Estimation of the difference in crown vigour for 2280 coniferous trees in Norway from 1989 to 1994, adjusted for the effects of ageing.

The considerable deposition of airborne pollutants over Norway has caused concern about the effect on the country's conifer forests. Monitoring of the forest vitality is carried out as an annual assessment of the crown vigour of a representative sample of conifer trees. Crown vigour is a subjective and imprecise indicator, but reasonably precise when change is concerned. A method is described where the expected decline in vitality due to ageing is removed. The remaining, age-adjusted change in crown vigour is then assessed under the prevalent hypothesis of ongoing forest decline. The uncertainties involved with the method are discussed in the conclusion.