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The effects of air pollution on the growth of mountain trees were investigated at Buna-daira (1,180 m a.s.l.), about half the way up Mt. Tateyama, located in Japan. Every year, about 1 million tourists are transported by highland buses through the Tateyama–Kurobe Alpine route. Since the route opened in 1971, some tree species along the road have declined and have been blighted, suggesting that bus exhaust was the cause. However, the level of regional and long-range transboundary air pollution has also increased significantly over the last few decades. The atmospheric NO2 concentration at the roadside in the forest was highly correlated with the traffic density of buses and penetration of the exhaust into the forest was detected. However, the maximum average NO2 concentration was lower than 3.5 ppbv during the peak traffic period in the year. At Buna-daira, the total stem cross-sectional area at breast height (BA) of the forest was nearly unchanged from 1999 to 2006, but the BA of Fagus crenata decreased 10% and that of Cryptomeria japonica increased 6%. Neither tree growth nor tree death was significantly correlated with distance from the road. The cause of the decline of F. crenata could not be attributed to the effects of road, i.e., air pollution emitted from the buses or edge effects of the road. This area was more affected by regional, long-range transport of air pollution (O3, SO2, etc.). The average atmospheric O3 concentration in autumn was higher than 40 ppbv and the recent increase in the O3 concentration may be an important factor of F. crenata decline through the changes in the interspecific relationships between F. crenata and C. japonica, O3 sensitive and tolerant species, respectively. [ABSTRACT FROM AUTHOR]