Effect of clear cutting on nutrient fluxes in a subalpine forest at Fraser, Colorado

Abstract. Nutrient fluxes were investigated on a forested and a clearcut plot in a mixed conifer high elevation (2900 m) forest at the Fraser Experimental Forest in Fraser, Colorado, USA. Plots were located on a coarse loamy mixed Dystric Cryochrept with relatively high base saturation (30-90%) and underlain by an impermeable clay subsoil. Following harvest in late 1984, annual mean NO₃ concentrations of 195 to 198 μmol l^-1 were observed from 1988 through 1990 and concentrations were still above reference levels in 1993. Total nitrogen loss attributable to leaching following harvest was estimated at 48kg ha^-1 over 8 years. Over this same period, atmospheric nitrogen inputs exceeded annual outflow of NH₄ plus NO₃ from the control plots by approximately 11 kg N ha^-1. A slight enrichment Of SO₄ and Cl was observed from the harvested plot in 1986 but concentrations later fell below control plot levels, apparently due to dilution by the increased discharge from the harvested plot which was three to four times that from the control plot.
Elevated Ca, Mg, and Na concentrations followed a similar pattern to NO₃ due to exchange reactions, while a depression in alkalinity of about one-third the amount of NO₃ found was also observed. Enrichment of K occurred primarily in water collected at less than 1 m depth. Increases in base cation loss due to leaching after harvest were about twice the amount that can be accounted for by the increased flux of NO₃, SO₄, and Cl anions. The excess reflects the increased water flux and consequent leaching of base cations in association with HCO₃ and organic anions.