| This study examined paired watersheds in forested wilderness, partial wilderness and second growth condition in the Skagit and Stilliguamish river basins in the North Cascade Mountains of Washington State, USA. The study examined the effects of watershed condition on: (1) water quality measured directly by dissolved oxygen, pH, and conductivity, and the patterns of stream flow; (2) the aquatic macroinvertebrate and plant populations; (3) the terrestrial floodplain plant populations; and (4) the substrate for these populations in the form of streamside large woody debris, stream boulders, and the floodplain surface. All sites were in remnant old-growth condition to isolate the hydrologic effects of watershed condition from other anthropogenic disturbances.; Field methods included surber-sampling in riffles to collect macroinvertebrates that were preserved and identified in the laboratory. Water quality was sampled directly instream with electronic instruments. All plant species including vascular plants, lichens and bryophytes were identified and characterized on fifty meter transects of the gravel bars and the floodplain terrace. Boulder size was measured by hand and plant cover was estimated with calibrated visual observation. Historical flow and landslide data was collected from USGS and University of Washington sources. Water quality, species richness, and community composition data was compared between the watershed types and classes using ANOVA and principal component analysis techniques. GIS and statistical tests were used to examine flow and landslide data.; The results of this study indicate that biodiversity and structural complexity are significantly lower at sites in wilderness watersheds than in second growth watersheds. Community structure shifted toward fewer disturbance-tolerant generalist species and away from more disturbance-sensitive specialist species. Water quality was also significantly different by watershed category, with dissolved oxygen higher in wilderness watersheds and conductivity and pH higher in second growth watersheds. Peak-flow and landslide rates increased significantly with cumulative area in second growth condition. Instream invertebrate communities showed shifts in species composition in second growth watersheds, showing declining Ephemeroptera, Plecoptera, Trichoptera diversity with declining wilderness. Instream and floodplain vascular plants and bryophytes had significantly lower species richness, and lower area of cover even when adjusted for substrate differences. Substrate changes helped to explain some of these shifts: significantly more, older and larger woody debris and larger, more stable boulders characterize watersheds with wilderness forests. |
