Comparing the geography of changing crown density from two sampling systems for Picea abies in Norway

This study examined the geographical pattern of changing forest condition i n Norway by comparing two nation-wide, independent sets of Norway spruce (P icea abies L. Karst) plots with 8 yr of crown density data. Each set had so me limitations regarding its ability to provide reliable information about the pattern of its changes in crown density. This problem mainly concerned the sampling strategy and the grid density. One set was a random sample and the other was a subjective sample. However, a consistent geographical patt ern was found at the regional scale, with the greatest reductions in crown density in mid Norway and south-eastern Norway. These areas are little to m oderately affected by long-range air pollution, and the results draw the at tention towards a range of other stress factors. There was no relationship between the two samples at the local scale. The change in crown density bet ween 1990 and 1997 estimated from the two samples was significantly differe nt. The results demonstrated the influence of different sampling techniques on the forest health statistics, but also indicated that the two samples t ogether can form a basis for further studies of forest condition in Norway, since the subjective sample complements the random sample at the regional level.