Relationships between climatic factors and δ 18 O values of tree rings in two species with different root systems were investigated in the northern area in Hokkaido Island, northern Japan. This region has a heavy snow cover in winter and has a humid climate throughout the year. In the case of spruce (Picea jezoensis), which has shallow roots, the δ 18 O values are negatively correlated with precipitation in July and August, probably due to higher relative humidity in summer and lower δ 18 O of summer precipitation in a year of higher summer precipitation, which is governed by the precipitation amount effect. Thus, the δ 18 O value of tree-ring cellulose in spruce must be valuable as a proxy of summer precipitation in this area. On the other hand, the δ 18 O value in oak (Quercus crispula), which has deeper roots than spruce, is not very sensitive to summer precipitation, but is negatively correlated mainly with relative humidity in July and August; this can be attributed to the smaller 18 O enrichment in leaf water in the summer of higher relative humidity. Our results indicate that δ 18 O values of tree rings in two species with different root systems from the same forest have the potential to reconstruct the two different climate factors.