Crown density and growth relationships between stands of Picea abies in Norway

Relationships between the crown density and growth of Norway spruce (Picea abies L. Karst.) stands are presented, after removal of the effects of major natural influences. Crown density was assessed annually from 1991 to 1996 on 569 monitoring plots comprising 40000 trees. Stand growth was determined from measurements of diameter and height in 1991 and 1996. Various models explaining mean crown density and annual growth of the stands as a function of natural factors, such as age and site index, were compared. The influence of the natural factors was then removed by recalculating crown density to residual values from one preferred model, and by recalculating growth to relative Values given as a percentage of model predictions. Crown density and its residuals were positively correlated to growth. These relationships were weak in terms of their ability to explain variation (low R-2). However, the various relationships consistently indicated that approximately 1% change in crown density corresponded to 1% change in growth. This relationship also included common spatial variation over Norway: a large part of south-east Norway had unexplained low crown density and unexplained low growth. Some other, smaller regional consistencies were also found. The study supports the use of crown density assessments and encourages the use of growth data in the search for major stress factors responsible for present forest condition.