Evaluation of sunlight penetration through riparian forest and its effects on stream biota

Riparian streams contain various types of terrestrial organic and inorganic matter that are continuously transported from hillsides. Riparian forest forms alongside streams and blocks direct sunlight. It also supports the formation of various in-stream habitats, leading to the diversification of in-stream benthic invertebrates. In this study, sunlight passing through riparian forest canopy was measured and its relationship with the biota in the stream was analysed. Then, the light conditions favourable to stream biota were clarified for the purpose of riparian forest management. Differences in light conditions, chlorophyll a levels, and the richness of benthic invertebrates were observed, even in areas with stream widths of less than 5 m. As the canopy was closed in the upper stream area, intense sunlight was required to increase the solar radiation and chlorophyll a levels. Family richness was related to amounts of chlorophyll a. In contrast, in lower stream areas, because of the many gaps in the forest canopy, intense light was not necessary to increase the solar radiation, amounts of chlorophyll a, or family richness. Afternoon sunlight led to greater illuminance and higher chlorophyll a levels than did morning sunlight. The riparian forest on the west side of the stream was essential, as it blocked strong direct afternoon sunlight in summer. In riparian forest management, sunlight direction should be considered while paying attention to the composition of riparian forest.