Effects of ozone and/or nitrogen load on the growth of Larix kaempferi, Pinus densiflora and Cryptomeria japonica seedlings

To obtain basic information on the critical levels of O3, for the protection of Japanese coniferous forests, the growth responses of Larix kaempferi, Pinus densiflora and Cryptomeria japonica seedlings to 03 under different nitrogen (N) loads were investigated. The seedlings were grown in 12 experimental treatments, which were comprised of four gas treatments (charcoal-filtered air and three levels of 03 at 1.0, 1.5 and 2.0 times ambient concentration) in combination with three N treatments (0, 20 and 50 kg ha-1 year 1), during the two growing seasons. Although no significant interaction between O3 and N load was detected for the growth of P. densiflora and C. japonica seedlings, significant interaction between O3 and N load for the growth of L. kaempferi seedlings was found at the end of the second growing season. The N load reduced the sensitivity of growth of L. kaempferi seedlings to O3. Critical levels of O3 expressed as AOT40 for one growing season from April to September, corresponding to a 10% reduction in the whole-plant dry mass increment, were evaluated to be 28 (NO), 29 (N 20) and 53 (N 50) iimol mol 1 h in L. kaempferi, 38 ƒEmol mol- 1 h in P. densiflora and 111 umol mot- 1 h in C. japonica. In the three tree species, a high correlation was found between the sensitivity to O3 of the whole- plant dry mass increment and that of the total net photosynthetic rate (Atotal), expressed as the product of needle dry mass and net photosynthetic rate in the summer of the second growing season. It was concluded that N deposition must be taken into account in the evaluation of the critical level of O3 for the protection of L. kaempferi forests. The Aotal can be used as an indexical plant factor for estimating the sensitivity to O3 of Japanese coniferous forest tree species under different N loads.