Aluminum chemistry downstream of a whole-tree-harvested watershed

From fall 1983 through spring 1984, watershed 5 at the Hubbard Brook Experimental Forest in New Hampshire was commercially whole-tree harvested. Stream chemistry was monitored within the cut watershed, in an adjacent reference watershed (watershed 6), and below the confluence of these two streams for a period of 15 months prior to completion of the cut and 12 months following the cut. Whole-tree harvesting acidified watershed 5, due to increased soil nitrification, resulting in stream acidification within the disturbed watershed and for a distance downstream that encompassed a drainage area approximately 5 times that of watershed 5. Concentrations of NO₃^-, inorganic Al, and basic cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) increased following the cut, whereas SO₄^2- concentrations decreased. Concentrations of inorganic Al below the confluence of watersheds 5 and 6 exceeded values toxic to fish, but were temporally variable. Inorganic Al concentrations downstream of watershed 5 appeared to be controlled by dilution following the cut. There was no indication that hydrolysis was induced by mixing of streamwater from the acidic experimental watershed and undisturbed adjacent watershed. Absence of hydrolysis was due to low acid-neutralizing capacity of streamwater from the undisturbed drainage.