Effects of riparian forest harvesting on the aquatic ecosystem in northern Minnesota streams

Background/Question/Methods In forested headwater streams, instream processes and aquatic communities are closely coupled with the adjacent terrestrial environment.  Timber harvesting in the adjacent forested riparian management zone has potential to disrupt the intimate connection between the terrestrial and aquatic ecosystems.  In 1996, a collaboration of Minnesota researchers and managers initiated a long-term research project investigating the effects of riparian forest buffers and harvesting methods on the riparian ecosystem (terrestrial and aquatic) in the Pokegama watershed (4 streams) in northern Minnesota.  In 2003, an additional eight sites, spread across northern Minnesota, were included into the study to investigate the effects of two levels of residual basal area (RBA) on the riparian ecosystem.  A primary purpose for these studies was to assess the effectiveness of Minnesota’s timber harvesting guidelines at mitigating impacts to aquatic communities.  For all sites, data were collected one year pre-harvest and up to 10 years post-harvest. 
We took a synthesized research approach to examine changes in riparian fish and aquatic invertebrate communities as a function of timber harvesting intensity (as measured by RBA) and the response of those communities post-harvest.  We examined several metrics of effect size including change in species richness, diversity, community dominance, tolerant/intolerant species, and guild structure for each year post-harvest.
Results/Conclusions Overall, trends were largely heterogeneous (i.e., varied among sites) but there was indication that slight decreases in intolerant fish and intolerant invertebrate taxa followed the highest degree of removal of the riparian forest cover.  Many of the water quality and other habitat variables exhibited a large degree of year-to-year variation but did not exhibit significant (P>0.05) harvest effects.  These results suggest that timber harvesting that retains the lowest recommended level of RBA along a stream may be adequate to minimize disruption to the aquatic ecosystem in northern Minnesota streams.