- This research was designed to broaden the understanding of how timber-harvest affects aquatic macroinvertebrates in perennial and intermittent headwater streams. This study compared emergent and benthic macroinvertebrate assemblages from 20 headwater streams in the central Oregon Coast Range that varied by harvest condition and flow duration. Through comparison of the community and functional characteristics of the macroinvertebrates in similar streams under different harvest conditions, logging impacts on adult and immature aquatic macroinvertebrates in perennial and intermittent streams were assessed.
Adult insects emerging from the 20 study streams were collected using emergence traps set for four weeks in a 40-meter reach during three sampling periods: August-September 2003, October-November 2003, and April-May 2004. Adult aquatic insects were identified to genus and family taxonomic levels and results were expressed as number of individuals emerging per m2 per day. Benthic samples were taken from each reach in May 2004, insects and non-insects were identified to various taxonomic levels, and results were expressed as number of individuals per m2. Ten streams were located in catchments that were clearcut to the stream bank no more than a year before sampling began; three of these streams were intermittent and seven were perennial. Intermittent streams were summer-dry and retained surface-water for a minimum of eight months during the year, while perennial streams had surface flow year-round. The other ten streams (four intermittent and six perennial) were located in forested catchments that have not been logged for at least 34 years. The physical habitat of each stream reach was assessed in August 2003.
Regardless of flow-duration or season, more aquatic insects emerged from streams in logged catchments. The emergence rate of Diptera, predominantly Chironomidae, was especially high from clearcut sites. More Trichoptera also emerged from clearcut sites. Ephemeroptera and Plecoptera did not emerge at rates that varied by logging condition. Plecoptera had higher emergence from intermittent than perennial sites in spring. Taxa richness was higher at clearcut sites, driven primarily by rare taxa encountered in less than 5 of the 10 clearcut sites. Emergent insect functional feeding group composition was affected by harvest condition with collector-filterers emerging at higher rates from clearcut sites.
Examination of community patterns through nonmetric multidimensional scaling (NMS) ordination analyses indicated that emergent aquatic macroinvertebrate assemblages differ by season and by harvest condition. The duration of stream flow did not strongly influence adult assemblages. Channel dimensions (active channel and bankfull width) and catchment area were correlated with ordination-axes, but were not different between logged and forested sites. The amount of riparian cover (canopy, understory, and groundcover), stand age, slash, and stream cover were all considerably different between logged and forested sites and were highly influential on emergent assemblages. Substrate size was also influential on adult assemblages. Mean water temperature in summer, fall, and spring was highly correlated with seasonal variation in assemblages.
Regardless of harvest-condition, intermittent streams had higher overall benthic macroinvertebrate density. Trichoptera had higher densities at intermittent sites than perennial sites, while Diptera, Plecoptera, and non-insects had similar densities between categories of flow duration and harvest condition. Benthic macroinvertebrate functional composition was also affected by flow-duration, with collector-filterers, collector-gatherers, and predators occurring at higher densities at intermittent sites than perennial sites. The densities of scrapers and shredders did not differ between classes of flow duration or harvest condition. The majority of taxa (72%) were common to both perennial and intermittent sites; however, taxa richness was lower at intermittent sites.
NMS-ordination analyses of the benthic macroinvertebrates showed that community patterns differed by harvest condition within the perennial flow-duration class. Benthic communities of intermittent streams did not differ between harvest conditions. Channel dimensions (active channel and bankfull width), mean substrate diameter, percent stream-cover, mean water temperature, elevation, and percent small riparian trees influenced benthic community assemblages. The majority of perennial-clearcut sites had macroinvertebrate community patterns more similar to intermittent-forested and intermittent-clearcut streams than to perennial-forested sites. The two perennial-clearcut sites that did not have benthic assemblages similar to the other five perennial-clearcut sites had larger diameter substrate, higher elevation, larger catchment area, and cooler mean water temperature than other perennial-clearcut sites.
This study provides evidence that Oregonâs current timber harvesting practices impact macroinvertebrate assemblages of perennial and intermittent headwaters in the Central Oregon Coast Range within a year following harvest. When the studied streams were clearcut to the streambank, there was increased emergence of adult macroinvertebrates, principally midges, at clearcut sites. Benthic assemblages differed primarily between intermittent and perennial flow-duration. However, within flow-classes, assemblages of benthic aquatic macroinvertebrates in perennial streams were impacted by clearcut logging to a higher degree than intermittent streams. Perennial-clearcut streams had different benthic assemblages from perennial-mature streams and perennial-clearcut streams with coarser substrate and cooler temperatures. Intermittent streams, which generally experience fewer flushing events and accumulate finer substrates than perennial streams, may be affected to a lesser degree by any sedimentation caused by logging activity. Correlations of emergent and benthic macroinvertebrate communities with the measured environmental gradients are suggestive that reduced canopy cover and substrate composition are major influential factors.