Effect of Subalpine Canopy Removal on Snowpack, Soil Solution,
and Nutrient Export, Fraser Experimental Forest, Colorado
Robert Stottlemyer1 and Charles A. Troendle2
Long-term research on the effects of vegetation manipulation on snowpack, soil water, and stream-
water chemistry and flux is a major objective at the alpine/subalpine Fraser Experimental Forest
(FEF), Colorado. In Rocky Mountain subalpine ecosystems, revegetation following disturbance is
slow. There is particular interest in the fate of increasing atmospheric inorganic nitrogen (N) inputs,
and the interaction between canopy removal and ecosystem N loss. Greater than 95% of FEF snow-
melt passes through the watersheds as subsurface flow, and soil processes significantly alter melt-
water chemistry. To better understand the mechanisms accounting for annual variation in watershed
streamwater ion concentration and flux with snowmelt, we studied subsur-face water flow, its ion
concentration, and flux in conterminous forested and clear-cut plots. Re-sults were compared to
streamwater ion flux and concentration from an adjacent watershed. The plots were established in
1978 and 1979 and monitored in an undisturbed state until 1984 when one was clear-cut. The effect
of clear-cutting was then studied for a decade. Repetitive patterns in subsurface flow and chemistry
were apparent. Control plot subsurface flow chemistry had the high-est ion concentrations in late
winter and fall. When shallow subsurface flow occurred, its Ca2+, SO42, and HCO3 concentra-
tions 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 percent-age of total flow. Canopy
removal increased precipitation reaching the forest floor by about 40%, increased snowpack peak
water equivalent (PWE) >35%, increased average snowpack CA2+, NO3, and NH4+ content, re-
duced snowpack K+ content, and increased runoff fourfold. 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 of total Ca2+, SO42, and
HCO3 flux in shallow depths was less than the percentage of shallow sub-surface flow, but K+ and
NO3 flux were greater. Relative to the control, canopy removal increased the percentage of total
Ca2+ flux at shallow depths 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, HCO3 flux was less, and NO3 and K+ greater. Increased subsurface
flow accounted for most of the increase in nonlimiting nutrient loss. For limiting nutrients, loss of
plant uptake and increased shallow subsurface flow accounted for the greater loss. The increase in
NO3 flux from the clear-cut (net N loss 6 kg N ha1 yr1) was the most pronounced response. After
a decade of study, NO3 concentrations in subsurface flow from the clear-cut remained above the
control plot (mean of 49 versus 2 eq L1). Streamwater and control plot NO3 concentrations and
variation were identical during the 10-year study. Patterns of seasonal ion concentration in subal-
pine streamwater and plot subsurface flow were similar, indi-cating that processes defined in the
plot study may regulate seasonal change in streamwater chemistry.
1
U.S. Geological Survey, 240 W. Prospect Road, Fort Collins, Colorado 80526 USA
2
U.S. Forest Service, Rocky Mountain Research Station, 240 W. Prospect Road, Fort Collins, Colorado
80526 USA
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