Phylogenetic relatedness drives litter-mediated diversity-invasibility relationships.

Native plant diversity typically enhances community resistance to biological invasions by reducing resource availability and by accumulating natural enemies. However, after senescence, litter-mediated processes may either strengthen resistance through pathogen spillover or promote invasion through nutrient enrichment. We experimentally partitioned nutrient and pathogen effects to test how litter diversity and phylogenetic relatedness between native and invasive species shape invasion outcomes. Three invasive Asteraceae species were exposed to diseased litter from native plants that were either closely or distantly related, while manipulating litter diversity (1, 2, or 6 species) and pathogen presence (with or without fungicide). Higher litter diversity from closely related natives suppressed invader biomass. Fungicide treatment eliminated this effect, demonstrating that pathogen spillover mediated diversity-driven invasion resistance. Conversely, litter from distantly related natives enhanced invader biomass with increasing diversity due to nutrient enrichment. Our results show that phylogenetic relatedness structures the balance between pathogen spillover and nutrient enrichment, determining whether native diversity reinforces or undermines invasion resistance through litter pathways.