Local and landscape effects of past forest harvest on stream habitat and fish assemblages

Forest harvests have been shown to have negative effects on stream fish and habitat; however, the relationship between these factors, and the magnitude of these effects, has received little study. We investigated the influence that various land-cover types (including recent forest harvest) have on fish assemblages at multiple spatial scales and compared these results to the influences of local instream habitat variables. Satellite land-cover data and land management harvest maps were used to characterize the land-cover types throughout the Knife River basin in northeast Minnesota. Eleven spatial scales (with 30-m and 100-m buffer widths), including site, reach, stream corridor, and catchment, were evaluated. Forward stepwise regression was used to relate land cover to coldwater index of biotic integrity scores and metrics. Land-cover relationships varied with spatial scale, but land cover at the catchment and corridor scales explained the most variation in fish and habitat variables. Generally, increases in forest cover and decreases in water/wetland were associated with higher quality fish assemblages and instream habitat. No negative effects of forest harvest were found at the site or reach scales. Forest harvest 5-8 years old was negatively related to fish assemblage quality at the stream corridor and catchment scales, possibly related to changes in temperature and substrate at the corridor scale, and increases in fine sediments and unstable banks at the catchment scale. The cumulative effect of increasing forest harvest from 0 to 8 years old throughout the catchment was associated with lower quality fish assemblages and instream habitat, indicating that large increases in similar timed forest harvests throughout a catchment (not just in the riparian zone) can have negative effects on stream fish and habitat.