In the Tanzawa Mountains located in the Kanagawa Prefecture, Japan, the decline of beech forests has been observed since 1980. Various studies were conducted thereafter, and O₃ and water shortage in addition to beech sawfly, were suspected of the decline factors. We examined the ecophysiological characteristics of beech seedlings using environment-controlled cabinets, to clarify the acute and chronic effects of O₃ and water stress singly or combined on this species. The seedlings were exposed to 70, 50 or 0 ppb O₃ with different water treatment such as 1200, 840 or 600 mm in terms of growing period precipitation, and harvested after 12 weeks. With increasing O₃ concentration or water stress, the dry weights were decreased, while the NAR similar to the RGR showed a decreasing tendency to O₃ concentration or water stress. However, effect of water stress on 70ppb O₃ treated seedlings and that of O₃ concentration on 600 mm treated seedlings were ambiguous. In the present experiment, some ecophysiological activities and biomass production were reduced with increasing concentration of O₃ and with increasing water stress, while these two factors acted additively from the statistical analysis, which was strongly suggested that O₃ and water shortage induced the observed decline of beech forests in the Tanzawa Mountains.