Crown density assessments, control surveys and reproducibility

This study aims to evaluate the quality of crown density data, based on ind ependent, pairwise tree assessments. The data originates from monitoring of forest health (crown condition) in Norway; 250 plots, comprising 12 000 in dividual trees of Norway spruce, have been reassessed by a single observer during 1990-95. Of the trees, 2300 were controlled more than twice, providi ng the possibility of evaluating the quality of assessed temporal changes o f crown density. True errors (standard deviation) are estimated to be about 10% for single trees and 5% for plot means, while the real standard deviat ion of the differences were slightly higher. The errors of the temporal cha nges of crown density were of similar magnitude. Systematic differences in crown density were found between sites and plot types, partly resulting fro m observer bias. However, the results suggest that observer bias is really the result of each observer's personal style in assessment.