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Japanese larch ( Larix kaempferi (Lamb.) Carr.) and its hybrid are economically important coniferous trees widely grown in the Northern Hemisphere. Ground-level ozone (O 3 ) concentrations have increased since the pre-industrial era, and research projects showed that Japanese larch is susceptible to elevated O 3 exposures. Therefore, methodologies are needed to (1) protect Japanese larch against O 3 damage and (2) conduct biomonitoring of O 3 in Japanese larch forests and, thus, monitor O 3 risks to Japa-nese larch. For the first time, this study evaluates whether the synthetic chemical ethylenediurea (EDU) can protect Japanese larch against O 3 damage, in two independent experiments. In the first experiment, seedling communities, simulating natural regeneration, were treated with EDU (0, 100, 200, and 400 mg L ?1 ) and exposed to either ambient or elevated O 3 in a growing season. In the second experi-ment, individually-grown saplings were treated with EDU (0, 200 and 400 mg L ?1 ) and exposed to ambient O 3 in two growing seasons and to elevated O 3 in the succeeding two growing seasons. The two experiments revealed that EDU concentrations of 200–400 mg L ?1 could protect Japanese larch seedling communities and individual saplings against O 3 -induced inhibition of growth and productivity. However, EDU concentrations ≤ 200 mg L ?1 did offer only partial pro-tection when seedling communities were coping with higher level of O 3 -induced stress, and only 400 mg EDU L ?1 fully protected communities under higher stress. Therefore, we conclude that among the concentrations tested the con-centration offering maximum protection to Japanese larch plants under high competition and O 3 -induced stress is that of 400 mg EDU L ?1 . The results of this study can provide a valuable resource of information for applied forestry in an O 3 -polluted world.