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

摘要

Nutrient fluxes were investigated on a forested anda clearcut plot in a mixed conifer high elevation (2900 m) forest at theFraser Experimental Forest in Fraser, Colorado, USA. Plots were locatedon a coarse loamy mixed Dystric Cryochrept with relatively high base saturation(30-90%) and underlain by an impermeable clay subsoil. Following harvestin late 1984, annual mean NO₃ concentrations of 195 to 198 μmol l-1 wereobserved from 1988 through 1990 and concentrations were still above referencelevels in 1993. Total nitrogen loss attributable to leaching followingharvest was estimated at 48kg ha-1 over 8 years. Over this same period,atmospheric nitrogen inputs exceeded annual outflow of NH₄ plus NO₃ fromthe control plots by approximately 11 kg N ha-1. A slight enrichment OfSO₄ and Cl was observed from the harvested plot in 1986 but concentrationslater fell below control plot levels, apparently due to dilution by theincreased discharge from the harvested plot which was three to four timesthat from the control plot. Elevated Ca, Mg, and Na concentrations followeda similar pattern to NO₃ due to exchange reactions, while a depressionin 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 mdepth. Increases in base cation loss due to leaching after harvest wereabout twice the amount that can be accounted for by the increased fluxof NO₃, SO₄, and Cl anions. The excess reflects the increased water fluxand consequent leaching of base cations in association with HCO₃ and organicanions.