Long-term stream chemistry response to harvesting in a northern hardwood forest watershed experiencing environmental change
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abstract
Previous studies of harvest impacts on stream chemistry from deciduous forests with seasonal snow cover have shown variable responses and recovery times. We hypothesized that this variability in response and recovery was due to the interactive effects of forest harvesting with regional and global environmental changes. We investigated harvest impacts on stream solute (K⁺, Na⁺, Ca²⁺, Mg²⁺, Cl–, SO₄²–, DOC, Si, NO₃–, NH₄⁺, total P) concentrations, pH and conductivity under clearcut, selection, and shelterwood harvesting using a before-after control-impact experimental approach. The experiment was conducted at the Turkey Lakes Watershed, a shade-tolerant sugar maple forest within the Boreal Shield Ecozone, over a 36 year period (15 year pre-harvest and 21 year post-harvest) that has experienced climate change and acidification recovery. Harvest impacts on stream chemistry were greatest in the clearcut, moderate in the selection cut, and least in the shelterwood cut. The largest impacts on stream solutes typically lasted one to six years following harvest, although some solutes were impacted during the entire 21 year post-harvest period. Different patterns in the response of stream solute concentrations included: initial deviations (increase or decrease) followed by a return to pre-harvest levels within two to four years (Na⁺, Ca²⁺, Mg²⁺, conductivity); initial increase followed by a decrease but still not recovered (K⁺); initial increase followed by a decrease to below pre-harvest levels (NO₃–, Na⁺, Cl⁻); sustained elevated concentrations (dissolved organic carbon [DOC], Si); and a noisy response (TP, SO₄²⁻, pH). The different patterns were consistent with expected changes in nutrient demand, sources, and transport (including changes in hillslope-stream hydrologic connectivity due to harvesting). Comparing harvest impacts to long-term climate change and acidification recovery showed that harvesting had antagonistic (mitigative) effects to environmental changes for some solutes (e.g., cations), while for others (e.g., DOC) harvesting had agonistic (synergistic) effects to environmental changes. These results emphasize the need for long-term monitoring to accurately interpret the consequences of forest harvesting practices on the resilience of forests experiencing environmental change.
