Riparian forests have a complex disturbance regime, which is often related to a hypothesis that frequency and intensity of fluvial disturbance may have important consequences on the forest structure and dynamics. In this study, we investigated tree distributions in two, adjacent montane creeks, Kanumazawa and Oarasawa, that differ in stand structure and probably in disturbance regime, and asked (1) to what extent the distributional shift (reciprocation) occur for major species and (2) how different the frequencies of fluvial disturbances are in each creek.

We established 15.25-ha survey area over the two-creek landscape, and mapped the location of three representative tree species in the landscape. Stem cores (Oarasawa) and direct observation record for 20 years (Kanumazawa) were used for evaluation of disturbance regimes.

Different distribution patterns were found; species found mainly in Kanumazawa, species found mainly in Oarawasa, and species found evenly in both. Stem cores suggested that the average interval of fluvial disturbance was 4.3 yr in Oarasawa, whereas in Kanumazawa only two debris sedimentation was observed at 19-yr interval. The possible processes were discussed that can cause such distributional differences in view of recent theory in community ecology.