Leaf defense capacity of Japanese elm (Ulmus davidiana var. japonica) seedlings subjected to a nitrogen loading and insect herbivore dynamics in a free air ozone-enriched environment

Japanese elm ( Ulmus davidiana var. japonica ) is a native species in cool-temperate forests in Japan. We investigated growth, physiological reactions, and leaf defense capacity of Japanese elm seedlings under nitrogen (N) loading (45.3 kg N ha −1 year −1 ) and seasonal insect dynamics in a free-air ozone (O 3 )-enriched environment (about 54.5 nmol O 3 mol −1 ) over a growing season. Higher leaf N content and lower condensed tannin content in the presence of N loading and lower condensed tannin content in elevated O 3 were observed, suggesting that both N loading and elevated O 3 decreased the leaf defense capacity and that N loading further enhanced the leaf quality as food resource of insect herbivores. Two major herbivores were observed on the plants, elm leaf beetle ( Pyrrhalta maculicollis ) and elm sawfly ( Arge captiva ). The peak number of observed insects was decreased by N loading. Visible foliar injury caused by N loading might directly induce the reduction of number of the observed elm sawfly individuals. While elevated O 3 slightly suppressed the chemical defense capacity, significantly lower number of elm leaf beetle was observed in elevated O 3 . We conclude that N loading and elevated O 3 can alter not only the leaf defense capacity of Japanese elm seedlings but also the dynamics of elm leaf beetle and sawfly herbivores.