The health of Norwegian forests has deteriorated since the mid-1980s. Scientists are registering lower crown density and larger numbers of yellowing needles, signs that forests are being adversely affected. It is impossible to point to a single reason for the death of trees, loss of crown density or discoloration of needles. One or several types of pollution may be involved, combined with climatic stress factors such as drought, wind or frost. The crown density of the righthand spruce tree is normal given normal conditions where it is growing, whereas the crown density of the left hand tree is clearly lower.
Norwegian soils have slowly but surely become more acidic as a result of natural processes.
During the last couple of hundred years, human activity has speeded up the pace of acidification. The removal of timber causes acidification because it results in basic cations being removed from the soil more rapidly than they are added by natural weathering processes. As long as trees are growing, they tend to acidify the soil because they absorb minerals, thus removing basic ions from the soil. The process is reversed as trees die and rot, releasing minerals back to the soil where they can neutralize acidic ions. When foresters remove the timber, not all the nutrients absorbed by the tree are returned to the soil.