Tree species composition and richness in a mature, warm-temperate riparian forest in Shikoku, Japan

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.