Local and sub-basin effects of timber harvests on stream macroinvertebrates in Hinkle Creek watershed

Abstract

We studied the impacts of clear cut timber harvest over six-years, focusing on aquatic macroinvertebrate response using a BACI design. Data were collected before and after logging throughout a stream network to examine both local and downstream effects of upstream treatments. Sites were located in fishless headwater tributaries, fish-bearing tributaries and main stems in Hinkle Creek’s north fork (reference) and south fork (sequentially harvested) where benthic macroinvertebrates (from 2004 to 2009) and emergent insects (in 2005–2007 and 2009) were sampled. Pre-harvest (2004–2005) benthic macroinvertebrate composition was similar across sub-basins, with distinct longitudinal differences from tributaries to the main stems. At clear cut sites in fishless tributaries, benthic macroinvertebrate densities and percent Chironomidae increased, while taxa richness decreased, and these effects remained consistent for four years after the first harvest; aquatic insect emergence rates were also significantly higher in harvested v. reference fishless tributaries. No downstream macroinvertebrate responses were detected after the first harvest event. Following main stem harvest, confounded by a dam-break flood in December 2008, responses at main stem harvested sites were consistent with the localized responses observed in fishless tributaries, except benthic densities did not increase. While strong differences persisted among macroinvertebrates along the longitudinal gradient in the reference sub-basin, localized decreases in taxa richness and increased proportional contributions of generally disturbance tolerant chironomid midges among harvested sites resulted in altering the structure of invertebrate communities along the stream continuum. We conclude that additive effects of localized changes to instream resources along the stream continuum following successive harvest activities should be considered in plans for basin-wide timber harvests.