As tropospheric ozone (O₃) concentrations over Japan are increasing, there is concern over its potential adverse effects on forest trees induced by stomatal uptake in the daytime. Although atmospheric data, including O₃ levels, have been frequently collected at air pollution monitoring stations in urban areas since the 1970s, continuous recording of O₃ concentration data above forests has been less frequent. Therefore, O₃ concentrations above forests were monitored at three flux towers in Sapporo, Appi, and Yamashiro from 2012 onwards. Furthermore, past daytime O₃ concentrations in the forests were estimated by a multiple regression model using atmospheric and meteorological data of the forests and nearby monitoring stations. The obtained daytime (sunrise to sunset) forest O₃ concentrations were positively correlated with urban O₃ concentrations, although less correlated early in the morning. In the multiple regression analysis, the model with the minimum Akaike information criterion was employed using data from 2012 and validated using data from 2013. To assess past O₃ risk, we estimated O₃ concentrations over the forests from 2000 to 2011 and calculated the cumulative exposure index AOT40 during daylight hours (≥ 50 Wm^-2 global radiation). Among the three sites, AOT40 was lowest in the Sapporo forest site and did not exceed the critical level for the dominant canopy species (Betula platyphylla var. japonica and Quercus mongolica var. crispula) from 2000 to 2011. Conversely, since the dominant canopy species of the Appi forest site is Fagus crenata, categorized as a highly O₃-sensitive species, the forest of Appi was subject to AOT40 above the critical level throughout the period from 2001 to 2011. As AOT40 of Yamashiro has increased and exceeded the critical level of the dominant canopy species, Q. serrata (a moderately O₃-sensitive species), since 2006, the forest of Yamashiro might have suffered some adverse effects of O₃ exposure.