tree species are also colonized by beneficial microorganisms such as mycorrhizal fungi, however the role of mutualists in the Janzen–Connell effect has not often been studied. Integrated studies on the potentially opposite roles of pathogenic and mycorrhizal fungi for the maintenance of tree species richness are even much rarer, possibly because mycorrhizal ecology and ecological phytopathology largely developed as independent ecological subdisciplines. Liang et al. (2015) showed that arbuscular mycorrhizal fungi could neutralize the Janzen–Connell effect and ectomycorrhizal fungi have also been shown to counteract the Janzen–Connell effect (Segnitz et al., 2020). Bennett et al. (2017) showed that in the temperate forests ectomycorrhizal tree seedlings were mostly affected by positive plant–soil feedback, where adult plants generate favourable conditions for the next generation of that species through increasing mutualist abundance, whereas arbuscular mycorrhizal trees were mostly affected by negative plant–soil feedback. The same result was obtained in a subtropical forest by Chen et al. (2019), however both studies did not experimentally manipulate seedling density