Increased phytotoxic O(3) dose accelerates autumn senescence in an O(3)-sensitive beech forest even under the present-level O(3)

Ground-level ozone (O(3)) concentrations are expected to increase over the 21(st) century, especially in East Asia. However, the impact of O(3) has not been directly assessed at the forest level in this region. We performed O(3) flux-based risk assessments of carbon sequestration capacity in an old cool temperate deciduous forest, consisting of O(3)-sensitive Japanese beech (Fagus crenata), and in a warm temperate deciduous and evergreen forest dominated by O(3)-tolerant Konara oak (Quercus serrata) based on long-term CO(2) flux observations. On the basis of a practical approach for a continuous estimation of canopy-level stomatal conductance (G(s)), higher phytotoxic ozone dose above a threshold of 0 uptake (POD0) with higher G(s) was observed in the beech forest than that in the oak forest. Light-saturated gross primary production, as a measure of carbon sequestration capacity of forest ecosystem, declined earlier in the late growth season with increasing POD0, suggesting an earlier autumn senescence, especially in the O(3)-sensitive beech forest, but not in the O(3)-tolerant oak forest.