Diversity effects increase across trophic levels during forest growth

Plant diversity affects ecosystem functioning via bottom-up effects propagating across trophic interactions. However, little is known about how these mechanisms change over time. This is particularly relevant during forest growth, where biomass and deadwood accumulation, canopy closure, and increased structural complexity transform habitat conditions, filtering species and reorganizing interactions. Using a decade of data from the world’s largest forest biodiversity experiment (‘BEF-China’), we tested how tree diversity effects propagate through multitrophic interactions over time to regulate a higher-order ecosystem function (parasitism). We provided artificial nesting aids to solitary bee and wasp communities from 2014 to 2023, tracking abundance, diversity, and interactions of 75 host and 116 natural enemy species from 65,000 brood cells. We found that parasitism in growing forests was jointly driven by tree diversity and stand age via multitrophic diversity, mediated by stand biomass accumulation. Forest growth amplified diversity effects across the trophic cascade, culminating in a stronger dependence of parasitism on diverse enemy communities. Tree diversity thus yielded increasing ecological benefits across multitrophic communities over time, highlighting the advantages of species-rich communities for ecosystem recovery. These temporal dynamics show that the full ecological value of biodiversity – and the costs of its loss – may only unfold over years or decades.