Growth of Japanese and hybrid larch seedlings grown under free-air O₃ fumigation—an initial assessment of the effects of adequate and excessive nitrogen

Abstract

　Ground-surface ozone (O₃) and nitrogen (N) deposition in forests are increasing rapidly in East Asia. The Japanese larch (JL) has been transplanted from central Japan to northern areas. Because the JL suffers from shoot blight disease and grazing by voles, the hybrid larch (F₁) was developed to overcome these weak points. For sustainable use of these larches, we should know the growth responses of larches to elevated O₃ (eO₃) and excessive N deposition in northern Japan. Two-year-old JL and F₁ seedlings were potted in well-weathered immature volcanic ash soil with basal dressing of balanced fertilizer containing 47 kgN ha^-1 yr^-1, due to a lack of nutrients in the soil. To simulate eO₃, seedlings were grown in a free-air O₃ fumigation system (60 nmol mol^-1 in daytime). Also, to simulate acid rain, NH₄NO₃ (50 kgN ha^-1 yr^-1 in total) was applied to the plants. Independent of the N loading, the dry mass of needle litter in both JL and F₁ was less at eO₃ than at ambient O₃. Re-translocation of N before the needle shedding of both larches was lower under eO₃. As a result, the aboveground biomass per needle (i.e. productive efficiency) was higher at eO₃.