Aquatic Insect Emergence from Headwater Streams Flowing through Regeneration and Mature Forests in Western Oregon

We examined the effect of canopy cover on adult aquatic insect emergence by collecting bi-weekly samples from twelve headwater stream reaches flowing either under a mature conifer canopy or streams flowing through ten-year-old regeneration in western Oregon from February to November 1997. Density and biomass generally followed a bimodal curve with peaks during early spring and sununer. Richness was unimodally distributed across time, peaking during mid-season, when abundance began to decline. However, there was a trend showing that headwater streams flowing through young forest supported 1.5-fold-higher insect density and biomass than streams flowing through mature forest (response confined primarily to Diptera). Many families of Diptera were indicators of open-canopy, whereas very few Ephemeroptera, Plecoptera, and Trichoptera (EPT) genera were indicators of either open or closed canopy. The proportional guild and ordinal composition of both density and biomass of the EPT guilds showed few differences between streams flowing through mature and regenerating forests.