In cool-temperate regions of Japan, riparian forests tend to have high tree species richness, but it is unknown if this trend is consistent in warm-temperate regions. Here, we assessed tree species richness and forest structure in three topographic units reflecting the ridge to valley gradient (slope crests and upper and lower slopes) in a mature broadleaf evergreen forest in southwest Japan. We established a 0.59-ha valley bottom plot that was primarily comprised of a lower slope unit, then surveyed and mapped all living trees with a diameter at breast height > 5 cm. Resulting compositional and species richness data were compared to those from an adjacent permanent plot that included the slope crest and upper slope units, provided by the Japanese Ministry of the Environment. In the valley bottom, debris flow terraces had the highest tree density and served as important habitat for broadleaf deciduous trees. Tree species compositions were broadly similar in that nearly all species were found in each topographic unit. However, there were obvious differences in species densities among topographic units, with the lowest densities found in the lower slope unit. Species–area curves indicated that tree species richness was lowest in the lower slope unit. Species–individual curves indicated that all three units had very similar total species richness. Even when forbs were included, understory species richness in the lower slope unit remained low. Our results suggest that unlike broadleaf deciduous cool-temperate forests, broadleaf evergreen riparian forests in warm-temperate regions may not necessarily relate to greater species richness.

在日本的冷温带地区，河岸森林的树种丰富度往往很高，但这种趋势在暖温带地区是否一致尚不清楚。在这里，我们评估了三个地形单元中的树种丰富度和森林结构，这些地形单元反映了日本西南部成熟阔叶常绿林中山脊到山谷的梯度（坡顶和上下坡）。我们建立了一个 0.59 公顷的谷底地块，该地块主要由一个较低的坡度单元组成，然后对所有胸高 > 5 厘米的活树进行了调查和绘制。将得到的组成和物种丰富度数据与日本环境省提供的相邻永久地块的数据进行比较，该地块包括坡顶和上坡单元。在谷底，泥石流梯田的树木密度最高，是落叶阔叶树的重要栖息地。树种组成大致相似，因为在每个地形单元中都发现了几乎所有的树种。但不同地形单元的物种密度存在明显差异，低坡单元的物种密度最低。物种面积曲线表明，低坡单元的树种丰富度最低。物种个体曲线表明，所有三个单位的总物种丰富度都非常相似。即使包括杂草，下坡单元的林下物种丰富度仍然很低。我们的研究结果表明，与阔叶落叶冷温带森林不同，暖温带地区的阔叶常绿河岸森林可能不一定与更大的物种丰富度有关。