Detecting differences in crown transparency assessments between countries using the image analysis system CROCO.

Estimates of crown transparency calculated using the semi-automatic image analysis system CROCO were compared with the visual estimates of survey teams from 12 European countries. For each of five European species (Norway spruce (Picea abies (L.) Karst.), silver fir (Abies alba Mill.), Scots pine (Pinus sylvestris L.), beech (Fagus sylvatica L.), oak (Quercus robur L.)) 15 trees were assessed. In 64% of the comparisons, CROCO estimates and the survey teams' scores differed significantly, while differences between countries were significant in 58% of all comparisons. We calculated Spearman's rank correlation and the standard deviation of the differences between CROCO and each country and between pairs of countries. We then computed the medians for CROCO and each country. CROCO and only a few countries had for all species a higher median correlation and lower median standard deviation than the median for the comparison between countries. In addition no country had a consistently higher correlation or lower standard deviation than CROCO. We conclude that CROCO can provide more consistent and less variable estimates of crown transparency than visual assessments and that it can serve as a reference to detect differences in visual transparency assessments between countries over time.