Indirect reciprocal facilitation promotes species coexistence in plant communities worldwide

Plant-plant facilitation is key to supporting species persistence within communities. However, as beneficiary species mature and compete with their benefactors, competition appears to dominate, suggesting that facilitation has a limited role in long-term community structuring. Here, we propose that facilitation can consistently enhance diversity when it occurs reciprocally between interacting species. Leveraging spatial associations between recruits and canopy species to infer recruiting interactions, we analyse a global dataset of 96 plant community recruitment networks, encompassing 2,403 non-transient populations across 23 countries. We show that ~90% of facilitation events were reciprocated, primarily mediated by intermediary species that transmit benefits through indirect facilitation loops, ultimately returning these benefits to the original benefactors. We find stronger positive effects within longer facilitation loops coupled with fewer negative interactions, which are essential conditions for benefactors to receive increased returns and, consequently, for the long-term persistence of indirect reciprocal facilitation. A positive empirical relationship between indirect reciprocity and species richness, substantiated by numerical simulations, indicates that these conditions also foster species coexistence. Our findings highlight the underappreciated role of indirect reciprocal facilitation in maintaining biodiversity.