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 Experim ental Forest (FEF), CO, since 1982. Greater than 95% of FEF snowmelt passes through watersheds as subsurface flow where soil processes significantly a lter meltwater chemistry. To better understand the mechanisms accounting fo r annual variation in watershed streamwater ion concentration and flux with snowmelt, we studied subsurface water flow, its ion concentration, and flu x in conterminous forested and clear cut plots. Repetitive patterns in subs urface flow and chemistry were apparent. Control plot subsurface flow chemi stry had the highest ion concentrations in late winter and fall. When shall ow subsurface flow occurred, its Ca2+, SO42-, and HCO3- concentrations were lower and K+ higher than deep flow. The percentage of Ca2+, NO3-, SO42-, a nd HCO3- flux in shallow depths was less and K+ slightly greater than the p ercentage of total flow. Canopy removal increased precipitation reaching th e 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 bot h shallow and deep flow. The percentage change in total Ca2+, SO42-, and HC O3- flux in shallow depths was less than the change in water flux, while th at of K+ and NO3- flux was greater. Relative to the control, in the clear c ut 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- f rom 2.7 to 17%. The increases in Ca2+ and SO42- flux were proportional to t he increase in water flux, the flux of HCO3- increased proportionally less than water flux, and NO3- and K+ were proportionally greater than water flu x. Increased subsurface flow accounted for most of the increase in non-limi ting nutrient loss. For limiting nutrients, loss of plant uptake and increa sed shallow subsurface flow accounted for the greater loss. Seasonal ion co ncentration patterns in streamwater and subsurface flow were similar. Copyr ight (C) 1999 John Wiley & Sons, Ltd.