We investigated how hoarding behavior by squirrels and wood mice affected the distribution of walnut trees in a riparian forest along the Kushiro River in eastern Hokkaido, Japan. We established a 2 0 0 × 1 0 0 m plot on the left bank of the river, mapped the locations of walnut trees, and estimated their ages(DBH > 1 0 cm)using annual ring cores. The topography was a predominantly flat site within 6 0 m of the river and sloped 6 0-1 0 0 m from the river. Walnut trees were distributed

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... the plot. The oldest walnut tree in the plot was 6 8 years old and located 1 0 m from the river. Walnut trees older than 5 0 years were located within 4 0 m of the river, while the trees at the upper part of the slope tended to be younger. These age differences suggested that the areas were not colonized at the same time. Five of the seven small trees in the plot grew near old trees, probably their mothers; the other two grew near trees of other species. Given the distribution and age structure, the walnut trees spread from the flat area to the slope. In flat areas, seeds disperse via the river, gravity, and squirrels or wood mice. On slopes, gravity and squirrels or wood mice disperse the seeds. We inferred that squirrels and wood mice helped the walnut trees expand into places where they could not spread via the river and gravity alone, especially to locations far from the river and on upper slopes.