Acidity consequent on root uptake was calculated as the excess cation accumulation in both stems and all above-ground tree components in a range of forest types, and values were derived for acidity resulting from the accumulated humus. In addition data from one pine stand was used to construct models of rate of accumulation of excess cations in trees and humus throughout the forest rotation. Rates of hydrogen ion production reaches a maximum very early in the life of a forest and the average annual permanent acidification resulting from removal of harvested material covers a wider range than either measured inputs in rainfall, including throughfall and stemflow, or estimated weathering rates. However, it is suggested that because rainwater inputs are episodic, include a mobile anion and may be channelled through the profile, whereas root generated acidity varies only gradually, does not involve the movement of an anion and occurs in intimate contact with the soil surfaces, rainwater acidity is the more likely to lead to streamwater acidification whereas root uptake will primarily acidify the soil.