Leaf traits-mediated effects of tree diversity on insect herbivory on Populus laurifolia in a riparian forest ecosystem

•Insect herbivory increases with landscape-level forest isolation and tree height.•Poplars in pure forest stands suffer greater insect damage than in mixed stands.•Tree diversity effects are significant in stands at the near distance from river.•Insect herbivory responds sensitively to poplar leaf area and specific leaf area.•Tree diversity effects on herbivory act indirectly through changes in leaf traits.