Previously, it has been observed that the internal circulation (ion leakage) of calcium from a coniferous forest is caused by uptake of sulphur dioxide (SO2). Here we show that this correlation was not changed when the forest floor is covered with a roof. The reaction takes place in the canopy and is not influenced by deposition and root uptake of calcium and sulphate. The ion leakage of calcium is linked to the loss of acidity in throughfall. The process can, for one of the catchments, schematically be written: SO2 + H2O + 0.5 O2 + 0.58 CaA2 → SO42−+ 0.94 H++ 0.58 Ca2++ 1.16 HA, in which A denotes the anion to a weak acid. This reaction also takes place today when the SO2 concentration is very low, but when the precipitation is still acidic. The ion leakage of manganese also is caused by the uptake of SO2, but only 0.12 manganese ions are released per SO2 molecule.