Ground-level ozone (O3) pollution is a persis-tent environmental issue that can lead to adverse effects on trees and wood production,thus indicating a need for forestry interventions to mediate O3 effects.We treated hybrid larch (Larix gmelinii var.japonica × L.kaempferi)saplings grown in nutrient-poor soils with 0 or 400 mg L-1 water solutions of the antiozonant ethylenediurea(EDU0,EDU400) and exposed them to ambient O3 (AOZ;08:00-18:00 ≈ 30 nmol mol-1) or elevated O3 (EOZ;08:00-18:00≈ 60 nmol mo1-1) over three growing sea-sons.We found that EDU400 protected saplings against most effects of EOZ,which included extensive visible foliar injury,premature senescence,decreased photosyn-thetic pigment contents and altered balance between pig-ments,suppressed gas exchange and biomass production,and impaired leaf litter decay.While EOZ had limited effects on plant growth (suppressed stem diameter),it decreased the total number of buds per plant,an effect that was not observed in the first growing season.These results indicate that responses to EOZ might have implications to plant competitiveness,in the long term,as a result of decreased potential for vegetative growth.However,when buds were standardized per unit of branches biomass,EOZ significantly increased the number of buds per unit of biomass,suggest-ing a potentially increased investment to bud development,in an effort to enhance growth potential and competitiveness in the next growing season.EDU400 minimized most of these effects of EOZ,significantly enhancing plant health under O3-induced stress.The effect of EDU was attributed mainly to a biochemical mode of action.Therefore,hybrid larch,which is superior to its parents,can be significantly improved by EDU under long-term elevated O3 exposure,providing a perspective for enhancing afforestation practices.