This study was designed to evaluate if long term signals of prior forest harvest exist between paired basins for macroinvertebrate species and communities.
Results from this study suggest that neither richness nor densities differed between streams flowing through young growth versus old growth forests. Despite similarities among these metrics, multivariate ordination techniques helped elucidate slight differences in benthic community composition between paired streams when red alder was present in riparian zones of previously harvested basins. Indicator Species Analysis of community composition and abundance revealed that no taxa were exclusively indicative of either forest type.
Macroinvertebrate life-history traits among headwater streams were related to stream size, stream substrates, or stream discharge. Macroinvertebrates that ingest leaf litter (shredders) decreased at sites with increasing stream width, while macroinvertebrates that scrape off algae and biofilms from instream substrates (scrapers) increased with increasing stream width. Differences in macroinvertebrate habit-trait groups were related to differences in stream substrates or stream discharge between very small headwater streams (< 20 ha basin area) and larger headwater streams (50-100 ha basin size). Seasonal variation in stream discharge also influenced macroinvertebrate communities.
Patterns in adult insect emergence in these streams displayed pulses of activity that varied among streams. During summer 2003, total emergence was greatest in one high elevation stream in this study. During spring 2004, total emergence began earlier in one low elevation and one mid elevation stream, but linkages with stream water temperature were tenuous; cumulative emergence remained higher in these streams than all others through early summer 2004. Although forest harvest can often have effects on macroinvertebrate communities, differences in community composition may persist long-term if regrowth of riparian vegetation includes shifts to other species, such as red alder (Alnus rubra).
Instream and riparian conditions: Instream and riparian attributes were measured at the time of benthic sampling. Six transects, perpendicular to stream flow, were established in study reaches and wetted stream widths were measured at each transect. At 11 points across each transect stream depths were measured and substrate types recorded. Substrate type was characterized by size as silt, sand, gravel, cobble, boulder, bedrock, or wood (modified from Wolman 1954). Percent canopy cover was measured using a canopy densiometer mid-channel at each transect with the observer facing upstream, downstream, right, and left (Platts et al. 1987). For each reach, average proportion of substrate type and average percent canopy cover were calculated. Composition of overstory and understory riparian vegetation was noted within 5 m of the stream. Allochthonous litterfall was collected continuously from July through October 2003. At each site, six litter traps, circular baskets (0.12 m2) with 0.6 mm mesh nets, were placed along the edges of the stream. Leaf litter was collected monthly, dried at 60ºC for 24-36 hours, and weighed. Litter was categorized as red alder leaves, other deciduous leaves, conifer needles, or miscellaneous (catkins, cones, lichens, and twigs).
Macroinvertebrate sampling: Benthic sampling was conducted four times and during periods of relatively stable stream flow; June 2003, November 2003, March 2004, and May 2004. Emerging macroinvertebrates were collected a total of 13 times; at the time of benthic sampling, at midpoints between benthic samples in 2004, and approximately 3-week intervals during the summer and early fall 2003. Locations of benthic sampling were randomized within study reaches at each sample date; emergence sample locations were only randomized once at the start of the study. On each sampling date, six benthic samples were collected per reach. All samples were taken with a 0.25 mm mesh Surber sampler (0.093 m2) in riffle/cascade stream units on substrates ranging from silt to cobble. Four 0.25 m2 emergence traps, consisting of PVC frames draped with 0.6 mm mesh nets, were set over the stream bed and left in place for 6-8 days and then sampled. A small amount of unscented, biodegradable soap was added to the water in emergence trap collecting cups to decrease surface tension; in cold weather, rock salt was added to serve as both an antifreeze and a preservative agent. When invertebrates were collected, they were sieved through 0.25 mm mesh and stored in 95% ethanol. Emergence rates were calculated as densities of invertebrates collected in traps during each 6-8 day collection interval and expressed as number of individuals.m-2.day-1.
Benthic density (number/m2) is calculated by the number of benthic individuals in surber / surber size (0.093 m2).
Emergence rate (number per m2 per day) is calculated by the number of individuals emerging from trap during sampling interval / trap size (0.25 m2) / trap duration.
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