Forest Management Options to Maintain Hydrologic Connectivity in Boreal Forest Streams of Ontario

Forest management is a major component of the economy of Northwestern Ontario and the government has a mandate to regulate harvest activities in a manner that both protects aquatic resources and maintains the supply of forest products. Analyses of forest harvesting in the region indicates that, on average, 15% of headwater catchments were disturbed by harvest during the past 5 years.  The median catchment area harvested was 17.5%; however, 13.4% of the catchments had more than half their area harvested.  There is concern that such harvest levels may result in hydrologic alterations that impact fish habitat but there is also uncertainty about how the pattern of harvest disturbance within a catchment contributes these potential impacts. Habitat in small streams is influenced by both longitudinal (upstream) and lateral runoff and allocthonous inputs which differ along the length of a stream.  The objectives of our research were to: 1) Evaluate associations between lateral contributions to a stream and characteristics of stream habitat and biota, 2) Examine changes in these terrestrial-aquatic linkages following forest harvesting and 3) Develop GIS based techniques to quantify levels of lateral contribution along streams.  Reaches associated with larger areas of lateral contributions, or “Reach Contributing Area” (RCA), had finer substrate and more wood debris, greater thermal variability (up to 12^oC), a distinct benthic invertebrate community structure, and higher brook trout abundance compared to reaches with smaller RCAs.  Impacts on terrestrial-aquatic linkages were evaluated by monitoring water and nutrient inputs into a stream before and after experimental harvest. Water yield from RCAs approximately doubled relative to pre-harvest and water flux increased significantly relative to a control.  Concentrations of DOC and some nutrients (e.g. K, N, P) and metals (e.g. Mn, Fe) increased 2 to 3 times in the year after harvest, and the flux rate of mercury approximately double that of reference conditions. Identification of potentially significant RCAs will facilitate planning of forest management activities to preserve terrestrial-aquatic linkages and potentially mitigate adverse impacts to fish habitat.