Effect of subalpine canopy removal on snowpack, soil solution, and nutrient export, Fraser Experimental Forest, CO

Research on the effects of vegetation manipulation on snowpack, soil water, and streamwater chemistry and flux has been underway at the Fraser Experimental Forest (FEF), CO, since 1982. Greater than 95% of FEF snowmelt passes through watersheds as subsurface flow where soil processes significantly alter meltwater chemistry. To better understand the mechanisms accounting for annual variation in watershed streamwater ion concentration and flux with snowmelt, we studied subsurface water flow, its ion concentration, and flux in conterminous forested and clear cut plots. Repetitive patterns in subsurface flow and chemistry were apparent. Control plot subsurface flow chemistry had the highest ion concentrations in late winter and fall. When shallow subsurface flow occurred, its Ca²⁺, SO₄²⁻, and HCO₃⁻ concentrations were lower and K⁺ higher than deep flow. The percentage of Ca²⁺, NO₃⁻, SO₄²⁻, and HCO₃⁻ flux in shallow depths was less and K⁺ slightly greater than the percentage of total flow. Canopy removal increased precipitation reaching the forest floor by about 40%, increased peak snowpack water equivalent (SWE)>35%, increased the average snowpack Ca²⁺, NO₃⁻, and NH₄⁺ content, reduced the snowpack K⁺ content, and increased the runoff four-fold. Clear cutting doubled the percentage of subsurface flow at shallow depths, and increased K⁺ concentration in shallow subsurface flow and NO₃⁻ concentrations in both shallow and deep flow. The percentage change in total Ca²⁺, SO₄²⁻, and HCO₃⁻ flux in shallow depths was less than the change in water flux, while that of K⁺ and NO₃⁻ flux was greater. Relative to the control, in the clear cut the percentage of total Ca²⁺ flux at shallow depths increased from 5 to 12%, SO₄²⁻ 5·4 to 12%, HCO₃⁻ from 5·6 to 8·7%, K⁺ from 6 to 35%, and NO₃⁻ from 2·7 to 17%. The increases in Ca²⁺ and SO₄²⁻ flux were proportional to the increase in water flux, the flux of HCO₃⁻ increased proportionally less than water flux, and NO₃⁻ and K⁺ were proportionally greater than water flux. Increased subsurface flow accounted for most of the increase in non-limiting nutrient loss. For limiting nutrients, loss of plant uptake and increased shallow subsurface flow accounted for the greater loss. Seasonal ion concentration patterns in streamwater and subsurface flow were similar .