Release of aluminum following whole-tree harvesting at the Hubbard Brook Experimental Forest, New Hamsphire

From fall, 1983, through spring, 1984, an experimental watershed at the Hubbard Brook Experimental Forest in New Hamsphire was commercially whole-tree harvested. Stream chemistry was monitored along an elevational gradient for approximately 1 yr prior to and 2 yr following the treatment in both the experimental watershed and an adjacent reference watershed. Whole-tree harvesting initially resulted in a large increase in stream NO₃^- and basic cation (Ca²⁺, Mg²⁺, Na⁺, and K⁺) concentrations and a decrease in stream sulfate concentrations. A decrease in pH and increase in Al concentrations followed. Elevational trends in stream chemistry were qualitatively similar before and after the cut, although the absolute concentrations of solutes changed markedly. These responses are explained by increased soil nitrification coupled with decreased vegetative uptake following the whole-tree harvest. Acidity generated by nitrification facilitated the mobilization of basic cations and increased anion adsorption. When NO₃^- production exceeded the release of basic cations, stream pH declined and Al concentrations increased. Aluminum was released in an entirely inorganic form resulting in potentially toxic concentrations in streamwater.