Based on previous studies, we have come to expect certain distributions and diversities of stream organisms and communities along longitudinal gradients and temporal patterns of physical and biological stream attributes (Anderson and Sedell 1979, Vannote et al. 1980, Hawkins and Sedell 1981, and Minshall et al. 1983, Minshall et al. 1985, Bunn 1986, Ward 1986, Grubaugh et al. 1996). Currently, small headwater streams are receiving more attention in stream ecology and other related fields because they may represent unique communities of organisms across the landscape and along the stream continuum. In small forested streams, riparian vegetation often composes the food base for many stream invertebrates. Changes in the structure and abundance of riparian vegetation will likely affect stream invertebrates using these resources.
Shifts in stream invertebrate composition have been documented after removal of riparian vegetation (Murphy and Hall 1981, Hawkins et al. 1982, Stone and Wallace 1998). Various forest age classes may exhibit differences in taxonomic composition (Ward 1998), and richness of some taxonomic groups may be higher in streams with complete canopy removal of riparian vegetation (Stone and Wallace 1998). In the western Cascade Range of Oregon, Red alder (Alnus rubra) is the dominant early seral stage species in riparian zones. This provides a distinct contrast to analogous old growth areas. Invertebrate community patterns however, may vary seasonally in response to resource availability and in accordance with various life history traits of specific organisms (Mackay and Kalff 1969, Cummins and Klug 1979, Hawkins and Sedell 1981, Minshall et al. 1983, Anderson and Wallace 1984, Minshall et al. 1985, Bunn 1986, Dudgeon 1993). Invertebrate communities may exhibit greater abundance or taxa richness during times of abundant allochthonous resources (i.e., autumn leaf-fall). Additionally, it is likely that invertebrate community structure varies seasonally. It is therefore imperative to quantify seasonal dynamics of invertebrate communities in order to compare and/or contrast communities among various forest age classes and types.
If stream invertebrate abundance, taxa richness, or community structure differs between young and old forests, are legacies of these differences evident 20-40 years post-disturbance? If so, are these differences uniform through time, or are they temporally dependant? This study focuses on seasonal differences in stream invertebrate abundance, taxa richness, and community structure between old growth (200+ yrs) and young (20-40 yrs) forest stands in defined reaches within small-stream forested basins (12.4 to 98.1ha) in the H.J. Andrews Experimental Forest (HJA).
Six small-stream basins were selected. These basins were selected non-randomly according to the following criteria. Four with hydrologic gauges were selected based on the current objectives of the Long-Term-Ecological-Research program at HJA. These four basins have experienced a multitude of hydrologic, stream nutrient, and vegetative studies, yet our knowledge of benthic invertebrates in these areas is largely undocumented. Two additional non-gauged sites were selected to complete a paired basin study with a randomized complete block design. At three elevations (500, 700, and 1000m), the paired basins represent differences in local landscape characteristics including aspect, dominant overlying geology, and local vegetation. These three elevations serve as blocks. Within blocks, adjacent small-stream basins represent a young growth and old growth pair of observational research units. Each study reach consists of one, 50m stream section upstream of a stream gauge (in gauged basins) or access road (in non-gauged basins). These study reaches were selected based on ease of access.
Multiple sampling methods were used to assess stream invertebrate communities. Benthic (Surber sampler) sampling dates (4) were selected once per season (quarter) beginning in June 2003. Benthic samples were collected in summer (late June) before stream flow was too low to effectively collect samples. Autumn samples were not collected until stream flow returned to levels great enough to force stream water through the net (late November). Winter and spring samples were collected during periods of relatively stable stream flow thus avoiding increased stream flow during rain events (early March and May, respectively). Six benthic samples were collected in each 50m study reach in each season. Emergence traps were used to collect adult emergent insects throughout one calendar year. Sampling dates spread throughout from June 2003 to June 2004 were selected to correspond to benthic sampling dates plus additional seasonal data.
Preliminary results from benthic samples suggest strong changes in community structure between seasons as some taxa not found in summer (i.e., Agathon, Anagapetus, Chernokrilus, and Rhyacophila "angelita group") were collected in autumn and winter samples. However, results from summer comparisons demonstrate no differences in benthic and emergence abundance (p = 0.47, and p = 0.98, respectively) or benthic richness (p = 0.41) between old growth and second growth invertebrate assemblages.
Differences in abundance, taxa richness, or community structure between old and young growth stream ecosystems may be detected shortly after disturbance, but 20-40 years post-disturbance (in this case forest harvest) may be enough time for invertebrate communities to recover to similar assemblages as those found in corresponding undisturbed areas. Stone and Wallace (1998) demonstrated that although particular functional groups of invertebrates differed between a logged and un-logged site one year after harvest, EPT (Ephemeroptera, Plecoptera, and Trichoptera) taxa richness did not differ 1, 5, or 16 years after harvest. Another study found similar results with estimates of taxa richness being similar after harvest while proportions of various functional groups were different between old growth and harvested stream communities (Price et al. 2003). Current analyses are focused on seasonal differences of invertebrate communities between forest age classes with respect to functional properties of various invertebrate assemblages and guilds.
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