All through these years, there was a fierce discussion about what caused the damage. Scientists still do not agree entirely, but no-one doubts that acid rain affects forests. Norwegian forests are mainly affected indirectly, by the acidification of soils as nutrients are lost by leaching. Nevertheless, direct injury has been observed, for instance in the Pasvik valley in Finnmark county, where sulphur dioxide in the acid smoke from the nickel refineries on the Kola Peninsula causes burns on pine needles and the foliage of deciduous trees. The indirect effects of acid rain on forests are caused by the loss of soil nutrients.
Acid rain can displace nutrients such as calcium and magnesium from the soils.
Scientists have modelled these processes, and estimate that in forested areas in southern Sweden and Norway, acid rain may have leached out 20 to 70 per cent of the most important minerals. In very acid conditions, aluminium becomes soluble and is released from the soil. At high enough concentrations, aluminium is toxic and damages tree roots. Until recently, it was thought that soil acidification was primarily caused by sulphur deposition, but scientists have now realized that nitrogen compounds from exhaust fumes can also cause damage. Nitrogen inputs increase forest growth, but the trees need other nutrients as well.