Factors limiting the distribution of deciduous broadleaved trees in warm-temperate mountainous riparian forests

We examined differences in the responses of deciduous and evergreen broadleaved trees to fluvial disturbance and light environment near a river-facing forest edge in order to identify the factors limiting the distribution of deciduous trees, which are important components of warm-temperate mountainous riparian forests. Deciduous trees tended to be distributed on lower terraces of less than 2-m relative elevation from the water level, which corresponded with sites suffering from strong bank erosion due to high flood frequency. On the other hand, evergreen trees showed an opposite trend of distribution, indicating that high flood frequency associated with strong erosion-dominated soil disturbances might be a constraint for their establishment and/or survival. Furthermore, crowns of deciduous trees tended to be in the canopy layer at the forest edge, with fewer individuals observed beneath the canopy of evergreen trees, even at the forest edge. In contrast, evergreen trees were concentrated in the understory of the forest interior. These observations suggested that the light environment is the predominant factor affecting the establishment and survival of deciduous trees in warm-temperate mountainous riparian forests; however, the river-facing forest edge beneath the evergreen canopy is not a suitable habitat for deciduous trees. We concluded that erosion-dominated soil disturbance on lower terraces provides suitable habitats for deciduous trees by facilitating their reaching the canopy layer by limiting the establishment and/or survival of evergreen trees.