The effects of riparian and catchment forest harvest on stream habitat and fish assemblages

Forest harvests can have negative effects on stream habitat and fish communities in high gradient streams, but few studies have been conducted in low gradient streams typical of the midwestern United States. I studied the effects of riparian forest thinning and the influence that land cover (including recent forest harvest) have on fish assemblages at multiple spatial scales in two low-gradient cold-water stream systems in northern Minnesota. I also examined the relationship between fish communities and instream habitat measures and temperature in these systems. In both systems fish community quality was positively related to lower temperatures and negatively related to increases in fine sediments. In the Pokegama Creek stream system, riparian thinning along 200-m streamside reaches reduced canopy cover, but few significant effects of harvest treatment were found on habitat and fish variables over 3-years post-harvest. Fine sediments increased significantly throughout the basin one year following riparian thinning and were correlated with decreases in fish community quality, suggesting that factors operating at broader scales may influence fish and habitat, or mask any local harvest effects. The effects of land cover at multiple scales was examined in the Knife River basin. Land cover at the catchment and corridor scales, rather than the site and reach scales, explained the most variation in fish and habitat variables. Recent forest harvest (0-15 years) showed no negative effects on stream habitat or fish communities at the site or reach scale, but harvests 5-8 years old were negatively related to fish assemblage quality at the stream corridor and catchment scales. The cumulative effect of increasing harvest completed within a timeframe of 0-8 years throughout the catchment was associated with lower quality fish assemblages and habitat.;Short segments (< 200-m long) of riparian thinning in low gradient streams do not appear to negatively influence stream habitat and fish communities, nor does recent forest harvest at local scales. The cumulative effect of increasing forest harvests throughout the basin was negatively related to habitat and fish community quality. While local harvest may have little influence on fish community quality, cumulative effects of harvest throughout a basin deserve more attention.